Article pubs.acs.org/Langmuir
Plastron-Mediated Growth of Captive Bubbles on Superhydrophobic Surfaces So Hung Huynh, Alifa Afiah Ahmad Zahidi, Murat Muradoglu, Brandon Huey-Ping Cheong, and Tuck Wah Ng* Laboratory for Optics and Applied Mechanics, Department of Mechanical and Aerospace Engineering, Monash University, Clayton, Victoria 3800, Australia ABSTRACT: Captive bubbles on a superhydrophobic (SH) surface have been shown to increase in volume via injection of air through the surrounding plastron. The experimental contact diameter against volume trends were found to follow that predicted by the Surface Evolver simulation generally but corresponded with the simulated data at contact angle (CA) = 158° when the volume was 20 μL but that at CA = 170° when the volume was increased to 180 μL. In this regime, there was a simultaneous outward movement of the contact line as well as a small reduction in the slope that the liquid−air interface makes with the horizontal as air was injected. At volumes higher than 180 μL, air injection caused the diameter to reduce progressively until detachment. The inward movement of the contact line in this regime allowed the bubble body to undergo shape deformations to stay attached onto the substrate with larger volumes (300 μL) than predicted (220 μL at CA = 170°) using simulation. In experiments to investigate the effect of translating the SH surface, movement of captive bubbles was possible with 280 μL volume but not with 80 μL volume. This pointed to the possibility of transporting gas-phase samples on SH surfaces using larger captive bubble volumes.
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plastrons have been shown to be highly stable.19 Unlike the case of a sessile drop, it is relatively easy to dispense a bubble on a SH surface that is replete with a plastron, because the bubble attaches more readily to the surface than the tip. Using an energy depiction, the work performed associated with the respective liquid−vapor, solid−liquid, and solid−vapor surface tensions γ, γSL, and γSV, liquid−vapor, solid−liquid, and solid− vapor area changes dALV, dASL, dASV, Laplace pressure change ΔP, and volume change dV (see Figure 1) can be given by
INTRODUCTION Gas bubbles that can stably exist on solid surfaces or captive bubbles have appearances similar to sessile drops. Because of this and the widely believed reciprocal behavior between the two, there have been reports of determining wettability by measuring the contact angle (CA) of captive bubbles instead of sessile drops, which is convenient because the instrumentation to do so is essentially the same.1,2 While there are abundant reported applications with the use of sessile drops,3,4 relatively fewer have been performed with captive bubbles. Nevertheless, it is noteworthy that the latter include useful operations, such as the transport of material5,6 and surface cleaning.7 Various ways to generate bubbles in a flow channel have been reported. These include the use of co-flows,8 ultrasonics,9 and heating.10 The methods mentioned, however, may not be suited for locating captive bubbles precisely on solid surfaces. It has been shown that air dispensation using a retracting tip offers a ready means to achieve this.11,12 Once a captive bubble is created, it is noteworthy that pressure can be used to vary its size.13 Superhydrophobic (SH) surfaces have been widely investigated because of the low levels of hysteresis that they offer. It is therefore not surprising that sessile drops dispensed on SH surfaces have been increasingly used in various applications.14−16 When a SH surface is placed underwater, it generates a visible film of air called plastron. This has been widely studied in terms of understanding the ability of certain insects to breathe underwater;17 albeit, its use in drag reduction for liquid flows has been attempted.18 It is noteworthy that © 2015 American Chemical Society
δw = γ dALV + γSL dASL + γSV dASV + ΔP dV
(1)
If the conditions of δw = 0, ΔV = 0, dASL = dASV, and dALV/ dASL = cos θ for a semi-spherical shape are enforced with
Figure 1. Schematic depiction of a captive bubble located on a solid substrate with a contact diameter 2r and CA θ. Received: January 7, 2015 Revised: May 14, 2015 Published: May 18, 2015 6695
DOI: 10.1021/acs.langmuir.5b00058 Langmuir 2015, 31, 6695−6703
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proximity to the outer edge of the bubble to dispense the remaining gas volume. There were three sets of experimental data collected. The first set was to examine the maximum volume against variation of hydrostatic pressure ranging from 14 to 45 mm H2O (from 137.3 to 441.3 Pa). In this experiment, air is dispensed at increments of 1 μL, followed by a rest period of 3 s each time to simulate a quasi-static condition. This is performed until the captive bubble departs from the substrate. Readings of the critical volume were obtained using the accurate dosing mechanism provided by the DSA100S system. In the other experiment, air is dispensed at step volumes of 20 μL, followed by a rest period of 3 s each time to simulate a quasi-static condition. This is performed until the captive bubble departs from the substrate under hydrostatic pressures of 14 and 30 mm H2O (from 137.3 to 294.2 Pa). Images after each step were captured using the DSA100S built-in camera, and an open source image analysis software (ImageJ) was used to determine the contact diameter of the captive bubble on the substrate. In all instances, the lens on the DSA100S was laser-guided to align at 90° with the needle tip to prevent any projection from occurring in measurement. In a third experiment, we first created a 150 μL captive bubble on the surface. As air was injected through the plastron, we then recorded the captive bubble development using a high-speed camera (Fastec Troubleshooter) at 125 frames per second. The CA was then analyzed using the software Tracker (https://www. cabrillo.edu/~dbrown/tracker/). Bubble Transportation. This experiment was conducted to demonstrate the ability of bubbles generated on the SH surface to move under an inertial forcing function. The SH substrate was mounted underwater at 690 Pa of hydrostatic pressure. A pneumatic actuator (Numatics, G453A3SK0050A00) driven by compressed air with a gauge pressure of 45 kPa was used to translate the platform horizontally through the setup shown in Figure 2. The movement was
surface tension dominating over buoyancy, eq 1 simplifies to the easily recognizable Young’s equation of γ cos θ = γSV − γSL
(2)
Both eqs 1 and 2 are often taken to be useable interchangeably with sessile drops and captive bubbles. As in the case with sessile drops, one would normally increase the volume by first depositing a captive bubble onto the surface, followed by pipetting additional air volumes onto it. There will be a significant degree of hysteresis this way because of the manner in which air is delivered and the pipet drawn away. Consequently, the captive bubble will manifest a relatively wide range of contact diameters on the surface.20 To overcome this, an alternative method to deliver air to the captive bubble will be desirable. Here, we investigate the delivery of air into the plastron around the captive bubble to do so. Captive bubbles that are created uniformly are useable if they can be transported from one spatial location to another on the substrate. We additionally explore a substrate translating mechanism without the use of liquid flow to do this.
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MATERIALS AND METHODS
Sample Preparation. SH surfaces were created using an electroless galvanic deposition process. This process allowed for the rough tuning of wettability depending upon the variables in the preparation process. Polished copper surfaces were first cleaned with absolute ethanol and allowed to dry. They were then immersed in a solution of aqueous AgNO3. After this, the surfaces were cleaned with absolute ethanol and dried with compressed inert gas. They were subsequently immersed in a 1 mM solution of the surface modifier CF3(CF2)7CH2CH2SH in absolute ethanol. The samples were then thoroughly rinsed using at least 100 mL of distilled water, followed by rinsing with absolute ethanol. They were then allowed to air-dry. The sessile drop CAs were measured to be 158° (σ = 0.80°). The surface microstructure was examined using scanning electron microscopy (SEM, FEI, NovaNanoSEM 430). Computation. The Surface Evolver software was used to compute the predicted profile and size of inverted droplets. It does this using an energy minimization method that evolves a surface down the energy gradient. The assumption made in the computation was that an inverted static water drop on a surface should be identical to an immersed bubble on another surface with an opposing CA (as described in Figure 1). For example, the static profile of a hanging pendant drop on a surface with a CA of 10° was assumed to be identical to the profile of an immersed bubble on a surface with a CA of 170°. Experimental Section. Plastron Stability. To obtain an indication of plastron stability, the superhydrophic substrate was partially immersed into a container of distilled water. A camera was trained at this substrate around the immersion region, and its angle of view adjusted such that the plastron, which manifests as a reflective optical layer, could be seen. Images were recorded at intervals of 1 min for 30 min. A similar procedure was undertaken for a substrate that has lost its superhydrophobicity. The surface characteristic was confirmed prior by dispensing water drops on it, where they did not roll away easily. Air Dispensation on Captive Bubbles. The air dispensation studies were conducted on a drop shape analysis system (KRÜ SS DSA100S). The microsyringe used to dispense bubbles had a capacity of 500 μL and a 26 gauge needle (outer diameter of 0.46 mm) tip equipped with a Luer screw (Innovative Labor Systeme, 2607909 and 2849045). The substrate used was prepared as described in the previous section. A plastic container with dimensions of 87 × 87 × 75 mm (width × length × height) was used as a water reservoir. During the experiment, the temperature was monitored to be constant at 22 °C. To dispense bubbles through the plastron, first, a primary bubble whose size is significantly less than the critical volume was deposited on the substrate. Next, the needle tip was moved within close
Figure 2. Setup to observe the bubble transportation characteristics of bubbles deposited on SH surfaces. In a tank with water, impulsive movement (in the direction of the solid arrow) was provided by a pneumatic actuator acting through an adaptor. A high-speed camera was used to record the captive bubble movement under illumination by a bright light source. If the captive bubble was able to move relative to the substrate, it would do so in the direction of the dashed arrow. recorded using a high-speed imaging device (Fastec Imaging, Troubleshooter TS1000ME) at 500 frames per second. Bubble sizes of 280 and 80 μL were dispensed onto the substrate using the method, and the experiment was repeated 3 times for each volume. The pneumatic pressure was measured using a digital gauge (Digitron, 2002P) when the solenoid was in the closed state. An open-source motion analysis software (Tracker) was used to examine the recorded images.
