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Younghyun Cho, Tae Soup Shim, and Shu Yang* In the last century, demand for water has grown at twice the rate of the population. Nearly a quarter of the population faces economic water shortages, not only in arid regions but also in regions with low rainfall despite high humidity.[1] Therefore, creating a coating that can capture water, via condensation or trapping of droplets, is critical to address the water shortage issues. In nature, control of the heterogeneous nucleation and growth of water droplets on a surface has been crucial for survival and proliferation of the bioorganisms. For example, in extremely arid habitat such as the Namib Desert, beetles Stenocara sp. capture and collect water from the early morning fog on their backs, which are covered with a random array of bumps. The peaks of the bumps are smooth and hydrophilic, where the fog is captured, and the troughs consisting of flattened hemispheres (10 µm in diameter) are superhydrophoibic, which assists the roll-off of water droplets.[2,3] Management of water on surfaces is also important to water filtration, microfluidics, antibiofouling, and anti-icing.[4,5] There have been increasing efforts to create beetle-like patterned surfaces to manipulate water nucleation and growth.[6–14] For example, Garrod et al.[9] show that a pattern combining hydrophilic–hydrophobic surfaces enhances the water collection efficiency compared with a surface that is homogeneously hydrophobic or hydrophilic. An optimal feature size of hydrophilic area (pixel size and center-to-center distance of 500 and 1000 µm, respectively) is suggested. Dorrer and Rühe have investigated the influence of the superhydrophobicity/hydrophilicity contrast to the development of water droplets and suggested that the critical volume at which droplets coalescence and roll off from the hydrophilic spot is linearly proportional to the diameter of the spot.[7] Since the free energy barrier for the droplet formation and nucleation rate are strongly governed by the intrinsic wettability of surface,[15,16] wettability contrast between different regions plays a critical role in determining where droplet nucleation and growth occur selectively. Wettability is determined by surface topography and chemical compositions.[17,18] On a rough surface, there are two simplified (non)wetting Dr. Y. Cho, Prof. S. Yang Department of Materials Science and Engineering University of Pennsylvania 3231 Walnut Street, Philadelphia, PA 19104, USA E-mail: [email protected] Dr. T. S. Shim[+] Department of Chemical and Biomolecular Engineering University of Pennsylvania 220 South 33rd Street, Philadelphia, PA 19104, USA [+]Present address: Department of Chemical Engineering, Ajou University, Suwon 16499, Republic of Korea
DOI: 10.1002/adma.201504899
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states, Cassie–Baxter state[19] and Wenzel state,[20] and states inbetween. In the Cassie–Baxter state, air is trapped between the grooves of the rough structure. Therefore, water droplets sit on a composite surface, whereas in the Wenzel state water completely fills the grooves between the rough surfaces, making the water droplet immobile. Beysens and co-workers have investigated water droplet nucleation and growth on micropatterned surfaces.[21–25] They categorize the condensation behaviors into four stages depending on the droplet to pattern size, including initial nucleation, bridge formation, drying, and large droplet formation. As the droplet grows, transition from Cassie– Baxter state to Wenzel state occurs due to the coalescence of neighboring droplets, resulting in the completely wetted surface.[26–29] For larger droplets such as raindrops (from a few hundred µm to a few mm), which are much larger than the typical length scales of the surface patterns, if the structured surface is chemically hydrophobic enough, Cassie–Baxter state is usually observed, leading to superhydrophobicity as seen in lotus leaf. However, the droplet size is much smaller (≈1–40 µm) for water condensed from the atmosphere. Therefore, water droplets can be readily formed within the grooves of the structures, resulting in the loss of nonwettability even on a hydrophobic surface.[16,21] In the case of a surface with mixed hydrophilic and hydrophobic regions, due to the wettability contrast water droplets are preferentially nucleated and grown onto the hydrophilic regions.[6–14] So far, preferential wettability is mostly demonstrated by introducing chemical heterogeneity on a flat or structured surface, typically hydrophilic patches surrounded by hydrophobic regions[8–12,14,30–32] Despite the theory proposed by Beysens and co-workers that it is possible to selectively nucleate water on structured surfaces with homogeneous chemistry, which is much easier to fabricate compared to the ones with hydrophobic and hydrophilic regions, little has been demonstrated in experiments. This is because it requires much higher wettability contrast between the patterned regions and the nonpatterned regions than that from heterogeneous surface chemistry. We hypothesize multilevel hierarchical structures with isolated micro- and nanostructures would be ideal for this purpose. Nanoroughness can offer better spatial control to prevent the formation of extremely small water droplets condensed from atmosphere (≈1–40 µm in diameter). Further, if the nanostructures are located only on the microstructures, they can be easily isolated from each other, thus, increasing wettability contrast between the patterned regions and the nonpatterned regions. Along the line, the combination of micro- and nanostructures drastically increases the surface roughness, further enhancing the wettability contrast. Nevertheless, it is nontrivial to create isolated nanoroughness with high asperity. Herein, we develop a new and versatile approach to fabricate hierarchical polymeric microstructures consisting of
