Journal of Colloid and Interface Science 416 (2014) 280–288
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Micro-and nanostructured silicon-based superomniphobic surfaces Thi Phuong Nhung Nguyen a,b,c, Rabah Boukherroub a, Vincent Thomy b,⇑, Yannick Coffinier a,⇑ a
Institut de Recherche Interdisciplinaire (IRI), USR CNRS 3078, Université Lille1, Parc de la Haute Borne, 50 Avenue de Halley, 59658 Villeneuve d’Ascq, France Institut d’Electronique, de Microélectronique et de Nanotechnologie (IEMN – UMR 8520), Cité Scientifique, Avenue Poincaré, BP 60069, 59652 Villeneuve d’Ascq, France c PetroVietnam University, 30/4 Street, Vung Tau City, Vietnam b
a r t i c l e
i n f o
Article history: Received 21 August 2013 Accepted 31 October 2013 Available online 16 November 2013 Keywords: Silicon nanostructures Multi-scale roughness Organic coating Superoleophobic Surperhydrophobic
a b s t r a c t We report on the fabrication of silicon nanostructured superhydrophobic and superoleophobic surfaces also called ‘‘superomniphobic’’ surfaces. For this purpose, silicon interfaces with different surface morphologies, single or double scale structuration, were investigated. These structured surfaces were chemically treated with perfluorodecyltrichlorosilane (PFTS), a low surface energy molecule. The morphology of the resulting surfaces was characterized using scanning electron microscopy (SEM). Their wetting properties: static contact angle (CA) and contact angle hysteresis (CAH) were investigated using liquids of various surface tensions. Despite that we found that all the different morphologies display a superhydrophobic character (CA > 150° for water) and superoleophobic behavior (CA 140° for hexadecane), values of hysteresis are strongly dependent on the liquid surface tension and surface morphology. The best surface described in this study was composed of a dual scale texturation i.e. silicon micropillars covered by silicon nanowires. Indeed, this surface displayed high static contact angles and low hysteresis for all tested liquids. Ó 2013 Elsevier Inc. All rights reserved.
1. Introduction Nature offers a large variety of examples of surfaces with amazing wettability properties, like the extreme water repellency. From plant leaves to animals, this property is ascribed to the micro- and nanotexturation of the lotus leaves, the duck feather or the wings of butterflies [1]. When a rain drop is in contact, it rolls off the surface and is unable to wet it, collecting at the same time the dust particles; this behavior is called self-cleaning effect. These socalled superhydrophobic surfaces have been deeply studied and are now well known [2,3]. In order to mimic natural surfaces, roughness can be designed in a controlled fashion to significantly modify the wetting behavior. There exists a host of examples of surfaces for which a micron or sub-micron scale texturation leads to high liquid repellency [4,5]. This property can rely on the following: – The achievement of high effective contact area between the liquid and the solid, leading to a high surface energy cost if the solid is treated with a low surface energy coating (Fig. 1A). This corresponds to a situation where the liquid is impaled by the texture. This approach has historically been proposed by Wenzel and it is denoted as Wenzel (W) state thereafter [6]. ⇑ Corresponding authors. Fax: +33 3 20 19 78 84. E-mail addresses: [email protected] (V. Thomy), coffi[email protected] (Y. Coffinier). 0021-9797/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2013.10.065
Yannick.
– The entrapment of air pockets between the liquid and the surface, ensuring that the liquid below the drop is mostly in contact with air (Fig. 1B). This is denoted as the Cassie–Baxter (CB) state thereafter [7]. This situation offers a very low liquid–solid friction, and this feature is particularly interesting for versatile droplet motion [8–11]. Thus to distinguish between Wenzel and Cassie–Baxter states, it is necessary to characterize the degree of impalement of the droplet sitting on or impaled inside the structuration. That is possible by measuring the difference between the advancing and receding contact angles, corresponding to the contact angle hysteresis (Dh), which is related to the retention force of the drop on the substrate [12]. Even though the Wenzel and Cassie–Baxter states exhibit high apparent contact angles (up to 160°), they present a totally different behavior. On most of nanotextured surfaces, in the Wenzel state, Dh, measured by surface tilting, is larger than 30° and the droplet remains stuck on the surface, whereas in the Cassie–Baxter state, the droplet Dh can be lower than 5°. In the latter case, the liquid droplet rolls off instead of sliding once the surface is tilted by a few degrees [13]: it is called the ‘‘rolling-ball’’ effect [14–16]. Another point that needs to be detailed is the apparition of metastable states where liquid droplet can transit from Cassie– Baxter to Wenzel state, the more energetically stable state. While, in practice, Cassie–Baxter state is most of the time expected, one of the most widespread techniques to insure this state is to develop multi-scale structures, combining micro- and nano-metric
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Fig. 1. A schematic illustration of the formation of composite interfaces with a droplet in the Wenzel (A) and Cassie–Baxter (B) state.