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RESULTS AND DISCUSSION At the outset, we sought to establish that the CAs developed were high on the surface created. In using a droplet and bubble of 50 μL, the respective equilibrium CAs were found to be 160° 6696
DOI: 10.1021/acs.langmuir.5b00058 Langmuir 2015, 31, 6695−6703
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Figure 3. SEM images at different magnifications of the SH substrate showing multi-scale and dendritic microstructures. The structures, which are at different heights, allow for the creation of multiple three-phase points that allow the plastron to exist stably.
± 5° and 165° ± 5°, respectively. This appears to indicate the complementing CA behavior exhibited between sessile drops and captive bubbles. It has been pointed out that using CAs to describe surfaces that have plastrons residing on them or sometimes referred to as exhibiting underwater superhydrophobicity is not particularly meaningful.21 Therefore, distinction should be made with studies with captive bubbles in which plastrons have not been reportedly seen,22 because the underlying mechanics may be different. In addition, measuring the apparent CAs to denote captive bubble growth is likely to have high levels of uncertainty when the base value is closer to 180°,23 and this uncertainty is compounded if the process is dynamic. For this reason, we have used the contact diameter as the defining parameter. Figure 3 presents typical SEM images of the SH substrate recorded at varying magnifications. Multi-scale structures, which resemble dendrites and granular deposits can be seen. These structures provide opportunity for three-phase contact sites to form across the expanse of the surface from which the plastron can exist stably. A substrate that has lost its superhydrophobicity when partially immersed in water did not exhibit any strong light reflectivity (Figure 4a). However, a SH substrate partially immersed showed strong light reflection from the air-dominated plastron (see Figure 4b). The optical characteristics were unchanged after 30 min (Figure 4c), indicating that the plastron was still intact. This showed that the plastron was stable if the substrate was left undisturbed in water.
Figure 5a shows the image recorded of an air bubble initially deposited on the SH substrate using a retracting syringe tip. The syringe tip was then used to deliver air via the plastron adjacent to the bubble, causing the bubble to grow in size (Figure 5b). This is obviously due to a higher pressure at the tip as opposed to the bubble. This parallels a process previously reported.24 However, in that situation, it is not stated if a plastron layer was observed; if not, the underlying mechanism may be different. The bubble was found to be slightly perturbed during the process. This was unlike the approach of placing the tip directly into the bubble, inserting air, and subsequently withdrawing the tip, which was found to perturb the bubble significantly. After achievement of a significant size, the bubble could detach from the surface through buoyancy (Figure 5c). Figure 6a provides distributions of the diameter in contact with the substrate surface against the volume of captive bubbles dispensed for CAs of 158°, 165°, and 170° using Surface Evolver simulation. The increasing trends extend up to the point where the heightened buoyancy force results in a nonsolution, which can be physically interpreted as the bubble detaching from the surface (220 μL for CAs of 165° and 170° and 210 μL for CA of 158°). From the experiments conducted by air delivery through the plastron (see Figure 6b), it can be seen that contact diameter against volume trends generally follow that predicted by simulation. However, there are notable differences. One of them lies with the experimental data corresponding with the simulated data at CA = 158° when the volume was 20 μL but then corresponding more closely with 6697
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not even exist. Hence, it is necessary to consider the physics more carefully. Rather than an isolated air layer sitting on top of the solid substrate, plastrons exist as a result of the formation of menisci over a multitude of three-phase contact points (see Figure 7a). If we consider just one meniscus, the pressure drop across the liquid−air interface is given by γ ΔP = LV (3) r where r is the radius of curvature of the interface. Clearly, if ΔP on the left-hand side of eq 3 is greater, r will have to correspondingly decrease to satisfy the equilibrium condition. However, when the meniscus is not able to form because of limited clearance from the substrate base, the plastron will then collapse. Hence, plastrons do have a lifespan that is dictated by the diffusion of air into the liquid, which lowers the available air clearance relative to the substrate base. At this juncture, it is also important to use eq 3 to argue against the continuous air layer of uniform thickness conception that totally separates the solid phase below from the liquid phase above. This would impute an almost infinite radius of curvature, because the presence of any captive bubble should, by virtue of the Laplace pressure, cause the air in it to deplete into the plastron. The fact that this does not occur indicates the contribution of the multitude of menisci formed, which is able to prevent this. Even without the captive bubble, this conception is difficult to reconcile because, without three-phase interactions, the air layer should detach from the solid surface and assume a spherical shape using surface tension, from which buoyancy will then cause it to float up. It may be argued that the edge of the sample might provide the three-phase contact points. However, that should result in a curvature extending all of the way to the edge when a captive bubble forms, which was not observed. Rather, the captive bubble could be localized to a region on the surface. Another moot point in this vein is that, if the surface were to be tilted (as in the case shown in Figure 4), air should drain upward quickly away from the plastron to form only one bubble at the apex. This did not occur even under observation over a long period of time. The situation here, of course, does not involve collapse of the plastron but rather how it interacts with a captive bubble formed. It is instructive to consider the case at the edge of the bubble in which the liquid−air interface manifests as a slope, as shown in Figure 7b. When air is delivered into the bubble, it should generate a slope that is marginally steeper to the horizontal to accommodate for the increase in volume if the contact line is pinned. Such a behavior, generally described as hysteresis, is possible if a solid phase is predominantly in contact with a liquid phase. In the presence of an air phase underneath, however, the extent of this pinning is not strong. Once the three-phase contact line depins, it will tend to move outward in search of an alternative point to form the edge of the bubble (see Figure 7c). Consequently, the bubble is able to assume a larger diameter than it can do so on a surface that does not contain any plastron. This lowers the slope but increases the CA at the same time. Such a behavior seemingly violates the condition depicted in eq 2. This is possible because the condition of dALV/dASL = cos θ, which is reserved only for a strictly semi-spherical shape, will no longer be in operation. Upon returning to Figure 6b, it can be seen that an interesting situation occurs when the bubble volume is higher than 180 μL. At this stage, the diameter now begins to reduce
Figure 4. Substrate that has lost its superhydrophobicity (a) when partially immersed in water did not exhibit any strong light reflectivity. For a SH substrate partially immersed in water after 1 min, (b) strong light reflection from the plastron was observed to the extent that the base of the container was lit up. The optical characteristics were retained when the same substrate was imaged after 30 min (c), indicating that the plastron was still intact.
Figure 5. Sequence of images showing (a) captive bubble deposited on the SH substrate, (b) growth of the bubble by injecting air into the plastron at some distance from the edge of the bubble, and (c) bubble in midst of detaching from the substrate at higher volumes as a result of buoyancy. The tip used was a 26 gauge (outer diameter of 0.46 mm) needle.