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Spatially Selective Nucleation and Growth of Water Droplets on Hierarchically Patterned Polymer Surfaces
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conditions, which involve the use of highly acidic solutions and high electric potential. In addition, in these experiments, nanopores are typically generated on a thin aluminum layer rather from the bulk aluminum plate, thus limiting the achievable height of the AAO nanopores and possibility to generate the highly ordered pore array via the second anodization process (see detailed discussion in Supporting Information). To circumvent these issues, here, we utilized a highly stable epoxy micropattern as a mask to generate nanopores in the nonmasked regions; later the hierarchical membrane was used as a template for obtaining the hierarchical polymer structure to direct water nucleation and growth. We first fabricated a highly ordered concave AAO pattern by hard anodization, followed by pore etching to obtain interpore distance (Dint) of 350 nm. Then a PDMS mold with a micropost array replicated from a SU-8 master was attached to the hard-anodized aluminum plate with conformal contact, forming microchannels to infiltrate epoxy (Figure 1).[50,51] Figure 1. Schematic illustrations of the process for a) preparation of an epoxy pattern on the AAO nanopores with a high aspect ratio aluminum plate and b) preparation of SPUA hierarchical polymer structures. (height/pore diameter, ≈3.7) were selectively fabricated on the nonepoxy regions by the second mild anodizananopillars. Nanopillars are replica molded from the hierarchition (Figure S1, Supporting Information). The pore thickness cally porous anodic aluminum oxide (AAO) membranes fabricould be fine-tuned by anodization time and the pore diamcated by anodization of bulk aluminum. The surface is treated eter can be further enlarged by the additional pore etching hydrophobic everywhere. On such surfaces, we observe selecprocess. Pore size and shape can be tailored by control on anotive nucleation and growth of water droplets within the grooves dization time, pore-etching time, and the number of reaction between the patterns. Both the geometry and size of micro- and cycles following the procedure described earlier.[52] Here, we nanostructures can be fine-tuned. In contrast, on patterned surfaces consisting of micro- or nanostructures only we show that prepared stepwise nanopores with the top diameter of 85 nm, water is nucleated everywhere. Importantly, the spatial control bottom diameter of 300 nm, and height of 1.1 µm since they is achieved without introducing hydrophilic patches. offer higher stability than cylindrical pillars against capillary As seen in Figure 1, hierarchically porous AAO templates force during drying.[52] After infiltrating the hierarchical AAO are realized by the anodization on epoxy patterned aluminum templates with SPUA, followed by removal of the template by plate, which is prepared by capillary infiltration of epoxy prepolchemical etching, we obtained hierarchical polymer structures ymer into the microchannels defined by poly(dimethylsiloxane) as seen in Figure 2. The structural parameters of the epoxy (PDMS) molds, followed by photopolymerization. The patmicropatterns could be easily tuned by changing the initial SU-8 terned aluminum plate is then anodized, where the AAO nanomaster fabricated by photolithography, we prepared hierarchical pores are formed selectively in the nonpatterned regions. The structures from different bottom microstructures, including a hierarchical AAO porous structure is subsequently utilized square array of microposts with diameter of 100 (Figure 2a,b) as a template for the preparation of hierarchical polymeric and 10 µm (Figure 2c,d), respectively, and zigzag herringbone structures from photocurable prepolymers, soft polyurethane structure with 100 µm in width, 100 µm in spacing, and 20 µm acrylate (SPUA). We note that our method of fabrication hierarin height, respectively (Figure 2e,f). chical AAO template is quite different from the literature. AAO Compared to other methods reported in the literature to templates with nanopore arrays are promising templates for localize AAO formation in the micropatterns, our approach has fabrication of high aspect ratio nanostructures over a large area, several distinct advantages. (1) Epoxy is a good electric insuincluding nanodots, nanorods, nanowires, and nanotubes.[33–41] lator with dielectric constant ≥3.0 and has strong adhesion to metal surfaces.[53] The ability to directly mold epoxy patterns The fabrication is simple, inexpensive, and highly reproducible and controllable. Typically, AAO templates are prepared from onto a bulk aluminum plate offers flexibility to control pore a bulk aluminum plate to generate deep and ordered nanopgrowth, allowing for well-ordered nanopores with an extremely ores. There have been a few tries to realize nanopore formahigh aspect ratios. (2) PDMS mold is reusable, thus, greatly tion on a selective area micropatterned by conventional photoreducing the experimental steps and cost. (3) Various types lithography.[42–49] However, it remains challenging due to the of hierarchical structures can be fabricated by combination of photolithography and soft lithography with AAO templating poor stability of generated pattern during harsh anodization
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COMMUNICATION Figure 2. Field-emission scanning electron microscopy (FE-SEM) images of various SPUA polymer structures a,b) stepwise nanopillars (top diameter of 85 nm, bottom diameter 300 nm, and height of 1.1 µm) on microposts with a diameter of 100 µm, c,d) stepwise nanopillars on microstructure with a diameter of 10 µm, e,f) stepwise nanopillars on zigzag herringbone structures, and g,h) localized nanopillar arrays without microstructure.