structuration. This point has been largely described and validated using different technological processes [17,18]. To extend the ‘‘rolling-ball effect’’ to low surface tension liquids, a structure with a reentrant angle is required as shown by pioneering work of Tuteja et al. [19]. Theoretically, under these conditions, it is possible to obtain high contact angle even with hydrophilic surfaces. But, from an experimental view point, it is preferable to enhance the robustness of the surface (i.e., preventing any transition from Cassie to Wenzel state) and then to coat the re-entrant structure with an omniphobic layer with a surface energy as low as possible. Thus, even if hydrocarbon molecules present surface energies of 35–20 mN/m, it is preferable to use fluorinated molecules that can achieve low surface energy (28–6 mN/m) [20,21]. Furthermore, Tuteja et al. recently evidenced that superoleophobicity can be achieved through an interplay with both surface geometry and chemistry. Until now, such superoleophobic surfaces were prepared by creating either a ‘‘re-entrant’’ curvature structure resembling micro-hoodoos, by designing a number of different nano-fiber surfaces [22,23], by an alumina nanowire forests [24] or by ZnO nanostructures [25]. Very recently, Tuteja’s group has developed a stainless steel mesh uniformly coated with electrospun microbeads [26]. This surface presents a clear hierarchical structure with re-entrant features on both scales: fibers and microbeads. This surface displays amazing wetting properties: with a heptane droplet (surface tension of 20 mN/m), they measured a contact angle of 150° and a quasi-null contact angle hysteresis. In that case, the double structuration is not a key feature because similar properties were obtained using only microbeads. Another example of hierarchical omniphobic surfaces was recently reported by Ellinas et al. [27] Such interfaces have been fabricated, via nanosphere lithography (polystyrene beads) on poly(methyl methacrylate) (PMMA) substrate and bearing ordered microcolumns surmounted by nanostructured polystyrene particles and coated by C4F8 [27]. However, these interfaces presented static CA values of 101° and 41° for hexadecane and decane, respectively and no hysteresis measurements were performed. Zhao et al. [28] have recently described superomniphobic surfaces based on silicon micropillars. They have studied the influence of surface texturation by varying the solid area fraction (i.e., by changing pillar diameter, pitch and height) and the effects of the overhang thickness and coatings on the wetting properties for water and hexadecane. Their surfaces displayed high static contact angle (SCA) and contact angle hysteresis (CAH) values of 150° and 20°, respectively, for hexadecane and SCA and CAH values of 150° and 10°, respectively for water [28,29]. All these studies evidenced that multi-scale topography is beneficial for the development of omniphobic surfaces. The objective of this study is to develop silicon-based omniphobic surfaces and to study the role of the surface morphology on the repellency of liquids of low surface tension. For that, we report here the fabrication of omniphobic surfaces realized by different techniques achieved on silicon surfaces, leading to single or dual scale surface morphologies. We have designed six interfaces and investigated their wetting properties. Their morphologies are described below:
- Silicon micropillars (lP-Si) obtained by standard optical lithography and dry reactive ion etching (RIE) [30]. - Silicon micropillars (lP-Si) subjected to metal-assisted electroless etching, presenting a double scale micro- and nanostructuration (lP-NanoSi). - Silicon nanowires synthesized via the vapor–liquid–solid (VLS) growth mechanism [8,15,16,30,31–33]. By varying the furnace pressure and reaction time two different surface morphologies with either one or two silicon nanowires layers, SiNW-A or SiNW-B are prepared. - These two different silicon nanowire morphologies were also grown on silicon micropillars (lP-SiNW-A and lP-SiNW-B). The resulting surfaces exhibit a double micro-nanostructuration as already described in a previous study [30]. All these interfaces were chemically modified with perfluorodecyltrichlorosilane (PFTS), a low surface energy molecule. The surface morphology was characterized using scanning electron microscopy (SEM), and the wetting properties were investigated using various molecules owing to their different surface tensions (from 72 to 21 mN/m). 2. Materials and methods All cleaning and etching reagents were of VLSI grade. Sulfuric acid, 96% (H2SO4), hydrogen peroxide 30% (H2O2), nitric acid 69% (HNO3), and hydrofluoric acid 48% (HF) were supplied by Amplex. All chemicals were of reagent grade or higher and were used as received unless otherwise specified. Acetone, isopropanol, silver nitrate (AgNO3) and sodium tetrafluoroborate (NaBF4) were obtained from Aldrich. 2.1. Fabrication of silicon micropillars (lP-Si) Single side polished silicon (1 0 0) oriented n-type wafers (Siltronix) (phosphorus-doped, 0.009–0.01 Ohm-cm resistivity) were used as substrates. The surface was first degreased in acetone and isopropanol, rinsed with Milli-Q water and then cleaned in a piranha solution (3:1 concentrated H2SO4/30% H2O2) for 15 min at 80 °C followed by copious rinsing with Milli-Q water. A 3.5 lm thick negative resist AZnLOF 2035 (Clariant, France) is spin-coated at 3000 rpm. A soft bake for 1 min on a hot plate at 90 °C is necessary to reduce the solvent content and to prevent resist adhesion to the exposure mask. Then, the exposure, post-exposure bake and development, are conducted for transferring the mask patterns to the resist (latent image). Then, the silicon wafer was etched using Deep Reactive Ion Etching (DRIE, Silicon Technology System) to produce high aspect ratio micropillars. 2.2. Fabrication of double scale micro- and nanosilicon interface (lPNanoSi) To obtain these surfaces, l-pillars, fabricated as described above, were subjected to a metal-assisted electroless etching. Silicon nanostructures are easily prepared, in a reproducible manner,