CA = 170° when the volume was increased to 180 μL. This imputes that as air was added to the captive bubble, there was a simultaneous outward movement of the contact line as well as a reduction in slope that the liquid−air interface makes with the horizontal. The latter is indicative of course to the attainment of an advancing state. If the plastron is visualized as a thin air layer on which the captive bubbles sits (see Figure 1), this would not be possible because a three-phase contact line should 6698
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Figure 6. Results from (a) Surface Evolver simulation of diameter contact with the substrate surface relative to the volume of a captive bubble for CAs of 158°, 165°, and 170°. The experimental results found (b) show general similarity but with differences. The diameter reaches a peak at a volume of 180 μL (indicated by the dashed line) from which it reduces with increasing volume. It eventually reaches a volume of 300 μL before detaching from the surface. The error bars depict one standard deviation from five readings made.
accurately. One of them lies in the difficulty of predicting the pinning behavior of the three-phase contact points where there is a strong neighboring presence of air. This should be considerably different from the typical case when the threephase contact line has a strong neighboring presence of the solid phase. In addition, the menisci that make up the plastron will likely be deformed at the vicinity of the three-phase contact points, resulting in Laplace pressure changes that can have an effect on the pinning behavior. The modeling of this will also be challenging. Clearly, when the captive bubble volume increases, the attendant buoyancy increase will lead to it losing the ability to remain on the surface. Hence, the bubble has to undergo changes to accommodate for this energy change. It is pertinent to note the difference in nature of the captive bubbles located on the surface with plastron here and that on typical surfaces. The latter generally undergoes a pinch off process such that radial symmetry is preserved.25−27 As a result of the pinch off, the contact line remains pinned on the surface. Because pinch off does not occur here, the tendency of detachment causes the contact line to be drawn back inward toward the bubble center. Because the ability to pin is reduced by the plastron, a reverse process from panels c to b of Figure 7 occurs to bring the contact line inward (which reduces the diameter). The ability for the contact line to move inward offers ability for the bubble body to undergo shape deformations such that it is able to stay attached to the substrate, notwithstanding a larger volume that tends to cause detachment due to buoyancy. This is attested to by the experimental value of 300 μL achieved, which from a mechanistic viewpoint is the condition in which buoyancy is able to overcome the pinning afforded by the multitude of three-phase contact points. This is in opposition to the value of 220 μL at CA = 170° found using Surface Evolver simulation. In the simulations, detachment is inferred to when no solution with a particular volume can be attained. The disparity between experimental and theoretical limit imputes contributions to contact bubble stability offered by the more “pliable” characteristics at the three-phase contact points where there is a strong presence of air. In the process, the contact bubble is able to embark on alternative routes to
Figure 7. Schematic depiction of (a) menisci linking various pinning points on the multi-dimensional surface structure that traps an air layer to make up the plastron. When a bubble is introduced, the liquid−air interface makes a steeper contact (exaggerated here) with the structure at the front edge (b) if pinning is present. In the presence of plastron, the three-phase contact point depins readily to seek another point to pin further up the edge (c). This process has the effect of increasing the contact diameter of the bubble. During the late stages when buoyancy dominates, a reverse situation occurs from panels c to b. This has the effect of decreasing the bubble diameter.
progressively with the volume increase. This seemingly appears to contradict the explanation to account for the case where the bubble volume is lower than 180 μL. It is important to note that, with increasing volumes delivered to the bubble, the contribution of buoyancy, which tends to cause upward movement, increases. Hence, there are two competing processes at work: a tendency of the bubble to increase its areal coverage as a result of low pinning and a propensity to detach as a result of buoyancy. The latter dominates when the volume is larger, and the results here indicate that the 180 μL volume is the threshold when this noticeably occurs. There will be significant challenges faced in trying to model this behavior 6699
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Langmuir surface energy minimization such that it will be able to sustain a larger volume, notwithstanding buoyancy. The ability to do this has been demonstrated in the context of soap film rearrangements.28 These routes will likely engender subtle shape departures, which are too small to observe with the naked eye. The ability to do this appears to be in contrast with the behavior of liquid bodies on SH surfaces, where contact line pinning is used to achieve significant shape changes in a continuous flow delivery.29 At this juncture, we seek to account for the much larger diameters (relative to volume) attained previously, with values that are even higher than that computed for CA = 179° using Surface Evolver.20 There, the mechanical interaction between the tip and bubble would reasonably have caused more significant shape changes to the gaseous body, thereby causing a larger departure from the dALV/dASL = cos θ condition. This affords capacity for the contact line to attain a state described in Figure 6c at the outset. The manner of increased shape departure from semi-spherical can be linked to the direct injection of air into the captive bubble, which in some ways resembles the behavior of low impact velocity of drops on surfaces, which causes increased spread of the contact line,30 notwithstanding the subsequent withdrawal of the tip. In doing so, an eventual diameter is attained by opportune pinning corresponding to the situation when the contact line velocity is sufficiently low. A similar situation has been observed in the case of drops on surfaces subjected to lateral vibration.31 The opportune pinning aspect is attested to by the greater spread in diameter values recorded with the previous approach where the mechanical perturbation is higher than the method used here. Regardless of the method applied, both will result in an outcome of the bubble diameter being larger. It should be noted that, when a tip was used to interact with the bubble directly, it is possible to selectively collapse the plastron at specific regions if not careful. This will essentially be similar to creating wetting metastable states through the introduction of high hydrostatic pressure on SH surfaces, first shown with sessile drops32 and more lately with plastrons.19 It is noteworthy that the collapse at certain regions does not destroy the plastron. In addition, the diameter of the bubble is determined by the mechanism occurring over a multitude of points rather than that illustrated for simplicity in Figure 6. With the method here, it is still important to be mindful that there is an adjoined air network between the point of air delivery and the bubble. Otherwise, it will not be possible to use the plastron to deliver air (at higher pressure) into the gas bubble (at lower pressure). We have found that, when the tip was moved much further away from the edge of the bubble, any injected air could not increase its growth. Hence, there is correspondence of this with the insurmountable losses in pipes, which similarly inhibit flow. We attempt to apply some context to the mechanism in which the contact line moves, as described in Figure 7, which, with pinning, there ought to be CA increases. In reality, these events are expected to be highly transient, and because the forces involved are weak, only very minute changes in the CA would appear. The results to trace the development of the CA as air is fed into the captive bubble using a high-speed camera (see Figure 8) showed the CA changes to be barely discernible. Nevertheless, there is a cycling characteristic exhibited that confirms the assertion of stick−slip processes taking place. Considering the possible impact of hydrostatic pressure determined previously,13 the results here do not show
Figure 8. Traces of the CA of the captive bubble against volume as air was injected in through the plastron layer. The image recordings were performed using a high-speed camera at 125 frames per second. The low levels of variation show cyclic features indicative of a stick−slip process.
significant variations or trends by varying this factor. This applies to the maximum volumes attained before detachment (Figure 9a) as well as the attendant diameters (Figure 9b)
Figure 9. Experimental results obtained for the maximum volume that the captive bubble can exist on the surface before detachment at various hydrostatic pressures and the contact diameter that it makes with the surface at that point. The parameters appear invariant to hydrostatic pressure. The error bars depict one standard deviation from five readings made.
found at these instances. This is likely attributed to the significantly lower range of magnitudes that we have used. From an application perspective, however, the ability to generate relatively consistent bubble shapes, notwithstanding this range of hydrostatic pressure, is useful. The effect of translating the SH surface with captive bubbles of different volumes deposited was investigated using the setup shown in Figure 2. Figures 10 and 11 present image sequences with time recorded for volumes 80 and 280 μL, respectively. The former indicates a case where the bubble is almost stationary relative to the substrate, although it can be seen that the shape was slightly deformed at various stages during the process. The latter alternatively depicts a case where the bubble is moveable relative to the substrate. Here, significant shape changes can be seen at various stages. Figure 12 provides quantitative distributions of displacement of the substrate and the bubble relative to the substrate determined for both cases. 6700
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Figure 10. Sequence of images at various times depicting the position of a captive bubble (located above the horizontal substrate) of 80 μL volume deposited on a SH surface substrate that is translated using a pneumatic actuator (moving from right to left). There is virtually no displacement of the captive bubble to the substrate.
Figure 12. Displacement distributions in relation to time of the substrate and captive bubble relative to the substrate derived from experiments conducted on bubbles with air volumes of (a) 80 μL and (b) 280 μL. Virtually, no relative movement was found with the small bubble, whereas the relative movement of the large bubble was almost frictionless. Bubble displacement was taken at the midpoint of the left and right points of contact with the surface.
relative to the solid surface on an obstacle at any particular instance in time, however, is given by 1 Fdrag = ρU 2AcC D (4) 2 where ρ is the density of the liquid, Ac is the cross-sectional area of the captive bubble, and CD is the drag coefficient (dependent upon shape). This force will function to translate the captive bubble relative to the surface. If we assume that this is the dominant factor in action, a larger captive bubble, which has a correspondingly larger value of Ac and, thus, drag force, presuming all other factors to be unchanged, will engender it to be more easily moved. This assumption is reasonable because of the much higher density of water (1000 kg/m3) than air (1.23 kg/m3). In other words, at the same velocity that a fluid moves over the substrate, the drag force generated on an obstacle of the same size and shape in liquid will exceed that generated in air by ∼O(103) according to eq 4. In view of this, the results do not necessarily indicate that smaller captive bubbles will possess higher adhesion forces to the surface as a result of surface tension. The capacity to transport captive bubbles in this manner offers the possibility of transporting gas-phase samples on surfaces, thus opening up vistas for lab-on-a-chip processes. Efforts in this vein have been attempted,34 while the usefulness of harnessing the gaseous phase in plastrons has been highlighted.35
Figure 11. Sequence of images at various times depicting the position of a captive bubble (located above the horizontal substrate) of 280 μL volume deposited on a SH surface substrate that is translated using a pneumatic actuator (moving from right to left). The captive bubble has clearly displaced relative to the substrate.