(see Figure 2). Furthermore, we show that nanopillars without the microstructure can be prepared in the localized regions (Figure 2g,h). After infiltration and curing of the prepolymer in the AAO template, if we remove AAO template only, we obtain the localized nanopillar array; if both AAO template and remaining epoxy pattern are removed, we obtain hierarchically patterned polymer structures (see Figure 1b). Armed with exquisitely controlled hierarchical structures, we investigated the wettability and water droplet formation on the surface. For the water droplet condensed from the atmosphere, droplet size typically ranges from 1 to 40 µm, while the size of typical sessile drop in contact angle measurement is a few hundreds of µm to a few mm. Therefore, in order to efficiently prevent the droplet condensation on the surface, it is essential to create nanoroughness smaller than 1 µm. The apparent contact angles of fluorosilane-treated SPUA structures from millimeter scale sessile drops are 137°, 146°, and 161° for microposts (100 µm in diameter, and 50 µm in spacing, Figure 2a,b), tapered nanopillars, and hierarchical structures consisting of microposts and nanopillars, respectively. Since they are chemically homogeneous, the difference in water contact angles originates from the surface texture only. The water droplet formation on various structured surfaces was investigated using in situ environmental scanning electron microscopy (ESEM). Figure 3a shows a series of ESEM images of water droplet formation on the hierarchical SPUA structure as a function of the vapor pressure inside the SEM chamber at a fixed temperature of 0 °C. For comparison, we observed the droplet formation on micropatterns without nanopillars at the same time. Occasionally, during fabrication nanopillars did not form on some of the microposts due to insufficient capillary infiltration, which indeed served even better purpose to explicitly show the effect of nanopillars. Therefore, we show the ESEM images in the defect region here. It was clear that droplet nucleation and subsequent growth selectively occurred only at the regions where there were no nanopillars. Water droplets
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were rarely observed on the micropost with nanopillars (indicated by red circles in Figure 3a) even when the nanopillar-free regions were fully filled with water droplets (Figure S2, Supporting Information). Since the water contact angle of the hierarchically structured surface (161°, see Table 1) is much higher than that of the flat surface (105°), even though the latter is hydrophobic, droplets are preferentially nucleated on the flat surface, that is the groove regions in-between the microposts and top of the microposts. This suggests that hydrophilic surface, which has been adopted in literature for selective water nucleation and growth, is not necessary. Rather, wettability contrast plays an essential role in determining the site for preferential nucleation and growth, whether it is on a composite surface with hydrophilic and hydrophobic regions,[6–10,12,16] or hierarchical structures with homogeneously hydrophobic chemistry. The size of water droplets nucleated at the beginning of condensation is only a few µm. Therefore, they are easily nucleated and grown in between microposts without the confinement restriction. As the vapor pressure increases, larger droplets are formed due to the coalescence of neighboring droplets and thus fill the grooves between the microposts eventually as shown in Figure 3a. According to the prediction by Beysens and coworkers,[21–25,29] if the droplet size is smaller than the length scale of the surface pattern, they can be simultaneously nucleated from all surfaces, here, the grooves and the top and side walls of the microposts. In the case of micropost only surface, the water was nucleated between the grooves as the theory predicted (Figure 3b). Once they completely fill the grooves, water level rises up to the top surface and droplets coalesce on the top to form droplets that is much larger than the individual micropost, in agreement with literature observation.[21,22,29] However, with the presence of nanopillars on microposts in the present study, droplet nucleation and growth was completely different. Water droplets were nucleated only in the grooves in-between microposts where there
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Figure 3. ESEM images of water nucleation and growth on the fluorosilane-treated a) hierarchical SPUA structures (microposts with nanopillars) and b) microposts only (without nanopillars) as a function of vapor pressure from 4.8 to 7 torr. The temperature is fixed at 0 °C. Red circles indicate the microposts with nanopillars, while others having no nanopillars.
were no nanopillars. On a flat surface, water condensation from moist air is governed by the free energy barrier ΔG and nucleation rate J, which are strongly depend on the intrinsic wettability of the surface[12] ΔG = πσ lvγ *2 ( 2 − 3cosθ + cos3 θ )
(1)
where σ1v, γ *, and θ are the liquid-vapor surface energy, critical radius (around 2 nm at T = 274 K), and apparent contact angle, respectively J = J 0 exp ⎡⎣πσ lv γ *2 ( 2 − 3cosθ + cos3 θ ) /3kT ⎤⎦
(2)
where J0, k, and T are the kinetic constant, Boltzmann constant, and temperature, respectively. A surface with a larger contact angle requires more energy to nucleate water droplet, and thus, has a lower nucleation rate compared to the surface with a smaller static contact angle. When different intrinsic wettability coexists on the surface, water droplets will be preferentially nucleated and grown onto the surface with a smaller contact angle. Nucleation is accelerated on the surface with a larger wettability contrast between those regions. As mentioned before, wettability of a surface is determined by surface energy (i.e., chemical compositions) and surface roughness. Different from most studies reported in literature, where nucleation of water is controlled by the difference in surface energy in a selective area,[6–10,12,16] our system clearly show that it is
Table 1. Static water contact angles on various structures made from SPUA.