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by electroless etching of crystalline silicon substrate in aqueous solution of AgNO3 and SiOx etching reagent [34–41]. The metalassisted chemical etching displays several advantages, as it takes place at relatively low temperature and does not require any specific equipment. By varying the etching time, temperature, etching reagent types (HF, NH4F or NaBF4), concentrations of reagents and the type of silicon substrate (doping, crystalline orientation. . .), different silicon nanostructures have been produced presenting different shapes, lengths, diameters and porosities [37,41]. In this study, the lP interface was placed directly in NaBF4 (4 M)/AgNO3 (0.04 M) aqueous solution for 2 h at 80 °C. Ag+ ions capture electrons from the valence band of silicon, reducing Ag+ ion into Ag0, leading to the precipitation of silver nanoparticles onto the Si surface followed by the formation of SiO2 layer below of the Ag particles. Then, SiO2 layer was etched away by NaBF4 (F ) to yield the silicon nanostructures (NanoSi). Finally, the Ag particles are removed by dipping in HCl-HNO3-H20 solution overnight at RT. Safety considerations The mixture H2SO4/H2O2 (piranha) solution is a strong oxidant. It reacts violently with organic materials. It can cause severe skin burns. It must be handled with extreme care in a well-ventilated fume hood while wearing appropriate chemical safety protection. HF is a hazardous acid which can result in serious tissue damage if burns were not appropriately treated. Etching of silicon should be performed in a well-ventilated fume hood with appropriate safety considerations: face shield and double layered nitrile loves. 2.3. Silicon nanowires synthesis via the vapor–liquid–solid (VLS) mechanism (SiNW-A, SiNW-B, lP-SiNW-A and lP-SiNW-B) The silicon nanowires are synthesized using the vapor–liquid– solid (VLS) growth mechanism as previously described [8,15,16,30–33]. The SiNW synthesis was achieved on two different surfaces: – On flat silicon (Siflat), leading to surfaces SiNW-A and SiNW-B. – On silicon micropillars, leading to surfaces lP-SiNW-A and lPSiNW-B. Briefly, a 40 Å Au layer was thermally evaporated on Si/SiO2 (300 nm) and then placed in a chemical vapor deposition (CVD) furnace. The temperature is raised up to 500 °C. At this temperature, the Au layer is dewetted forming gold nanoparticles. These gold nanoparticles serve as catalyst for the SiNW growth. Then, a silane gas (SiH4) is injected at 40 sccm under a furnace pressure of 0.4 T during either 10 or 60 min. The SiH4 dissociates to give Si and H2 preferentially on the Au droplets, leading to the formation of AuSi liquid eutectic droplets. When, the AuSi droplets are oversaturated by Si, the silicon precipitates and forms a Si crystal at the solid–liquid interface.
2.5. Scanning electron microscopy SEM images were obtained using an electron microscope ULTRA 55 (Zeiss) equipped with a thermal field emission emitter, three different detectors (EsB detector with filter grid, high efficiency In-lens SE detector, Everhart-Thornley Secondary Electron Detector) and an energy dispersive X-ray analysis device (EDX analysis). 2.6. Wetting properties Surfaces have been characterized using a Drop Shape Analysis System (Krüss GmbH – DSA100). It comprises an automated tilting support integrating a light source, a CDD camera and an automatic dosing system. It is to be noted that the rotation axis of the tilting table and the optical axis (corresponding to CDD camera) are the same. Thus on the images and video taken while the surface stays horizontal, the droplet shape is deformed. A ± 1° error is assumed on contact angle computation and each result is averaged on 5 measurements. The wetting properties are characterized through the following steps: (i) A drop of 4 lL is deposited onto the surface, static apparent contact angle (CA) is measured. (ii) Then the tilting table rotates from 0° to 90° while drop deformation and contact angle variation are recorded. (iii) Advancing and receding angles are measured just before contact line depining during tilting. The difference between advancing and receding angles gives the contact angle hysteresis (Dh). 3. Results 3.1. Fabrication of different interfaces Here, we have investigated 6 different structured silicon surfaces presenting different morphologies. Fig. 2 displays the SEM image of silicon l-pillars (lP-Si) with 10 lm5 lm10 lm (diameter*pitch*height) prepared by microfabrication techniques i.e. optical lithography followed by DRIE step. Dimensions of the l-pillars were chosen from a previous study presenting a Us value (solid area fraction) of 0.54 [30]. It has to be noted that a wavy side wall structure is observed and due to the successive steps of passivation/etching occurring during the DRIE process. Then, these l-pillars were subjected to a metal-assisted electroless etching (AgNO3/NaBF4) to produce silicon interfaces with a double scaled micro- and nanostructuration (lP-NanoSi). Fig. 3 shows SEM images of lP-NanoSi. We can easily see that the lP
2.4. Surface functionalization All the interfaces were UV/O3 treated for 30 min at room temperature to remove any organic contaminant and to generate surface silanol groups. Then the activated surfaces were directly dipped into a freshly 1H, 1H, 2H, 2H-perfluorodecyltrichlorosilane (PFTS, ABCR, GmbH, Germany) solution (10 3 M) in hexane for 4 h at room temperature in a dry nitrogen purged glove box. The resulting surfaces were rinsed first in hexane and then twice in dichloromethane, twice in ethanol and then dried under a gentle nitrogen flow.