It can be seen that the substrate undergoes an initial constant velocity phase before slowing to a stop subsequently. The fact that the 80 μL captive bubble was relatively immobile on the substrate is illustrated through the data of Figure 12a. Conversely, the capacity of the 280 μL captive bubble to move freely on the substrate is shown through Figure 12b, highlighting the usefulness of creating large captive bubbles. The almost mirror image distributions (substrate and bubble relative to the substrate) about the zero displacement line allude to a close to friction-free transport of the captive bubble on the surface. To understand this, it is necessary to reference previous work studying the ability of sessile drops to be shed from surfaces when subjected to cross-airflow.33 There, the critical air speed velocity for shedding has been found to decrease for large volumes, as the surface area increases faster than the contact length of the drop on the surface. Essentially, the drop will be able to shed away more readily as a result of the higher the degree of drag experienced. Because captive bubbles are differentiated from sessile drops only by the two fluid mediums exchanging places, a similar outcome can be expected. If the substrate were to be translated, there will be body forces involved, in addition to the drag developed from the relative movement of one fluid (air) over the other (water). The drag force developed at velocity U of liquid movement
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CONCLUSION A method of increasing the volume in a captive bubble on a SH surface via delivery of air through the surrounding plastron is demonstrated. From the experiments conducted, the contact diameter against volume trends were found to generally follow that predicted by Surface Evolver simulation. However, the 6701
DOI: 10.1021/acs.langmuir.5b00058 Langmuir 2015, 31, 6695−6703
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(7) Wang, J.; Zheng, Y.; Nie, F. Q.; Zhai, J.; Jiang, L. Air bubble bursting effect of lotus leaf. Langmuir 2009, 15, 14129−14134. (8) Xiong, R.; Bai, M.; Chung, J. N. Formation of bubbles in a simple co-flowing micro-channel. J. Micromech. Microeng. 2007, 17, 1002. (9) Ichikawa, N.; Chung, P.M.-Y.; Matsumoto, S.; Matsumoto, J.; Takada, N. Generation of gas bubbles in microflow using micropipette with ultrasound. Microgravity Sci. Technol. 2007, 19, 35−37. (10) Prakash, M.; Gershenfeld, N. Microfluidic bubble logic. Science 2007, 315, 832−835. (11) Huynh, H. S.; Guan, J. P.; Vuong, T.; Ng, T. W. Comparisons of micro liquid and gaseous drops deposited on surfaces via a retreating tip. Langmuir 2013, 29, 11615−11622. (12) Huynh, S. H.; Wang, J.; Yu, Y.; Ng, T. W. Hydrostatic pressure effect on micro air bubbles deposited on surfaces with a retreating tip. Langmuir 2014, 30, 6095−6103. (13) Li, J.; Chen, H.; Zhou, W.; Wu, B.; Stoyanov, S. D.; Pelan, E. G. Growth of bubbles on a solid surface in response to a pressure reduction. Langmuir 2014, 30, 4223−4228. (14) Cao, L.; Jones, A. K.; Sikka, V. K.; Wu, J.; Gao, D. Anti-icing superhydrophobic coatings. Langmuir 2009, 25, 12444−12448. (15) Ke, Q.; Jin, Y.; Jiang, P.; Yu, J. Oil/water separation performances of superhydrophobic and superoleophilic sponges. Langmuir 2014, 30, 13137−13142. (16) Vuong, T.; Cheong, B.H.-P.; Huynh, S. H.; Muradoglu, M.; Liew, O. W.; Ng, T. W. Drop transfer between superhydrophobic wells using air logic control. Lab Chip 2015, 15, 991−999. (17) Hebetsa, E. A.; Chapman, R. F. Surviving the flood: Plastron respiration in the non-tracheate arthropod Phrynus marginemaculatus (Amblypygi: Arachnida). J. Insect Physiol. 2000, 46, 13−19. (18) McHale, G.; Flynn, M. R.; Newton, M. I. Plastron induced drag reduction and increased slip on a superhydrophobic sphere. Soft Matter 2011, 7, 10100−10107. (19) Poetes, R.; Holtzmann, K.; Franze, K.; Steiner, U. Metastable underwater superhydrophobicity. Phys. Rev. Lett. 2010, 105, 166104. (20) Ling, W. Y. L.; Lu, G.; Ng, T. W. Increased stability and size of a bubble on a superhydrophobic surface. Langmuir 2011, 27, 3233− 3237. (21) Marmur, A. Underwater superhydrophobicity: Theoretical feasibility. Langmuir 2006, 22, 1400−1402. (22) Hong, S. J.; Chang, F. M.; Chou, T. H.; Chan, S. H.; Sheng, Y. J.; Tsao, H. K. Anomalous contact angle hysteresis of a captive bubble: Advancing contact line pinning. Langmuir 2011, 27, 6890−6896. (23) Ling, W. Y. L.; Ng, T. W.; Neild, A. The pendant bubble method for accurate characterization of superhydrophobic surfaces. Langmuir 2011, 27, 13978−13982. (24) Chang, F.-M.; Sheng, Y.-J.; Cheng, S.-L.; Tsao, H.-K. Tiny bubble removal by gas flow through porous superhydrophobic surfaces: Ostwald ripening. Appl. Phys. Lett. 2008, 92, 264102. (25) Burton, J. C.; Waldrep, R.; Taborek, P. Scaling and instabilities in bubble pinch-off. Phys. Rev. Lett. 2005, 94, 184502. (26) Di Bari, S.; Robinson, A. J. Experimental study of gas injected bubble growth from submerged orifices. Exp. Therm. Fluid Sci. 2013, 44, 124−137. (27) Schmidt, L. E.; Keim, N. C.; Zhang, W. W.; Nagel, S. R. Memory-encoding vibrations in a disconnecting air bubble. Nat. Phys. 2009, 5, 343−346. (28) Vandewalle, N.; Noirhomme, M.; Schockmel, J.; Mersch, E.; Lumay, G.; Terwagne, D.; Dorbolo, S. Hysteretic behavior in threedimensional soap film rearrangements. Phys. Rev. E: Stat., Nonlinear, Soft Matter Phys. 2011, 83, 021403. (29) Katariya, M.; Vuong, T.; Ng, T. W. Liquid body formation from a semi-spherical superhydrophobic well on a small incline. Langmuir 2014, 30, 13731−13736. (30) Gu, Y.; Li, D. Liquid drop spreading on solid surfaces at low impact speeds. Colloids Surf., A 2000, 163, 239−245. (31) Whitehill, J.; Neild, A.; Ng, T. W.; Martyn, S.; Chong, J. Droplet spreading using low frequency vibration. Appl. Phys. Lett. 2011, 98, 133503.
experimental data corresponded with the simulated data at CA of 158° when the volume was 20 μL but then corresponded more closely toward CA of 170° when the volume was increased to 180 μL. This indicated that, as air was added to the captive bubble, there was a simultaneous outward movement of the contact line. When the bubble volume was higher than 180 μL, the diameter began to reduce progressively with the volume increase. The ability for the contact line to move inward in this regime offers the bubble body ability to undergo shape deformations to the extent that it is able to stay attached to the substrate with larger volumes, despite buoyancy. This is attested to by the experimental value of 300 μL achieved, as opposed to that of 220 μL found using Surface Evolver simulation at CA = 170°. The changes in CA as air was fed into the captive bubble were barely discernible but showed a cycling characteristic that confirms stick−slip processes taking place. In experiments to investigate the effect of translating the SH surface with captive bubbles, movement of the captive bubbles with 280 μL volume was possible but not with the captive bubbles with 80 μL volume. This highlights the usefulness of creating large captive bubbles. It also infers the possibility of transporting gas-phase samples on SH surfaces. The future challenges in this endeavor lie in the capacity to introduce effective means of automation and control.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Tuck Wah Ng acknowledges funding from the Australian Research Council Discovery Project DP120100583. This work was performed in part at the Melbourne Centre for Nanofabrication (MCN) in the Victorian Node of the Australian National Fabrication Facility (ANFF). Access to usage of the KRÜ SS DSA100S system at the Institute of Frontier Materials, Deakin University, is appreciated. Discussions with O. W. Liew at the Cardiovascular Research Institute, Singapore, are also acknowledged.