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Flat SPUA
Micropatterned SPUA (100 µm in diameter)
SPUA nanopillars w/o micropattern
Micropatterned SPUA w/ nanopillars
Without fluorosilane treatment
89° ± 2.4°
105° ± 2.7°
124° ± 2.5°
137° ± 3.2°
With fluorosilane treatment
105° ± 1.9°
137° ± 2.2°
146° ± 2.1°
161° ± 4.2°
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COMMUNICATION Figure 4. ESEM images of water nucleation and growth on tapered fluorosilane-treated a) SPUA nanopillar-only surface and b) SPUA nanopillar arrays as a function of vapor pressure at a fixed temperature of 0 °C. Pressure was increased from 4.8 to 7 torr. Circles indicate the nanopillar regions.
due to geometry and size effect. The size of nanopillars (top diameter of 85 nm), which is much smaller than the initial water droplet size (typically a few µm), and the isolation of the nanopillars arrays by locating them onto the microposts further enhances the selectivity. Figure 4a shows the water condensation behavior on a surface with nanopillars only. Although the water contact angle on nanopillar surface is as high as of 146° (Table 1), because the surface is spatially uniform, nucleation of water droplets occurs randomly on the surface although droplets do not grow readily as those on a flat surface and a surface with microposts only. It can be attributed to the much smaller feature size of the nanopillars compared to the nucleated water droplet size, and the highly hydrophobic nature of the nanopillar array, leading to a slower droplet growth rate. Thus, coalescence of droplets on the nanopillars rarely occurred for the vapor pressure tested in our experiments, preventing the formation of larger droplets that are typically observed in grooves between microposts. To further study the spatial effect of the nanopillars, we patterned a square array of nanopillars on the flat surface, where the nanopillars were localized within circles (100 µm in diameter). Despite that the nanopillars were isolated from each other, and the difference of water contact angles between the nanopillar array and flat surface is rather large (≈41°), water droplets were found randomly nucleated over the entire surface as the vapor pressure increased (Figure 4b), much like the
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observations from surfaces with microposts only (Figure 3b), nanopillars only (Figure 4a), and flat surface (Figure S3, Supporting Informtion). Different from other surfaces, however, very few droplets appeared in the low vapor pressure range, which could be explained by both the small size of nanopilllars and their further isolation in circles. Appreciable water droplets appeared at a much high pressure (here 6.4 torr), and grew much faster than the surface with nanopillars everywhere. Eventually the whole surface was wetted. When the wettability difference between the nanopillared regions and the flat regions is not high enough, at a high vapor pressure, water eventually overcomes the capillary length and fills the grooves between nanopillars. Then the nanopillar regions act as defects that accelerate the droplet nucleation. In the case of micropost only surface, since the feature size of micropost (≈100 µm in diameter) is much larger than the initial droplet size (few µm), droplets are generated everywhere regardless of presence of the microposts when the vapor pressure increases (Figure 3b). These results unequivocally confirm that the high wettability contrast realized by the combination of micro- and nanostructures contributes to spatial selectivity of droplet nucleation and growth. In summary, we prepared a new type of hierarchical polymer pillar arrays templated from the hierarchically porous AAO template, which was realized by the anodization on the aluminum plate masked by patterned epoxy. Each structural component
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can be precisely fine-tuned as we show, for example, microstructured herringbone and dot patterns with various sizes and tapered nanopillars with different interpore spacing and height. On these surfaces, we investigated spatial control of the nucleation and growth of water droplets using in situ ESEM. The combination of microposts and nanopillar arrays strongly prevented the droplet formation without altering surface chemistry, which could be explained by the high wettability contrast between the hierarchically structured surface and flat regions. It is further supported by the similar observations on structures prepared from other polymers (e.g., epoxy). Supporting this, no preferential nucleation was observed on nanopillar-only or micropost-only surfaces due to the low wetting contrast to the flat surface. We expect that the demonstrated spatial control of water droplet formation on the hierarchical structures open a new door for water harvesting without creating chemically heterogeneous surfaces. The insights gained from here can be applied to diverse range of potential applications, including cell proliferation, microfluidics, antifogging, water harvesting, and enhanced heat transfer.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was supported in part by National Science Foundation (NSF) grant # CBET-1264808, and NSF/PIRE, # 1545884. The authors thank Prof. Hyunsik Yoon from Seoul National University of Science & Technology and Minuta Technology (Osan-si, Korea) for providing SPUA. The authors also thank Prof. John Crocker, Prof. Daeyeon Lee from University of Pennsylvania, and Prof. So-Jung Park from Ewha Womans University for helpful discussion. Received: October 5, 2015 Revised: October 28, 2015 Published online: December 4, 2015