Fig. 2. SEM image of lP-Si surface evidencing the wavy side wall structure.
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Fig. 3. SEM images of lP-NanoSi interface tilted view (A), zooms at the bottom (B) and top (C) of the sample.
Fig. 4. SEM images of the SiNW interfaces synthesized either on Siflat or on lP-Si substrates.
are now totally covered by silicon nanostructures of 500 nm in height. The metal-assisted electroless etching occurred not only on top and side walls of the lP-Si, but also on silicon surface between the pillars (Fig. 3B). The other surfaces were synthesized by CVD growth. By changing the growth parameters, different surface morphologies can be obtained [8,15,16,30–33]. Here, four different substrates coated with SiNWs are synthesized either on flat silicon (Siflat) or on Si-lP surfaces (see Section 2.3).
SiNWs have been synthesized on flat silicon surface (Siflat) following the protocol described in ‘‘materials and methods’’ section. A furnace pressure of 0.4 T and reaction times of 10 or 60 min have been used for SiNW-A and SiNW-B surfaces, respectively. SiNW-A consists of a layer of 7 lm-long NWs with diameters ranging from 80 to 150 nm. Their average orientation is about 80° with respect to the horizontal plane (Fig. 4) [31]. SiNW-B is composed of nanowires with diameters ranging from 80 to 150 nm and 45 lm in height. This surface presents two layers: an upper layer of straight
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nanowires (20 lm) and lower layer of tangled ones (25 lm) (Fig. 4) [8,15,16,30]. lP-SiNW-A and lP-SiNW-B surfaces consist of lP-Si, with same dimensions as described above, covered by either SiNW-A or SiNW-B (Fig. 4) [30]. For lP-SiNW-A, SiNWs are mostly localized on top of lpillars and between pillars and only a few wires were observed on the side wall, whereas lP-SiNW-B surface presents identical surface morphology than SiNW-B. Indeed, since the SiNW-B double layers are thick (45 lm of total thickness), they have completely covered the lP-Si surface, leading to their disappearance under this thick layer of wires (Fig. 4). 3.2. Wetting properties 3.2.1. Wavy side wall structure surface of lP-Si compared to double scale (micro and nano) surface (lP-NanoSi) Static Young angles on PFTS-coated flat silicon surface along with apparent static contact angles obtained on lP-Si and lPNanoSi surfaces for 9 different liquids of different surface tensions (Table S1 in Supplementary information (SI)) are displayed in Fig. S1 in SI. On the flat surface, Young angles are found to be larger than 90° for liquids with surface tension down to 34 mN/m, and lower than 90° for the others (21.6 < c < 34 mN/m, diamonds), which represents a good oleophobic character (hY 120° for water) (Fig. S1). The lP-Si surface coated with PFTS exhibits a superomniphobic character: apparent contact angles around 140° for Young angles below 90° (Fig. S1). However, this property is not achieved for all tested liquids since it is only observed for surface tensions down to 27.5 mN/m (hexadecane) (Fig. S1, triangles). This phenomenon may not exist starting with a Young angle less than 90° if we assume a perfect and smooth pillar surface. Indeed, in that case, the structure should result in total wetting. However, a closer look at these structures points out a nonperfect edge, but rather undulations evocating a re-entrant structure (wavy side wall) (Fig. 2). Such structuration was explained by the successive steps of passivation/etching performed and occurring during the DRIE process as already shown by Zhao and co-workers [28,29]. Indeed, the
authors have shown that such silicon lpillars presented SCA values superior to 150° for both water and hexadecane due to the presence of these wavy structuration. In fact, by removing this side wall structuration, the SCA for hexadecane decreased to 120°. When static contact angle values obtained for lP-Si were compared to those measured on lP-NanoSi surface, we observed changes in the wetting properties. Indeed, lP-NanoSi displayed contact angles of 135° even for a liquid with a surface tension as low as 21.6 mN/m (n-octane) (Fig. S1, crosses). The results suggest that the combination of two roughness scales: silicon micropillars covered by nanostructures (lP-NanoSi) gives (i) the same behavior as for lP-Si for cL down to 27.4 mN/m, and (ii) the threshold in cL corresponding to a sharp decrease in CA is estimated to be lower than 20 mN/m, whereas this threshold is equal to 27.4 mN/m for lP-Si. In that case, it is clear that a multi-scale-structure improved the wettability behavior by increasing the CA whatever the liquid tested, preventing the static CA decrease for low cL liquids. Corresponding contact angle hysteresis for the different surfaces and liquids is reported in Fig. S2. While the planar surface exhibits values between 20° and 50°, large CAH values are measured on lP-Si surface, which can be explained by contact line pining and deformation at the microscale (Fig. S2). These effects should be inhibited by coupling these different structuration levels [42]. It has to be mentioned that for decane and octane with surface tensions of 23.8 and 21.6 mN/m, respectively, square shape droplets are observed, suggesting total impalement of the liquid inside the microstructuration with a huge CAH. The lP-NanoSi surface displays a quasi-null hysteresis for water and glycerol (Fig. S2, crosses), which is consistent with expected values. Indeed, energy barrier during displacement of the contact line on pillars is lowered by the nanostructuration. However, for liquids of lower surface tension, the CAH is comprised between 40° and 70°. In that case, the high interface area provided by this surface cannot counter-balance the force developed by liquids with surface tension lower than 60 mN/m. Figs. S3 and 5 display SEM images of polydimethylsiloxane (PDMS) droplet deposited on PFTS-modified lP-Si and lP-NanoSi
Fig. 5. SEM of a PDMS droplet deposited on PFTS-modified lP-NanoSi.