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REFERENCES
(1) Drelich, J.; Miller, J. D.; Good, R. J. The effect of drop (bubble) size on advancing and receding contact angles for heterogeneous and rough solid surfaces as observed with sessile-drop and captive-bubble techniques. J. Colloid Interface Sci. 1996, 179, 37−50. (2) Zuo, Y. Y.; Ding, M.; Bateni, A.; Hoorfar, M.; Neumann, A. W. Improvement of interfacial tension measurement using a captive bubble inconjunction with axisymmetric drop shape analysis (ADSA). Colloids Surf., A 2004, 250, 233−246. (3) Senses, E.; Black, M.; Cunningham, T.; Sukhishvili, S. A.; Akcora, P. Spatial ordering of colloids in a drying aqueous polymer droplet. Langmuir 2013, 29, 2588−2594. (4) Lee, A.; Moon, M.-W.; Lim, H.; Kim, W.-D.; Kim, H.-Y. Water harvest via dewing. Langmuir 2012, 28, 10183−10191. (5) Orozco, J.; Jurado-Sánchez, B.; Wagner, G.; Gao, W.; VazquezDuhalt, R.; Sattayasamitsathit, S.; Galarnyk, M.; Cortés, A.; Saintillan, D.; Wang, J. Bubble-propelled micromotors for enhanced transport of passive tracers. Langmuir 2014, 30, 5082−5087. (6) Wang, S.; Wu, N. Selecting the swimming mechanisms of colloidal particles: Bubble propulsion versus self-diffusiophoresis. Langmuir 2014, 30, 3477−3486. 6702
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Plastron-Mediated Growth of Captive Bubbles on Superhydrophobic Surfaces So Hung Huynh, Alifa Afiah Ahmad Zahidi, Murat Muradoglu, Brandon Huey-Ping Cheong, and Tuck Wah Ng* Laboratory for Optics and Applied Mechanics, Department of Mechanical and Aerospace Engineering, Monash University, Clayton, Victoria 3800, Australia ABSTRACT: Captive bubbles on a superhydrophobic (SH) surface have been shown to increase in volume via injection of air through the surrounding plastron. The experimental contact diameter against volume trends were found to follow that predicted by the Surface Evolver simulation generally but corresponded with the simulated data at contact angle (CA) = 158° when the volume was 20 μL but that at CA = 170° when the volume was increased to 180 μL. In this regime, there was a simultaneous outward movement of the contact line as well as a small reduction in the slope that the liquid−air interface makes with the horizontal as air was injected. At volumes higher than 180 μL, air injection caused the diameter to reduce progressively until detachment. The inward movement of the contact line in this regime allowed the bubble body to undergo shape deformations to stay attached onto the substrate with larger volumes (300 μL) than predicted (220 μL at CA = 170°) using simulation. In experiments to investigate the effect of translating the SH surface, movement of captive bubbles was possible with 280 μL volume but not with 80 μL volume. This pointed to the possibility of transporting gas-phase samples on SH surfaces using larger captive bubble volumes.
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plastrons have been shown to be highly stable.19 Unlike the case of a sessile drop, it is relatively easy to dispense a bubble on a SH surface that is replete with a plastron, because the bubble attaches more readily to the surface than the tip. Using an energy depiction, the work performed associated with the respective liquid−vapor, solid−liquid, and solid−vapor surface tensions γ, γSL, and γSV, liquid−vapor, solid−liquid, and solid− vapor area changes dALV, dASL, dASV, Laplace pressure change ΔP, and volume change dV (see Figure 1) can be given by
INTRODUCTION Gas bubbles that can stably exist on solid surfaces or captive bubbles have appearances similar to sessile drops. Because of this and the widely believed reciprocal behavior between the two, there have been reports of determining wettability by measuring the contact angle (CA) of captive bubbles instead of sessile drops, which is convenient because the instrumentation to do so is essentially the same.1,2 While there are abundant reported applications with the use of sessile drops,3,4 relatively fewer have been performed with captive bubbles. Nevertheless, it is noteworthy that the latter include useful operations, such as the transport of material5,6 and surface cleaning.7 Various ways to generate bubbles in a flow channel have been reported. These include the use of co-flows,8 ultrasonics,9 and heating.10 The methods mentioned, however, may not be suited for locating captive bubbles precisely on solid surfaces. It has been shown that air dispensation using a retracting tip offers a ready means to achieve this.11,12 Once a captive bubble is created, it is noteworthy that pressure can be used to vary its size.13 Superhydrophobic (SH) surfaces have been widely investigated because of the low levels of hysteresis that they offer. It is therefore not surprising that sessile drops dispensed on SH surfaces have been increasingly used in various applications.14−16 When a SH surface is placed underwater, it generates a visible film of air called plastron. This has been widely studied in terms of understanding the ability of certain insects to breathe underwater;17 albeit, its use in drag reduction for liquid flows has been attempted.18 It is noteworthy that © 2015 American Chemical Society
δw = γ dALV + γSL dASL + γSV dASV + ΔP dV
(1)
If the conditions of δw = 0, ΔV = 0, dASL = dASV, and dALV/ dASL = cos θ for a semi-spherical shape are enforced with
Figure 1. Schematic depiction of a captive bubble located on a solid substrate with a contact diameter 2r and CA θ. Received: January 7, 2015 Revised: May 14, 2015 Published: May 18, 2015 6695
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proximity to the outer edge of the bubble to dispense the remaining gas volume. There were three sets of experimental data collected. The first set was to examine the maximum volume against variation of hydrostatic pressure ranging from 14 to 45 mm H2O (from 137.3 to 441.3 Pa). In this experiment, air is dispensed at increments of 1 μL, followed by a rest period of 3 s each time to simulate a quasi-static condition. This is performed until the captive bubble departs from the substrate. Readings of the critical volume were obtained using the accurate dosing mechanism provided by the DSA100S system. In the other experiment, air is dispensed at step volumes of 20 μL, followed by a rest period of 3 s each time to simulate a quasi-static condition. This is performed until the captive bubble departs from the substrate under hydrostatic pressures of 14 and 30 mm H2O (from 137.3 to 294.2 Pa). Images after each step were captured using the DSA100S built-in camera, and an open source image analysis software (ImageJ) was used to determine the contact diameter of the captive bubble on the substrate. In all instances, the lens on the DSA100S was laser-guided to align at 90° with the needle tip to prevent any projection from occurring in measurement. In a third experiment, we first created a 150 μL captive bubble on the surface. As air was injected through the plastron, we then recorded the captive bubble development using a high-speed camera (Fastec Troubleshooter) at 125 frames per second. The CA was then analyzed using the software Tracker (https://www. cabrillo.edu/~dbrown/tracker/). Bubble Transportation. This experiment was conducted to demonstrate the ability of bubbles generated on the SH surface to move under an inertial forcing function. The SH substrate was mounted underwater at 690 Pa of hydrostatic pressure. A pneumatic actuator (Numatics, G453A3SK0050A00) driven by compressed air with a gauge pressure of 45 kPa was used to translate the platform horizontally through the setup shown in Figure 2. The movement was
surface tension dominating over buoyancy, eq 1 simplifies to the easily recognizable Young’s equation of γ cos θ = γSV − γSL
(2)
Both eqs 1 and 2 are often taken to be useable interchangeably with sessile drops and captive bubbles. As in the case with sessile drops, one would normally increase the volume by first depositing a captive bubble onto the surface, followed by pipetting additional air volumes onto it. There will be a significant degree of hysteresis this way because of the manner in which air is delivered and the pipet drawn away. Consequently, the captive bubble will manifest a relatively wide range of contact diameters on the surface.20 To overcome this, an alternative method to deliver air to the captive bubble will be desirable. Here, we investigate the delivery of air into the plastron around the captive bubble to do so. Captive bubbles that are created uniformly are useable if they can be transported from one spatial location to another on the substrate. We additionally explore a substrate translating mechanism without the use of liquid flow to do this.
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MATERIALS AND METHODS
Sample Preparation. SH surfaces were created using an electroless galvanic deposition process. This process allowed for the rough tuning of wettability depending upon the variables in the preparation process. Polished copper surfaces were first cleaned with absolute ethanol and allowed to dry. They were then immersed in a solution of aqueous AgNO3. After this, the surfaces were cleaned with absolute ethanol and dried with compressed inert gas. They were subsequently immersed in a 1 mM solution of the surface modifier CF3(CF2)7CH2CH2SH in absolute ethanol. The samples were then thoroughly rinsed using at least 100 mL of distilled water, followed by rinsing with absolute ethanol. They were then allowed to air-dry. The sessile drop CAs were measured to be 158° (σ = 0.80°). The surface microstructure was examined using scanning electron microscopy (SEM, FEI, NovaNanoSEM 430). Computation. The Surface Evolver software was used to compute the predicted profile and size of inverted droplets. It does this using an energy minimization method that evolves a surface down the energy gradient. The assumption made in the computation was that an inverted static water drop on a surface should be identical to an immersed bubble on another surface with an opposing CA (as described in Figure 1). For example, the static profile of a hanging pendant drop on a surface with a CA of 10° was assumed to be identical to the profile of an immersed bubble on a surface with a CA of 170°. Experimental Section. Plastron Stability. To obtain an indication of plastron stability, the superhydrophic substrate was partially immersed into a container of distilled water. A camera was trained at this substrate around the immersion region, and its angle of view adjusted such that the plastron, which manifests as a reflective optical layer, could be seen. Images were recorded at intervals of 1 min for 30 min. A similar procedure was undertaken for a substrate that has lost its superhydrophobicity. The surface characteristic was confirmed prior by dispensing water drops on it, where they did not roll away easily. Air Dispensation on Captive Bubbles. The air dispensation studies were conducted on a drop shape analysis system (KRÜ SS DSA100S). The microsyringe used to dispense bubbles had a capacity of 500 μL and a 26 gauge needle (outer diameter of 0.46 mm) tip equipped with a Luer screw (Innovative Labor Systeme, 2607909 and 2849045). The substrate used was prepared as described in the previous section. A plastic container with dimensions of 87 × 87 × 75 mm (width × length × height) was used as a water reservoir. During the experiment, the temperature was monitored to be constant at 22 °C. To dispense bubbles through the plastron, first, a primary bubble whose size is significantly less than the critical volume was deposited on the substrate. Next, the needle tip was moved within close
Figure 2. Setup to observe the bubble transportation characteristics of bubbles deposited on SH surfaces. In a tank with water, impulsive movement (in the direction of the solid arrow) was provided by a pneumatic actuator acting through an adaptor. A high-speed camera was used to record the captive bubble movement under illumination by a bright light source. If the captive bubble was able to move relative to the substrate, it would do so in the direction of the dashed arrow. recorded using a high-speed imaging device (Fastec Imaging, Troubleshooter TS1000ME) at 500 frames per second. Bubble sizes of 280 and 80 μL were dispensed onto the substrate using the method, and the experiment was repeated 3 times for each volume. The pneumatic pressure was measured using a digital gauge (Digitron, 2002P) when the solenoid was in the closed state. An open-source motion analysis software (Tracker) was used to examine the recorded images.