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Younghyun Cho, Tae Soup Shim, and Shu Yang* In the last century, demand for water has grown at twice the rate of the population. Nearly a quarter of the population faces economic water shortages, not only in arid regions but also in regions with low rainfall despite high humidity.[1] Therefore, creating a coating that can capture water, via condensation or trapping of droplets, is critical to address the water shortage issues. In nature, control of the heterogeneous nucleation and growth of water droplets on a surface has been crucial for survival and proliferation of the bioorganisms. For example, in extremely arid habitat such as the Namib Desert, beetles Stenocara sp. capture and collect water from the early morning fog on their backs, which are covered with a random array of bumps. The peaks of the bumps are smooth and hydrophilic, where the fog is captured, and the troughs consisting of flattened hemispheres (10 µm in diameter) are superhydrophoibic, which assists the roll-off of water droplets.[2,3] Management of water on surfaces is also important to water filtration, microfluidics, antibiofouling, and anti-icing.[4,5] There have been increasing efforts to create beetle-like patterned surfaces to manipulate water nucleation and growth.[6–14] For example, Garrod et al.[9] show that a pattern combining hydrophilic–hydrophobic surfaces enhances the water collection efficiency compared with a surface that is homogeneously hydrophobic or hydrophilic. An optimal feature size of hydrophilic area (pixel size and center-to-center distance of 500 and 1000 µm, respectively) is suggested. Dorrer and Rühe have investigated the influence of the superhydrophobicity/hydrophilicity contrast to the development of water droplets and suggested that the critical volume at which droplets coalescence and roll off from the hydrophilic spot is linearly proportional to the diameter of the spot.[7] Since the free energy barrier for the droplet formation and nucleation rate are strongly governed by the intrinsic wettability of surface,[15,16] wettability contrast between different regions plays a critical role in determining where droplet nucleation and growth occur selectively. Wettability is determined by surface topography and chemical compositions.[17,18] On a rough surface, there are two simplified (non)wetting Dr. Y. Cho, Prof. S. Yang Department of Materials Science and Engineering University of Pennsylvania 3231 Walnut Street, Philadelphia, PA 19104, USA E-mail: [email protected] Dr. T. S. Shim[+] Department of Chemical and Biomolecular Engineering University of Pennsylvania 220 South 33rd Street, Philadelphia, PA 19104, USA [+]Present address: Department of Chemical Engineering, Ajou University, Suwon 16499, Republic of Korea
DOI: 10.1002/adma.201504899
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states, Cassie–Baxter state[19] and Wenzel state,[20] and states inbetween. In the Cassie–Baxter state, air is trapped between the grooves of the rough structure. Therefore, water droplets sit on a composite surface, whereas in the Wenzel state water completely fills the grooves between the rough surfaces, making the water droplet immobile. Beysens and co-workers have investigated water droplet nucleation and growth on micropatterned surfaces.[21–25] They categorize the condensation behaviors into four stages depending on the droplet to pattern size, including initial nucleation, bridge formation, drying, and large droplet formation. As the droplet grows, transition from Cassie– Baxter state to Wenzel state occurs due to the coalescence of neighboring droplets, resulting in the completely wetted surface.[26–29] For larger droplets such as raindrops (from a few hundred µm to a few mm), which are much larger than the typical length scales of the surface patterns, if the structured surface is chemically hydrophobic enough, Cassie–Baxter state is usually observed, leading to superhydrophobicity as seen in lotus leaf. However, the droplet size is much smaller (≈1–40 µm) for water condensed from the atmosphere. Therefore, water droplets can be readily formed within the grooves of the structures, resulting in the loss of nonwettability even on a hydrophobic surface.[16,21] In the case of a surface with mixed hydrophilic and hydrophobic regions, due to the wettability contrast water droplets are preferentially nucleated and grown onto the hydrophilic regions.[6–14] So far, preferential wettability is mostly demonstrated by introducing chemical heterogeneity on a flat or structured surface, typically hydrophilic patches surrounded by hydrophobic regions[8–12,14,30–32] Despite the theory proposed by Beysens and co-workers that it is possible to selectively nucleate water on structured surfaces with homogeneous chemistry, which is much easier to fabricate compared to the ones with hydrophobic and hydrophilic regions, little has been demonstrated in experiments. This is because it requires much higher wettability contrast between the patterned regions and the nonpatterned regions than that from heterogeneous surface chemistry. We hypothesize multilevel hierarchical structures with isolated micro- and nanostructures would be ideal for this purpose. Nanoroughness can offer better spatial control to prevent the formation of extremely small water droplets condensed from atmosphere (≈1–40 µm in diameter). Further, if the nanostructures are located only on the microstructures, they can be easily isolated from each other, thus, increasing wettability contrast between the patterned regions and the nonpatterned regions. Along the line, the combination of micro- and nanostructures drastically increases the surface roughness, further enhancing the wettability contrast. Nevertheless, it is nontrivial to create isolated nanoroughness with high asperity. Herein, we develop a new and versatile approach to fabricate hierarchical polymeric microstructures consisting of