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interfaces. PDMS is a silicon-based organic polymer with a surface tension of 27 mN/m. After thermal curing (60 °C), the droplet became solid. There is no doubt about the Wenzel state of the PDMS droplet on lP-Si interface (Fig. S3). Indeed, we can easily see that the micropillars are totally surrounded by PDMS. In the contrary, a Cassie–Baxter state is observed on lP-NanoSi interface. The PDMS did not fill the micropillars inter-spacing but rested on top of the nanostructured silicon lP (Fig. 5). The PDMS droplet is in contact only with the top of the posts of the lP-NanoSi surface, and forms microcapillary bridges between the main liquid drop and the top of the posts (Fig. 5A and B). This liquid bridge along the contact line has already been observed for ionic liquids on staggered rhombus (Si/SiOx) and for water/ethanol mixtures on PDMS–Si3N4 microstructures [43–45]. This suggests that superomniphobic interfaces can display a Cassie–Baxter state even with a high hysteresis for low surface tension liquids. SEM images in Fig. 5C and D display a close-view of the interaction between PDMS and nanostructures located on top of the micropillars. We can see that the PDMS is not impaled inside the silicon nanostructures. These SEM images show clearly the role of a double micro/nanostructured surface compared to a simple microstructuration. 3.2.2. Wetting properties of surfaces made by CVD grown silicon nanowires (SiNW-A, SiNW-B, lP-SiNW-A and lP-SiNW-B) We investigated 4 other surfaces consisting of SiNWs, synthesized by CVD process, either on flat silicon (Siflat) or on silicon micropillars (lP-Si) substrates (Fig. 4). Their wetting properties have been assessed in terms of static contact angles and contact angle hysteresis. Figs. 6 and S4 display the static CA values of SiNW-A, lP-SiNWA, SiNW-B, lP-SiNW-B and lP-NanoSi (for comparison) surfaces. For all these surfaces, the CA decreased from 160° (for water) to 140° for hexadecane (27.4 mN/m). It has to be noted that lPSiNW-A showed slightly higher CA values than SiNW-A. The CA decreased down to 98°, 110°, 125°, 126° and 135° for SiNW-A, lPSiNW-A, SiNW-B, lP-SiNW-B and lP-NanoSi, respectively for octane (c = 21.63 mN/m). Surprisingly, the best results, in terms of static contact angles, were obtained for the lP-NanoSi interface followed by those corresponding to SiNW-B and lP-SiNW-B due to their identical surface morphologies as already mentioned above. Concerning the SiNW-A and lP-SiNW-A, we can notice that the presence of micropillars improved only slightly the static contact angles especially for hexane and octane, the two liquids with the lowest surface tensions. Interestingly, in Fig. S5, all interfaces exhibited lower CAH than lP-NanoSi, one which has the highest CAH (higher than 50°) for most liquids except for water and glycerol (CAH 0°). The
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lP-NanoSi’ CAH increased brutally at a surface tension of 50 mN/ m to a value of 49° and then, slowly increased until a CAH value >60° (23.8 mN/m). For the SiNW-based interfaces, very low CAH values were measured for liquids of surface tension down to 34 mN/m. However, for liquids of surface tension lower than 34 mN/m, some differences, in terms of CAH values, appeared related to the surface morphologies. Indeed, the CAH increased brutally to 38 and 29° for SiNW-A and lP-SiNW-A, respectively for c = 32 mN/m whereas the CAH remained quite reasonable: 18° and 13° for SiNW-B and lP-SiNW-B, respectively. Then, the CAH for SiNW-A increased to a value >60° at 27.4 mN/m whereas the lP-SiNW-A displayed a CAH value >60° only at 23.8 mN/m with a CAH of 49° at 27.4 mN/m. Here, the micropillars seemed to slightly improve the CAH for liquids of c < 34 mN/m. Except for SiNW-A, all CAH presented values >60° at c = 23.8 mN/m. However, the SiNW-B and lP-SiNW-B CAH were 15° and 17°, respectively for c = 27.4 mN/m and increased for decane and octane to values >60°. In Fig. 7 are represented the advancing and receding contact angles measured by tilting experiments for SiNW-A, lP-SiNW-A and SiNW-B for liquids with c P 21.6 mN/m. We can see that for the advancing CA, all surfaces displayed comparable values 155° for c ranging from 72.2 to 27 mN/m. For liquids of c < 27 mN/m, lPNanoSi still presented a CA around 150° even for n-octane. The advancing CA of lP-SiNW-A and SiNW-B surfaces started to decrease at 23.8 mN/m and 21.6 mN/m, respectively. However, from a close look at the receding CA, we can deduce that lP-NanoSi gave the worst results as soon as c reached 50 mN/m (left part of the dashed line in Fig. 7), leading to higher CAH values. Concerning the lP-SiNW-A surface, the receding CA followed those corresponding to the advancing CA. However, for liquids with c = 32 mN/m, receding CA started to decrease to reach a value of 72° for n-octane with a CAH >20°. Finally, SiNW-B surface showed the same receding CA profile than lP-SiNW-A surface until c = 32 mN/m. However, below 32 mN/m, the receding CA did not drop sharply as for lP-SiNW-A surface, and exhibited a CAH 20°. Then for c < 27.4 mN/m, receding CA decreased to 80° and 70° for n-decane and n-octane, respectively, giving CAH values >20°. From these results, we can conclude that the wetting behavior is dominated by the surface morphology. The surfaces composed of SiNW and especially SiNW-B surface exhibited the best wetting properties i.e. high static CA and lower CAH. Interestingly and although CAH value of 50° was observed for lP-SiNW-A surface for hexadecane (27 mN/m), SEM images showed that a droplet of PDMS (27 mN/m) is still not impaled inside the structure (Fig. 8). Here, we can also observe the formation of microbridges between each double scale feature. In Fig. 8C, we can see a closer view of interaction between the wires and the droplet. Thanks to their ductility, SiNW can resist the pressure applied by the liquid droplet without breaking. It has been shown that SiNW dimensions such as length, diameter, but also the presence of defects and the growth orientation have a strong influence on the ductile to brittle transition i.e. bending without breaking property [46]. However, that has been demonstrated for only one wire. Here, we have a cooperative effect due to the presence of numerous wires that have been recruited to repel the liquid droplet. Wires can then interact with the droplet either by the upper part of the wire or their side walls as shown in Figs. 8C and S6. 4. Discussion