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RESULTS AND DISCUSSION At the outset, we sought to establish that the CAs developed were high on the surface created. In using a droplet and bubble of 50 μL, the respective equilibrium CAs were found to be 160° 6696
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Figure 3. SEM images at different magnifications of the SH substrate showing multi-scale and dendritic microstructures. The structures, which are at different heights, allow for the creation of multiple three-phase points that allow the plastron to exist stably.
± 5° and 165° ± 5°, respectively. This appears to indicate the complementing CA behavior exhibited between sessile drops and captive bubbles. It has been pointed out that using CAs to describe surfaces that have plastrons residing on them or sometimes referred to as exhibiting underwater superhydrophobicity is not particularly meaningful.21 Therefore, distinction should be made with studies with captive bubbles in which plastrons have not been reportedly seen,22 because the underlying mechanics may be different. In addition, measuring the apparent CAs to denote captive bubble growth is likely to have high levels of uncertainty when the base value is closer to 180°,23 and this uncertainty is compounded if the process is dynamic. For this reason, we have used the contact diameter as the defining parameter. Figure 3 presents typical SEM images of the SH substrate recorded at varying magnifications. Multi-scale structures, which resemble dendrites and granular deposits can be seen. These structures provide opportunity for three-phase contact sites to form across the expanse of the surface from which the plastron can exist stably. A substrate that has lost its superhydrophobicity when partially immersed in water did not exhibit any strong light reflectivity (Figure 4a). However, a SH substrate partially immersed showed strong light reflection from the air-dominated plastron (see Figure 4b). The optical characteristics were unchanged after 30 min (Figure 4c), indicating that the plastron was still intact. This showed that the plastron was stable if the substrate was left undisturbed in water.
Figure 5a shows the image recorded of an air bubble initially deposited on the SH substrate using a retracting syringe tip. The syringe tip was then used to deliver air via the plastron adjacent to the bubble, causing the bubble to grow in size (Figure 5b). This is obviously due to a higher pressure at the tip as opposed to the bubble. This parallels a process previously reported.24 However, in that situation, it is not stated if a plastron layer was observed; if not, the underlying mechanism may be different. The bubble was found to be slightly perturbed during the process. This was unlike the approach of placing the tip directly into the bubble, inserting air, and subsequently withdrawing the tip, which was found to perturb the bubble significantly. After achievement of a significant size, the bubble could detach from the surface through buoyancy (Figure 5c). Figure 6a provides distributions of the diameter in contact with the substrate surface against the volume of captive bubbles dispensed for CAs of 158°, 165°, and 170° using Surface Evolver simulation. The increasing trends extend up to the point where the heightened buoyancy force results in a nonsolution, which can be physically interpreted as the bubble detaching from the surface (220 μL for CAs of 165° and 170° and 210 μL for CA of 158°). From the experiments conducted by air delivery through the plastron (see Figure 6b), it can be seen that contact diameter against volume trends generally follow that predicted by simulation. However, there are notable differences. One of them lies with the experimental data corresponding with the simulated data at CA = 158° when the volume was 20 μL but then corresponding more closely with 6697
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not even exist. Hence, it is necessary to consider the physics more carefully. Rather than an isolated air layer sitting on top of the solid substrate, plastrons exist as a result of the formation of menisci over a multitude of three-phase contact points (see Figure 7a). If we consider just one meniscus, the pressure drop across the liquid−air interface is given by γ ΔP = LV (3) r where r is the radius of curvature of the interface. Clearly, if ΔP on the left-hand side of eq 3 is greater, r will have to correspondingly decrease to satisfy the equilibrium condition. However, when the meniscus is not able to form because of limited clearance from the substrate base, the plastron will then collapse. Hence, plastrons do have a lifespan that is dictated by the diffusion of air into the liquid, which lowers the available air clearance relative to the substrate base. At this juncture, it is also important to use eq 3 to argue against the continuous air layer of uniform thickness conception that totally separates the solid phase below from the liquid phase above. This would impute an almost infinite radius of curvature, because the presence of any captive bubble should, by virtue of the Laplace pressure, cause the air in it to deplete into the plastron. The fact that this does not occur indicates the contribution of the multitude of menisci formed, which is able to prevent this. Even without the captive bubble, this conception is difficult to reconcile because, without three-phase interactions, the air layer should detach from the solid surface and assume a spherical shape using surface tension, from which buoyancy will then cause it to float up. It may be argued that the edge of the sample might provide the three-phase contact points. However, that should result in a curvature extending all of the way to the edge when a captive bubble forms, which was not observed. Rather, the captive bubble could be localized to a region on the surface. Another moot point in this vein is that, if the surface were to be tilted (as in the case shown in Figure 4), air should drain upward quickly away from the plastron to form only one bubble at the apex. This did not occur even under observation over a long period of time. The situation here, of course, does not involve collapse of the plastron but rather how it interacts with a captive bubble formed. It is instructive to consider the case at the edge of the bubble in which the liquid−air interface manifests as a slope, as shown in Figure 7b. When air is delivered into the bubble, it should generate a slope that is marginally steeper to the horizontal to accommodate for the increase in volume if the contact line is pinned. Such a behavior, generally described as hysteresis, is possible if a solid phase is predominantly in contact with a liquid phase. In the presence of an air phase underneath, however, the extent of this pinning is not strong. Once the three-phase contact line depins, it will tend to move outward in search of an alternative point to form the edge of the bubble (see Figure 7c). Consequently, the bubble is able to assume a larger diameter than it can do so on a surface that does not contain any plastron. This lowers the slope but increases the CA at the same time. Such a behavior seemingly violates the condition depicted in eq 2. This is possible because the condition of dALV/dASL = cos θ, which is reserved only for a strictly semi-spherical shape, will no longer be in operation. Upon returning to Figure 6b, it can be seen that an interesting situation occurs when the bubble volume is higher than 180 μL. At this stage, the diameter now begins to reduce
Figure 4. Substrate that has lost its superhydrophobicity (a) when partially immersed in water did not exhibit any strong light reflectivity. For a SH substrate partially immersed in water after 1 min, (b) strong light reflection from the plastron was observed to the extent that the base of the container was lit up. The optical characteristics were retained when the same substrate was imaged after 30 min (c), indicating that the plastron was still intact.
Figure 5. Sequence of images showing (a) captive bubble deposited on the SH substrate, (b) growth of the bubble by injecting air into the plastron at some distance from the edge of the bubble, and (c) bubble in midst of detaching from the substrate at higher volumes as a result of buoyancy. The tip used was a 26 gauge (outer diameter of 0.46 mm) needle.