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Spatially Selective Nucleation and Growth of Water Droplets on Hierarchically Patterned Polymer Surfaces
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conditions, which involve the use of highly acidic solutions and high electric potential. In addition, in these experiments, nanopores are typically generated on a thin aluminum layer rather from the bulk aluminum plate, thus limiting the achievable height of the AAO nanopores and possibility to generate the highly ordered pore array via the second anodization process (see detailed discussion in Supporting Information). To circumvent these issues, here, we utilized a highly stable epoxy micropattern as a mask to generate nanopores in the nonmasked regions; later the hierarchical membrane was used as a template for obtaining the hierarchical polymer structure to direct water nucleation and growth. We first fabricated a highly ordered concave AAO pattern by hard anodization, followed by pore etching to obtain interpore distance (Dint) of 350 nm. Then a PDMS mold with a micropost array replicated from a SU-8 master was attached to the hard-anodized aluminum plate with conformal contact, forming microchannels to infiltrate epoxy (Figure 1).[50,51] Figure 1. Schematic illustrations of the process for a) preparation of an epoxy pattern on the AAO nanopores with a high aspect ratio aluminum plate and b) preparation of SPUA hierarchical polymer structures. (height/pore diameter, ≈3.7) were selectively fabricated on the nonepoxy regions by the second mild anodizananopillars. Nanopillars are replica molded from the hierarchition (Figure S1, Supporting Information). The pore thickness cally porous anodic aluminum oxide (AAO) membranes fabricould be fine-tuned by anodization time and the pore diamcated by anodization of bulk aluminum. The surface is treated eter can be further enlarged by the additional pore etching hydrophobic everywhere. On such surfaces, we observe selecprocess. Pore size and shape can be tailored by control on anotive nucleation and growth of water droplets within the grooves dization time, pore-etching time, and the number of reaction between the patterns. Both the geometry and size of micro- and cycles following the procedure described earlier.[52] Here, we nanostructures can be fine-tuned. In contrast, on patterned surfaces consisting of micro- or nanostructures only we show that prepared stepwise nanopores with the top diameter of 85 nm, water is nucleated everywhere. Importantly, the spatial control bottom diameter of 300 nm, and height of 1.1 µm since they is achieved without introducing hydrophilic patches. offer higher stability than cylindrical pillars against capillary As seen in Figure 1, hierarchically porous AAO templates force during drying.[52] After infiltrating the hierarchical AAO are realized by the anodization on epoxy patterned aluminum templates with SPUA, followed by removal of the template by plate, which is prepared by capillary infiltration of epoxy prepolchemical etching, we obtained hierarchical polymer structures ymer into the microchannels defined by poly(dimethylsiloxane) as seen in Figure 2. The structural parameters of the epoxy (PDMS) molds, followed by photopolymerization. The patmicropatterns could be easily tuned by changing the initial SU-8 terned aluminum plate is then anodized, where the AAO nanomaster fabricated by photolithography, we prepared hierarchical pores are formed selectively in the nonpatterned regions. The structures from different bottom microstructures, including a hierarchical AAO porous structure is subsequently utilized square array of microposts with diameter of 100 (Figure 2a,b) as a template for the preparation of hierarchical polymeric and 10 µm (Figure 2c,d), respectively, and zigzag herringbone structures from photocurable prepolymers, soft polyurethane structure with 100 µm in width, 100 µm in spacing, and 20 µm acrylate (SPUA). We note that our method of fabrication hierarin height, respectively (Figure 2e,f). chical AAO template is quite different from the literature. AAO Compared to other methods reported in the literature to templates with nanopore arrays are promising templates for localize AAO formation in the micropatterns, our approach has fabrication of high aspect ratio nanostructures over a large area, several distinct advantages. (1) Epoxy is a good electric insuincluding nanodots, nanorods, nanowires, and nanotubes.[33–41] lator with dielectric constant ≥3.0 and has strong adhesion to metal surfaces.[53] The ability to directly mold epoxy patterns The fabrication is simple, inexpensive, and highly reproducible and controllable. Typically, AAO templates are prepared from onto a bulk aluminum plate offers flexibility to control pore a bulk aluminum plate to generate deep and ordered nanopgrowth, allowing for well-ordered nanopores with an extremely ores. There have been a few tries to realize nanopore formahigh aspect ratios. (2) PDMS mold is reusable, thus, greatly tion on a selective area micropatterned by conventional photoreducing the experimental steps and cost. (3) Various types lithography.[42–49] However, it remains challenging due to the of hierarchical structures can be fabricated by combination of photolithography and soft lithography with AAO templating poor stability of generated pattern during harsh anodization
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COMMUNICATION Figure 2. Field-emission scanning electron microscopy (FE-SEM) images of various SPUA polymer structures a,b) stepwise nanopillars (top diameter of 85 nm, bottom diameter 300 nm, and height of 1.1 µm) on microposts with a diameter of 100 µm, c,d) stepwise nanopillars on microstructure with a diameter of 10 µm, e,f) stepwise nanopillars on zigzag herringbone structures, and g,h) localized nanopillar arrays without microstructure.