Fig. 6. Static contact angles for lP-NanoSi, lP-SiNW-A and SiNW-B surfaces as a function of surface tension (c).
A comparison between the different surfaces suggests some clues toward the optimal strategy to reach both high contact angle and low hysteresis for most liquids, including those with low surface tension. Fig. 7 shows the advancing and receding contact angles of various liquids of different surface tensions, from which
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Fig. 7. Advancing and receding contact angles for liquids of various surface tensions on the different double scale structured interfaces: lP-NanoSi, lP-SiNW-A and SiNW-B. Left part from the dashed black line represents CAH values >20°.
some trends can be drawn. The threshold in cL below which the CAH sharply increases, i.e. CAH P20°, is a relevant indication of the quality of the superomniphobicity of a surface: it has to be as small as possible for a better omniphobic character. In particular, a comparison between typical double structured surfaces (lPNanoSi, lP-SiNW-A) suggests that the height of the small scale elements crucially influences the threshold in cL. Indeed, lP-NanoSi surface presents non-flexible 1 lm high Si nanostructures, seems to have the poorest features for omniphobicity with a threshold value of 50 mN/m.
However, as seen in Fig. 5, even when high CAH is observed for liquids with low surface tensions, the droplet is not impaled inside the micro- and nanostructuration. That proves that Cassie–Baxter state can still exist even at high CA hysteresis, resulting from the depinning mechanism when the surface is tilted. The lP-SiNW-A surface consisting of micropillars covered by flexible 6 lm high silicon nanowires exhibits much better results for low surface tension liquids, with a threshold at 32 mN/m. An explanation could be that lP-SiNW-A exposed globally re-entrant features composed by SiNWs covering the top of lP, as drawn in Fig. 9 (white dashed
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Fig. 8. SEM images of a PDMS droplet deposited on PFTS-modified lP-SiNW-A.
Fig. 9. Evidence of re-entrant geometry on lP-SiNW-A surface.
line). Moreover, the presence of SiNWs between the micropillars reinforces the omniphobic character of this interface (horizontal white dashed line). However, the best results are obtained for the SiNW-B surface, comprising a double layer of nanowires with a height of 20 lm for the upper layer: a threshold is found at 32 mN/m as well, except that the CAH does not increase very much and stays around 20° instead of increasing to a value of 50°. Therefore, instead of being completely stuck on a surface, a low surface tension droplet can still slide relatively easily on SiNW-B surfaces. This difference evidences that the best performance is obtained with a surface presenting the key design parameters with: (i) long nanowires structured as a double layer: a lower dense layer of entangled nanowires and an upper loose layer of straight nanowires, (ii) small pitch and (iii) structures with small diameter (
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Micro-and nanostructured silicon-based superomniphobic surfaces Thi Phuong Nhung Nguyen a,b,c, Rabah Boukherroub a, Vincent Thomy b,⇑, Yannick Coffinier a,⇑ a
Institut de Recherche Interdisciplinaire (IRI), USR CNRS 3078, Université Lille1, Parc de la Haute Borne, 50 Avenue de Halley, 59658 Villeneuve d’Ascq, France Institut d’Electronique, de Microélectronique et de Nanotechnologie (IEMN – UMR 8520), Cité Scientifique, Avenue Poincaré, BP 60069, 59652 Villeneuve d’Ascq, France c PetroVietnam University, 30/4 Street, Vung Tau City, Vietnam b
a r t i c l e
i n f o
Article history: Received 21 August 2013 Accepted 31 October 2013 Available online 16 November 2013 Keywords: Silicon nanostructures Multi-scale roughness Organic coating Superoleophobic Surperhydrophobic
a b s t r a c t We report on the fabrication of silicon nanostructured superhydrophobic and superoleophobic surfaces also called ‘‘superomniphobic’’ surfaces. For this purpose, silicon interfaces with different surface morphologies, single or double scale structuration, were investigated. These structured surfaces were chemically treated with perfluorodecyltrichlorosilane (PFTS), a low surface energy molecule. The morphology of the resulting surfaces was characterized using scanning electron microscopy (SEM). Their wetting properties: static contact angle (CA) and contact angle hysteresis (CAH) were investigated using liquids of various surface tensions. Despite that we found that all the different morphologies display a superhydrophobic character (CA > 150° for water) and superoleophobic behavior (CA 140° for hexadecane), values of hysteresis are strongly dependent on the liquid surface tension and surface morphology. The best surface described in this study was composed of a dual scale texturation i.e. silicon micropillars covered by silicon nanowires. Indeed, this surface displayed high static contact angles and low hysteresis for all tested liquids. Ó 2013 Elsevier Inc. All rights reserved.