CA = 170° when the volume was increased to 180 μL. This imputes that as air was added to the captive bubble, there was a simultaneous outward movement of the contact line as well as a reduction in slope that the liquid−air interface makes with the horizontal. The latter is indicative of course to the attainment of an advancing state. If the plastron is visualized as a thin air layer on which the captive bubbles sits (see Figure 1), this would not be possible because a three-phase contact line should 6698
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Figure 6. Results from (a) Surface Evolver simulation of diameter contact with the substrate surface relative to the volume of a captive bubble for CAs of 158°, 165°, and 170°. The experimental results found (b) show general similarity but with differences. The diameter reaches a peak at a volume of 180 μL (indicated by the dashed line) from which it reduces with increasing volume. It eventually reaches a volume of 300 μL before detaching from the surface. The error bars depict one standard deviation from five readings made.
accurately. One of them lies in the difficulty of predicting the pinning behavior of the three-phase contact points where there is a strong neighboring presence of air. This should be considerably different from the typical case when the threephase contact line has a strong neighboring presence of the solid phase. In addition, the menisci that make up the plastron will likely be deformed at the vicinity of the three-phase contact points, resulting in Laplace pressure changes that can have an effect on the pinning behavior. The modeling of this will also be challenging. Clearly, when the captive bubble volume increases, the attendant buoyancy increase will lead to it losing the ability to remain on the surface. Hence, the bubble has to undergo changes to accommodate for this energy change. It is pertinent to note the difference in nature of the captive bubbles located on the surface with plastron here and that on typical surfaces. The latter generally undergoes a pinch off process such that radial symmetry is preserved.25−27 As a result of the pinch off, the contact line remains pinned on the surface. Because pinch off does not occur here, the tendency of detachment causes the contact line to be drawn back inward toward the bubble center. Because the ability to pin is reduced by the plastron, a reverse process from panels c to b of Figure 7 occurs to bring the contact line inward (which reduces the diameter). The ability for the contact line to move inward offers ability for the bubble body to undergo shape deformations such that it is able to stay attached to the substrate, notwithstanding a larger volume that tends to cause detachment due to buoyancy. This is attested to by the experimental value of 300 μL achieved, which from a mechanistic viewpoint is the condition in which buoyancy is able to overcome the pinning afforded by the multitude of three-phase contact points. This is in opposition to the value of 220 μL at CA = 170° found using Surface Evolver simulation. In the simulations, detachment is inferred to when no solution with a particular volume can be attained. The disparity between experimental and theoretical limit imputes contributions to contact bubble stability offered by the more “pliable” characteristics at the three-phase contact points where there is a strong presence of air. In the process, the contact bubble is able to embark on alternative routes to
Figure 7. Schematic depiction of (a) menisci linking various pinning points on the multi-dimensional surface structure that traps an air layer to make up the plastron. When a bubble is introduced, the liquid−air interface makes a steeper contact (exaggerated here) with the structure at the front edge (b) if pinning is present. In the presence of plastron, the three-phase contact point depins readily to seek another point to pin further up the edge (c). This process has the effect of increasing the contact diameter of the bubble. During the late stages when buoyancy dominates, a reverse situation occurs from panels c to b. This has the effect of decreasing the bubble diameter.
progressively with the volume increase. This seemingly appears to contradict the explanation to account for the case where the bubble volume is lower than 180 μL. It is important to note that, with increasing volumes delivered to the bubble, the contribution of buoyancy, which tends to cause upward movement, increases. Hence, there are two competing processes at work: a tendency of the bubble to increase its areal coverage as a result of low pinning and a propensity to detach as a result of buoyancy. The latter dominates when the volume is larger, and the results here indicate that the 180 μL volume is the threshold when this noticeably occurs. There will be significant challenges faced in trying to model this behavior 6699
DOI: 10.1021/acs.langmuir.5b00058 Langmuir 2015, 31, 6695−6703
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Langmuir surface energy minimization such that it will be able to sustain a larger volume, notwithstanding buoyancy. The ability to do this has been demonstrated in the context of soap film rearrangements.28 These routes will likely engender subtle shape departures, which are too small to observe with the naked eye. The ability to do this appears to be in contrast with the behavior of liquid bodies on SH surfaces, where contact line pinning is used to achieve significant shape changes in a continuous flow delivery.29 At this juncture, we seek to account for the much larger diameters (relative to volume) attained previously, with values that are even higher than that computed for CA = 179° using Surface Evolver.20 There, the mechanical interaction between the tip and bubble would reasonably have caused more significant shape changes to the gaseous body, thereby causing a larger departure from the dALV/dASL = cos θ condition. This affords capacity for the contact line to attain a state described in Figure 6c at the outset. The manner of increased shape departure from semi-spherical can be linked to the direct injection of air into the captive bubble, which in some ways resembles the behavior of low impact velocity of drops on surfaces, which causes increased spread of the contact line,30 notwithstanding the subsequent withdrawal of the tip. In doing so, an eventual diameter is attained by opportune pinning corresponding to the situation when the contact line velocity is sufficiently low. A similar situation has been observed in the case of drops on surfaces subjected to lateral vibration.31 The opportune pinning aspect is attested to by the greater spread in diameter values recorded with the previous approach where the mechanical perturbation is higher than the method used here. Regardless of the method applied, both will result in an outcome of the bubble diameter being larger. It should be noted that, when a tip was used to interact with the bubble directly, it is possible to selectively collapse the plastron at specific regions if not careful. This will essentially be similar to creating wetting metastable states through the introduction of high hydrostatic pressure on SH surfaces, first shown with sessile drops32 and more lately with plastrons.19 It is noteworthy that the collapse at certain regions does not destroy the plastron. In addition, the diameter of the bubble is determined by the mechanism occurring over a multitude of points rather than that illustrated for simplicity in Figure 6. With the method here, it is still important to be mindful that there is an adjoined air network between the point of air delivery and the bubble. Otherwise, it will not be possible to use the plastron to deliver air (at higher pressure) into the gas bubble (at lower pressure). We have found that, when the tip was moved much further away from the edge of the bubble, any injected air could not increase its growth. Hence, there is correspondence of this with the insurmountable losses in pipes, which similarly inhibit flow. We attempt to apply some context to the mechanism in which the contact line moves, as described in Figure 7, which, with pinning, there ought to be CA increases. In reality, these events are expected to be highly transient, and because the forces involved are weak, only very minute changes in the CA would appear. The results to trace the development of the CA as air is fed into the captive bubble using a high-speed camera (see Figure 8) showed the CA changes to be barely discernible. Nevertheless, there is a cycling characteristic exhibited that confirms the assertion of stick−slip processes taking place. Considering the possible impact of hydrostatic pressure determined previously,13 the results here do not show
Figure 8. Traces of the CA of the captive bubble against volume as air was injected in through the plastron layer. The image recordings were performed using a high-speed camera at 125 frames per second. The low levels of variation show cyclic features indicative of a stick−slip process.
significant variations or trends by varying this factor. This applies to the maximum volumes attained before detachment (Figure 9a) as well as the attendant diameters (Figure 9b)
Figure 9. Experimental results obtained for the maximum volume that the captive bubble can exist on the surface before detachment at various hydrostatic pressures and the contact diameter that it makes with the surface at that point. The parameters appear invariant to hydrostatic pressure. The error bars depict one standard deviation from five readings made.
found at these instances. This is likely attributed to the significantly lower range of magnitudes that we have used. From an application perspective, however, the ability to generate relatively consistent bubble shapes, notwithstanding this range of hydrostatic pressure, is useful. The effect of translating the SH surface with captive bubbles of different volumes deposited was investigated using the setup shown in Figure 2. Figures 10 and 11 present image sequences with time recorded for volumes 80 and 280 μL, respectively. The former indicates a case where the bubble is almost stationary relative to the substrate, although it can be seen that the shape was slightly deformed at various stages during the process. The latter alternatively depicts a case where the bubble is moveable relative to the substrate. Here, significant shape changes can be seen at various stages. Figure 12 provides quantitative distributions of displacement of the substrate and the bubble relative to the substrate determined for both cases. 6700
DOI: 10.1021/acs.langmuir.5b00058 Langmuir 2015, 31, 6695−6703
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Langmuir
Figure 10. Sequence of images at various times depicting the position of a captive bubble (located above the horizontal substrate) of 80 μL volume deposited on a SH surface substrate that is translated using a pneumatic actuator (moving from right to left). There is virtually no displacement of the captive bubble to the substrate.
Figure 12. Displacement distributions in relation to time of the substrate and captive bubble relative to the substrate derived from experiments conducted on bubbles with air volumes of (a) 80 μL and (b) 280 μL. Virtually, no relative movement was found with the small bubble, whereas the relative movement of the large bubble was almost frictionless. Bubble displacement was taken at the midpoint of the left and right points of contact with the surface.
relative to the solid surface on an obstacle at any particular instance in time, however, is given by 1 Fdrag = ρU 2AcC D (4) 2 where ρ is the density of the liquid, Ac is the cross-sectional area of the captive bubble, and CD is the drag coefficient (dependent upon shape). This force will function to translate the captive bubble relative to the surface. If we assume that this is the dominant factor in action, a larger captive bubble, which has a correspondingly larger value of Ac and, thus, drag force, presuming all other factors to be unchanged, will engender it to be more easily moved. This assumption is reasonable because of the much higher density of water (1000 kg/m3) than air (1.23 kg/m3). In other words, at the same velocity that a fluid moves over the substrate, the drag force generated on an obstacle of the same size and shape in liquid will exceed that generated in air by ∼O(103) according to eq 4. In view of this, the results do not necessarily indicate that smaller captive bubbles will possess higher adhesion forces to the surface as a result of surface tension. The capacity to transport captive bubbles in this manner offers the possibility of transporting gas-phase samples on surfaces, thus opening up vistas for lab-on-a-chip processes. Efforts in this vein have been attempted,34 while the usefulness of harnessing the gaseous phase in plastrons has been highlighted.35
Figure 11. Sequence of images at various times depicting the position of a captive bubble (located above the horizontal substrate) of 280 μL volume deposited on a SH surface substrate that is translated using a pneumatic actuator (moving from right to left). The captive bubble has clearly displaced relative to the substrate.