(see Figure 2). Furthermore, we show that nanopillars without the microstructure can be prepared in the localized regions (Figure 2g,h). After infiltration and curing of the prepolymer in the AAO template, if we remove AAO template only, we obtain the localized nanopillar array; if both AAO template and remaining epoxy pattern are removed, we obtain hierarchically patterned polymer structures (see Figure 1b). Armed with exquisitely controlled hierarchical structures, we investigated the wettability and water droplet formation on the surface. For the water droplet condensed from the atmosphere, droplet size typically ranges from 1 to 40 µm, while the size of typical sessile drop in contact angle measurement is a few hundreds of µm to a few mm. Therefore, in order to efficiently prevent the droplet condensation on the surface, it is essential to create nanoroughness smaller than 1 µm. The apparent contact angles of fluorosilane-treated SPUA structures from millimeter scale sessile drops are 137°, 146°, and 161° for microposts (100 µm in diameter, and 50 µm in spacing, Figure 2a,b), tapered nanopillars, and hierarchical structures consisting of microposts and nanopillars, respectively. Since they are chemically homogeneous, the difference in water contact angles originates from the surface texture only. The water droplet formation on various structured surfaces was investigated using in situ environmental scanning electron microscopy (ESEM). Figure 3a shows a series of ESEM images of water droplet formation on the hierarchical SPUA structure as a function of the vapor pressure inside the SEM chamber at a fixed temperature of 0 °C. For comparison, we observed the droplet formation on micropatterns without nanopillars at the same time. Occasionally, during fabrication nanopillars did not form on some of the microposts due to insufficient capillary infiltration, which indeed served even better purpose to explicitly show the effect of nanopillars. Therefore, we show the ESEM images in the defect region here. It was clear that droplet nucleation and subsequent growth selectively occurred only at the regions where there were no nanopillars. Water droplets
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were rarely observed on the micropost with nanopillars (indicated by red circles in Figure 3a) even when the nanopillar-free regions were fully filled with water droplets (Figure S2, Supporting Information). Since the water contact angle of the hierarchically structured surface (161°, see Table 1) is much higher than that of the flat surface (105°), even though the latter is hydrophobic, droplets are preferentially nucleated on the flat surface, that is the groove regions in-between the microposts and top of the microposts. This suggests that hydrophilic surface, which has been adopted in literature for selective water nucleation and growth, is not necessary. Rather, wettability contrast plays an essential role in determining the site for preferential nucleation and growth, whether it is on a composite surface with hydrophilic and hydrophobic regions,[6–10,12,16] or hierarchical structures with homogeneously hydrophobic chemistry. The size of water droplets nucleated at the beginning of condensation is only a few µm. Therefore, they are easily nucleated and grown in between microposts without the confinement restriction. As the vapor pressure increases, larger droplets are formed due to the coalescence of neighboring droplets and thus fill the grooves between the microposts eventually as shown in Figure 3a. According to the prediction by Beysens and coworkers,[21–25,29] if the droplet size is smaller than the length scale of the surface pattern, they can be simultaneously nucleated from all surfaces, here, the grooves and the top and side walls of the microposts. In the case of micropost only surface, the water was nucleated between the grooves as the theory predicted (Figure 3b). Once they completely fill the grooves, water level rises up to the top surface and droplets coalesce on the top to form droplets that is much larger than the individual micropost, in agreement with literature observation.[21,22,29] However, with the presence of nanopillars on microposts in the present study, droplet nucleation and growth was completely different. Water droplets were nucleated only in the grooves in-between microposts where there
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Figure 3. ESEM images of water nucleation and growth on the fluorosilane-treated a) hierarchical SPUA structures (microposts with nanopillars) and b) microposts only (without nanopillars) as a function of vapor pressure from 4.8 to 7 torr. The temperature is fixed at 0 °C. Red circles indicate the microposts with nanopillars, while others having no nanopillars.
were no nanopillars. On a flat surface, water condensation from moist air is governed by the free energy barrier ΔG and nucleation rate J, which are strongly depend on the intrinsic wettability of the surface[12] ΔG = πσ lvγ *2 ( 2 − 3cosθ + cos3 θ )
(1)
where σ1v, γ *, and θ are the liquid-vapor surface energy, critical radius (around 2 nm at T = 274 K), and apparent contact angle, respectively J = J 0 exp ⎡⎣πσ lv γ *2 ( 2 − 3cosθ + cos3 θ ) /3kT ⎤⎦
(2)
where J0, k, and T are the kinetic constant, Boltzmann constant, and temperature, respectively. A surface with a larger contact angle requires more energy to nucleate water droplet, and thus, has a lower nucleation rate compared to the surface with a smaller static contact angle. When different intrinsic wettability coexists on the surface, water droplets will be preferentially nucleated and grown onto the surface with a smaller contact angle. Nucleation is accelerated on the surface with a larger wettability contrast between those regions. As mentioned before, wettability of a surface is determined by surface energy (i.e., chemical compositions) and surface roughness. Different from most studies reported in literature, where nucleation of water is controlled by the difference in surface energy in a selective area,[6–10,12,16] our system clearly show that it is
Table 1. Static water contact angles on various structures made from SPUA.