1. Introduction Nature offers a large variety of examples of surfaces with amazing wettability properties, like the extreme water repellency. From plant leaves to animals, this property is ascribed to the micro- and nanotexturation of the lotus leaves, the duck feather or the wings of butterflies [1]. When a rain drop is in contact, it rolls off the surface and is unable to wet it, collecting at the same time the dust particles; this behavior is called self-cleaning effect. These socalled superhydrophobic surfaces have been deeply studied and are now well known [2,3]. In order to mimic natural surfaces, roughness can be designed in a controlled fashion to significantly modify the wetting behavior. There exists a host of examples of surfaces for which a micron or sub-micron scale texturation leads to high liquid repellency [4,5]. This property can rely on the following: – The achievement of high effective contact area between the liquid and the solid, leading to a high surface energy cost if the solid is treated with a low surface energy coating (Fig. 1A). This corresponds to a situation where the liquid is impaled by the texture. This approach has historically been proposed by Wenzel and it is denoted as Wenzel (W) state thereafter [6]. ⇑ Corresponding authors. Fax: +33 3 20 19 78 84. E-mail addresses: [email protected] (V. Thomy), coffi[email protected] (Y. Coffinier). 0021-9797/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2013.10.065
Yannick.
– The entrapment of air pockets between the liquid and the surface, ensuring that the liquid below the drop is mostly in contact with air (Fig. 1B). This is denoted as the Cassie–Baxter (CB) state thereafter [7]. This situation offers a very low liquid–solid friction, and this feature is particularly interesting for versatile droplet motion [8–11]. Thus to distinguish between Wenzel and Cassie–Baxter states, it is necessary to characterize the degree of impalement of the droplet sitting on or impaled inside the structuration. That is possible by measuring the difference between the advancing and receding contact angles, corresponding to the contact angle hysteresis (Dh), which is related to the retention force of the drop on the substrate [12]. Even though the Wenzel and Cassie–Baxter states exhibit high apparent contact angles (up to 160°), they present a totally different behavior. On most of nanotextured surfaces, in the Wenzel state, Dh, measured by surface tilting, is larger than 30° and the droplet remains stuck on the surface, whereas in the Cassie–Baxter state, the droplet Dh can be lower than 5°. In the latter case, the liquid droplet rolls off instead of sliding once the surface is tilted by a few degrees [13]: it is called the ‘‘rolling-ball’’ effect [14–16]. Another point that needs to be detailed is the apparition of metastable states where liquid droplet can transit from Cassie– Baxter to Wenzel state, the more energetically stable state. While, in practice, Cassie–Baxter state is most of the time expected, one of the most widespread techniques to insure this state is to develop multi-scale structures, combining micro- and nano-metric
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Fig. 1. A schematic illustration of the formation of composite interfaces with a droplet in the Wenzel (A) and Cassie–Baxter (B) state.