It can be seen that the substrate undergoes an initial constant velocity phase before slowing to a stop subsequently. The fact that the 80 μL captive bubble was relatively immobile on the substrate is illustrated through the data of Figure 12a. Conversely, the capacity of the 280 μL captive bubble to move freely on the substrate is shown through Figure 12b, highlighting the usefulness of creating large captive bubbles. The almost mirror image distributions (substrate and bubble relative to the substrate) about the zero displacement line allude to a close to friction-free transport of the captive bubble on the surface. To understand this, it is necessary to reference previous work studying the ability of sessile drops to be shed from surfaces when subjected to cross-airflow.33 There, the critical air speed velocity for shedding has been found to decrease for large volumes, as the surface area increases faster than the contact length of the drop on the surface. Essentially, the drop will be able to shed away more readily as a result of the higher the degree of drag experienced. Because captive bubbles are differentiated from sessile drops only by the two fluid mediums exchanging places, a similar outcome can be expected. If the substrate were to be translated, there will be body forces involved, in addition to the drag developed from the relative movement of one fluid (air) over the other (water). The drag force developed at velocity U of liquid movement
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CONCLUSION A method of increasing the volume in a captive bubble on a SH surface via delivery of air through the surrounding plastron is demonstrated. From the experiments conducted, the contact diameter against volume trends were found to generally follow that predicted by Surface Evolver simulation. However, the 6701
DOI: 10.1021/acs.langmuir.5b00058 Langmuir 2015, 31, 6695−6703
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(7) Wang, J.; Zheng, Y.; Nie, F. Q.; Zhai, J.; Jiang, L. Air bubble bursting effect of lotus leaf. Langmuir 2009, 15, 14129−14134. (8) Xiong, R.; Bai, M.; Chung, J. N. Formation of bubbles in a simple co-flowing micro-channel. J. Micromech. Microeng. 2007, 17, 1002. (9) Ichikawa, N.; Chung, P.M.-Y.; Matsumoto, S.; Matsumoto, J.; Takada, N. Generation of gas bubbles in microflow using micropipette with ultrasound. Microgravity Sci. Technol. 2007, 19, 35−37. (10) Prakash, M.; Gershenfeld, N. Microfluidic bubble logic. Science 2007, 315, 832−835. (11) Huynh, H. S.; Guan, J. P.; Vuong, T.; Ng, T. W. Comparisons of micro liquid and gaseous drops deposited on surfaces via a retreating tip. Langmuir 2013, 29, 11615−11622. (12) Huynh, S. H.; Wang, J.; Yu, Y.; Ng, T. W. Hydrostatic pressure effect on micro air bubbles deposited on surfaces with a retreating tip. Langmuir 2014, 30, 6095−6103. (13) Li, J.; Chen, H.; Zhou, W.; Wu, B.; Stoyanov, S. D.; Pelan, E. G. Growth of bubbles on a solid surface in response to a pressure reduction. Langmuir 2014, 30, 4223−4228. (14) Cao, L.; Jones, A. K.; Sikka, V. K.; Wu, J.; Gao, D. Anti-icing superhydrophobic coatings. Langmuir 2009, 25, 12444−12448. (15) Ke, Q.; Jin, Y.; Jiang, P.; Yu, J. Oil/water separation performances of superhydrophobic and superoleophilic sponges. Langmuir 2014, 30, 13137−13142. (16) Vuong, T.; Cheong, B.H.-P.; Huynh, S. H.; Muradoglu, M.; Liew, O. W.; Ng, T. W. Drop transfer between superhydrophobic wells using air logic control. Lab Chip 2015, 15, 991−999. (17) Hebetsa, E. A.; Chapman, R. F. Surviving the flood: Plastron respiration in the non-tracheate arthropod Phrynus marginemaculatus (Amblypygi: Arachnida). J. Insect Physiol. 2000, 46, 13−19. (18) McHale, G.; Flynn, M. R.; Newton, M. I. Plastron induced drag reduction and increased slip on a superhydrophobic sphere. Soft Matter 2011, 7, 10100−10107. (19) Poetes, R.; Holtzmann, K.; Franze, K.; Steiner, U. Metastable underwater superhydrophobicity. Phys. Rev. Lett. 2010, 105, 166104. (20) Ling, W. Y. L.; Lu, G.; Ng, T. W. Increased stability and size of a bubble on a superhydrophobic surface. Langmuir 2011, 27, 3233− 3237. (21) Marmur, A. Underwater superhydrophobicity: Theoretical feasibility. Langmuir 2006, 22, 1400−1402. (22) Hong, S. J.; Chang, F. M.; Chou, T. H.; Chan, S. H.; Sheng, Y. J.; Tsao, H. K. Anomalous contact angle hysteresis of a captive bubble: Advancing contact line pinning. Langmuir 2011, 27, 6890−6896. (23) Ling, W. Y. L.; Ng, T. W.; Neild, A. The pendant bubble method for accurate characterization of superhydrophobic surfaces. Langmuir 2011, 27, 13978−13982. (24) Chang, F.-M.; Sheng, Y.-J.; Cheng, S.-L.; Tsao, H.-K. Tiny bubble removal by gas flow through porous superhydrophobic surfaces: Ostwald ripening. Appl. Phys. Lett. 2008, 92, 264102. (25) Burton, J. C.; Waldrep, R.; Taborek, P. Scaling and instabilities in bubble pinch-off. Phys. Rev. Lett. 2005, 94, 184502. (26) Di Bari, S.; Robinson, A. J. Experimental study of gas injected bubble growth from submerged orifices. Exp. Therm. Fluid Sci. 2013, 44, 124−137. (27) Schmidt, L. E.; Keim, N. C.; Zhang, W. W.; Nagel, S. R. Memory-encoding vibrations in a disconnecting air bubble. Nat. Phys. 2009, 5, 343−346. (28) Vandewalle, N.; Noirhomme, M.; Schockmel, J.; Mersch, E.; Lumay, G.; Terwagne, D.; Dorbolo, S. Hysteretic behavior in threedimensional soap film rearrangements. Phys. Rev. E: Stat., Nonlinear, Soft Matter Phys. 2011, 83, 021403. (29) Katariya, M.; Vuong, T.; Ng, T. W. Liquid body formation from a semi-spherical superhydrophobic well on a small incline. Langmuir 2014, 30, 13731−13736. (30) Gu, Y.; Li, D. Liquid drop spreading on solid surfaces at low impact speeds. Colloids Surf., A 2000, 163, 239−245. (31) Whitehill, J.; Neild, A.; Ng, T. W.; Martyn, S.; Chong, J. Droplet spreading using low frequency vibration. Appl. Phys. Lett. 2011, 98, 133503.
experimental data corresponded with the simulated data at CA of 158° when the volume was 20 μL but then corresponded more closely toward CA of 170° when the volume was increased to 180 μL. This indicated that, as air was added to the captive bubble, there was a simultaneous outward movement of the contact line. When the bubble volume was higher than 180 μL, the diameter began to reduce progressively with the volume increase. The ability for the contact line to move inward in this regime offers the bubble body ability to undergo shape deformations to the extent that it is able to stay attached to the substrate with larger volumes, despite buoyancy. This is attested to by the experimental value of 300 μL achieved, as opposed to that of 220 μL found using Surface Evolver simulation at CA = 170°. The changes in CA as air was fed into the captive bubble were barely discernible but showed a cycling characteristic that confirms stick−slip processes taking place. In experiments to investigate the effect of translating the SH surface with captive bubbles, movement of the captive bubbles with 280 μL volume was possible but not with the captive bubbles with 80 μL volume. This highlights the usefulness of creating large captive bubbles. It also infers the possibility of transporting gas-phase samples on SH surfaces. The future challenges in this endeavor lie in the capacity to introduce effective means of automation and control.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Tuck Wah Ng acknowledges funding from the Australian Research Council Discovery Project DP120100583. This work was performed in part at the Melbourne Centre for Nanofabrication (MCN) in the Victorian Node of the Australian National Fabrication Facility (ANFF). Access to usage of the KRÜ SS DSA100S system at the Institute of Frontier Materials, Deakin University, is appreciated. Discussions with O. W. Liew at the Cardiovascular Research Institute, Singapore, are also acknowledged.
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