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Flat SPUA
Micropatterned SPUA (100 µm in diameter)
SPUA nanopillars w/o micropattern
Micropatterned SPUA w/ nanopillars
Without fluorosilane treatment
89° ± 2.4°
105° ± 2.7°
124° ± 2.5°
137° ± 3.2°
With fluorosilane treatment
105° ± 1.9°
137° ± 2.2°
146° ± 2.1°
161° ± 4.2°
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COMMUNICATION Figure 4. ESEM images of water nucleation and growth on tapered fluorosilane-treated a) SPUA nanopillar-only surface and b) SPUA nanopillar arrays as a function of vapor pressure at a fixed temperature of 0 °C. Pressure was increased from 4.8 to 7 torr. Circles indicate the nanopillar regions.
due to geometry and size effect. The size of nanopillars (top diameter of 85 nm), which is much smaller than the initial water droplet size (typically a few µm), and the isolation of the nanopillars arrays by locating them onto the microposts further enhances the selectivity. Figure 4a shows the water condensation behavior on a surface with nanopillars only. Although the water contact angle on nanopillar surface is as high as of 146° (Table 1), because the surface is spatially uniform, nucleation of water droplets occurs randomly on the surface although droplets do not grow readily as those on a flat surface and a surface with microposts only. It can be attributed to the much smaller feature size of the nanopillars compared to the nucleated water droplet size, and the highly hydrophobic nature of the nanopillar array, leading to a slower droplet growth rate. Thus, coalescence of droplets on the nanopillars rarely occurred for the vapor pressure tested in our experiments, preventing the formation of larger droplets that are typically observed in grooves between microposts. To further study the spatial effect of the nanopillars, we patterned a square array of nanopillars on the flat surface, where the nanopillars were localized within circles (100 µm in diameter). Despite that the nanopillars were isolated from each other, and the difference of water contact angles between the nanopillar array and flat surface is rather large (≈41°), water droplets were found randomly nucleated over the entire surface as the vapor pressure increased (Figure 4b), much like the
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observations from surfaces with microposts only (Figure 3b), nanopillars only (Figure 4a), and flat surface (Figure S3, Supporting Informtion). Different from other surfaces, however, very few droplets appeared in the low vapor pressure range, which could be explained by both the small size of nanopilllars and their further isolation in circles. Appreciable water droplets appeared at a much high pressure (here 6.4 torr), and grew much faster than the surface with nanopillars everywhere. Eventually the whole surface was wetted. When the wettability difference between the nanopillared regions and the flat regions is not high enough, at a high vapor pressure, water eventually overcomes the capillary length and fills the grooves between nanopillars. Then the nanopillar regions act as defects that accelerate the droplet nucleation. In the case of micropost only surface, since the feature size of micropost (≈100 µm in diameter) is much larger than the initial droplet size (few µm), droplets are generated everywhere regardless of presence of the microposts when the vapor pressure increases (Figure 3b). These results unequivocally confirm that the high wettability contrast realized by the combination of micro- and nanostructures contributes to spatial selectivity of droplet nucleation and growth. In summary, we prepared a new type of hierarchical polymer pillar arrays templated from the hierarchically porous AAO template, which was realized by the anodization on the aluminum plate masked by patterned epoxy. Each structural component
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can be precisely fine-tuned as we show, for example, microstructured herringbone and dot patterns with various sizes and tapered nanopillars with different interpore spacing and height. On these surfaces, we investigated spatial control of the nucleation and growth of water droplets using in situ ESEM. The combination of microposts and nanopillar arrays strongly prevented the droplet formation without altering surface chemistry, which could be explained by the high wettability contrast between the hierarchically structured surface and flat regions. It is further supported by the similar observations on structures prepared from other polymers (e.g., epoxy). Supporting this, no preferential nucleation was observed on nanopillar-only or micropost-only surfaces due to the low wetting contrast to the flat surface. We expect that the demonstrated spatial control of water droplet formation on the hierarchical structures open a new door for water harvesting without creating chemically heterogeneous surfaces. The insights gained from here can be applied to diverse range of potential applications, including cell proliferation, microfluidics, antifogging, water harvesting, and enhanced heat transfer.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was supported in part by National Science Foundation (NSF) grant # CBET-1264808, and NSF/PIRE, # 1545884. The authors thank Prof. Hyunsik Yoon from Seoul National University of Science & Technology and Minuta Technology (Osan-si, Korea) for providing SPUA. The authors also thank Prof. John Crocker, Prof. Daeyeon Lee from University of Pennsylvania, and Prof. So-Jung Park from Ewha Womans University for helpful discussion. Received: October 5, 2015 Revised: October 28, 2015 Published online: December 4, 2015
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[47] G. Sharma, S. C. Chong, L. Ebin, C. Hui, C. L. Gan, V. Kripesh, Thin Solid Films 2007, 515, 3315. [48] J. Park, J. Fataccioli, H. Fujita, B. Kim, Int. J. Precis. Eng. Man. 2012, 13, 765–770. [49] X. Li , Y. He , T. H. Zhang , L. Que , Opt. Express 2012 , 20 , 21272 . [50] K. Y. Suh, Y. S. Kim, H. H. Lee, Adv. Mater. 2001, 13, 1386.
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