structuration. This point has been largely described and validated using different technological processes [17,18]. To extend the ‘‘rolling-ball effect’’ to low surface tension liquids, a structure with a reentrant angle is required as shown by pioneering work of Tuteja et al. [19]. Theoretically, under these conditions, it is possible to obtain high contact angle even with hydrophilic surfaces. But, from an experimental view point, it is preferable to enhance the robustness of the surface (i.e., preventing any transition from Cassie to Wenzel state) and then to coat the re-entrant structure with an omniphobic layer with a surface energy as low as possible. Thus, even if hydrocarbon molecules present surface energies of 35–20 mN/m, it is preferable to use fluorinated molecules that can achieve low surface energy (28–6 mN/m) [20,21]. Furthermore, Tuteja et al. recently evidenced that superoleophobicity can be achieved through an interplay with both surface geometry and chemistry. Until now, such superoleophobic surfaces were prepared by creating either a ‘‘re-entrant’’ curvature structure resembling micro-hoodoos, by designing a number of different nano-fiber surfaces [22,23], by an alumina nanowire forests [24] or by ZnO nanostructures [25]. Very recently, Tuteja’s group has developed a stainless steel mesh uniformly coated with electrospun microbeads [26]. This surface presents a clear hierarchical structure with re-entrant features on both scales: fibers and microbeads. This surface displays amazing wetting properties: with a heptane droplet (surface tension of 20 mN/m), they measured a contact angle of 150° and a quasi-null contact angle hysteresis. In that case, the double structuration is not a key feature because similar properties were obtained using only microbeads. Another example of hierarchical omniphobic surfaces was recently reported by Ellinas et al. [27] Such interfaces have been fabricated, via nanosphere lithography (polystyrene beads) on poly(methyl methacrylate) (PMMA) substrate and bearing ordered microcolumns surmounted by nanostructured polystyrene particles and coated by C4F8 [27]. However, these interfaces presented static CA values of 101° and 41° for hexadecane and decane, respectively and no hysteresis measurements were performed. Zhao et al. [28] have recently described superomniphobic surfaces based on silicon micropillars. They have studied the influence of surface texturation by varying the solid area fraction (i.e., by changing pillar diameter, pitch and height) and the effects of the overhang thickness and coatings on the wetting properties for water and hexadecane. Their surfaces displayed high static contact angle (SCA) and contact angle hysteresis (CAH) values of 150° and 20°, respectively, for hexadecane and SCA and CAH values of 150° and 10°, respectively for water [28,29]. All these studies evidenced that multi-scale topography is beneficial for the development of omniphobic surfaces. The objective of this study is to develop silicon-based omniphobic surfaces and to study the role of the surface morphology on the repellency of liquids of low surface tension. For that, we report here the fabrication of omniphobic surfaces realized by different techniques achieved on silicon surfaces, leading to single or dual scale surface morphologies. We have designed six interfaces and investigated their wetting properties. Their morphologies are described below:
- Silicon micropillars (lP-Si) obtained by standard optical lithography and dry reactive ion etching (RIE) [30]. - Silicon micropillars (lP-Si) subjected to metal-assisted electroless etching, presenting a double scale micro- and nanostructuration (lP-NanoSi). - Silicon nanowires synthesized via the vapor–liquid–solid (VLS) growth mechanism [8,15,16,30,31–33]. By varying the furnace pressure and reaction time two different surface morphologies with either one or two silicon nanowires layers, SiNW-A or SiNW-B are prepared. - These two different silicon nanowire morphologies were also grown on silicon micropillars (lP-SiNW-A and lP-SiNW-B). The resulting surfaces exhibit a double micro-nanostructuration as already described in a previous study [30]. All these interfaces were chemically modified with perfluorodecyltrichlorosilane (PFTS), a low surface energy molecule. The surface morphology was characterized using scanning electron microscopy (SEM), and the wetting properties were investigated using various molecules owing to their different surface tensions (from 72 to 21 mN/m). 2. Materials and methods All cleaning and etching reagents were of VLSI grade. Sulfuric acid, 96% (H2SO4), hydrogen peroxide 30% (H2O2), nitric acid 69% (HNO3), and hydrofluoric acid 48% (HF) were supplied by Amplex. All chemicals were of reagent grade or higher and were used as received unless otherwise specified. Acetone, isopropanol, silver nitrate (AgNO3) and sodium tetrafluoroborate (NaBF4) were obtained from Aldrich. 2.1. Fabrication of silicon micropillars (lP-Si) Single side polished silicon (1 0 0) oriented n-type wafers (Siltronix) (phosphorus-doped, 0.009–0.01 Ohm-cm resistivity) were used as substrates. The surface was first degreased in acetone and isopropanol, rinsed with Milli-Q water and then cleaned in a piranha solution (3:1 concentrated H2SO4/30% H2O2) for 15 min at 80 °C followed by copious rinsing with Milli-Q water. A 3.5 lm thick negative resist AZnLOF 2035 (Clariant, France) is spin-coated at 3000 rpm. A soft bake for 1 min on a hot plate at 90 °C is necessary to reduce the solvent content and to prevent resist adhesion to the exposure mask. Then, the exposure, post-exposure bake and development, are conducted for transferring the mask patterns to the resist (latent image). Then, the silicon wafer was etched using Deep Reactive Ion Etching (DRIE, Silicon Technology System) to produce high aspect ratio micropillars. 2.2. Fabrication of double scale micro- and nanosilicon interface (lPNanoSi) To obtain these surfaces, l-pillars, fabricated as described above, were subjected to a metal-assisted electroless etching. Silicon nanostructures are easily prepared, in a reproducible manner,
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by electroless etching of crystalline silicon substrate in aqueous solution of AgNO3 and SiOx etching reagent [34–41]. The metalassisted chemical etching displays several advantages, as it takes place at relatively low temperature and does not require any specific equipment. By varying the etching time, temperature, etching reagent types (HF, NH4F or NaBF4), concentrations of reagents and the type of silicon substrate (doping, crystalline orientation. . .), different silicon nanostructures have been produced presenting different shapes, lengths, diameters and porosities [37,41]. In this study, the lP interface was placed directly in NaBF4 (4 M)/AgNO3 (0.04 M) aqueous solution for 2 h at 80 °C. Ag+ ions capture electrons from the valence band of silicon, reducing Ag+ ion into Ag0, leading to the precipitation of silver nanoparticles onto the Si surface followed by the formation of SiO2 layer below of the Ag particles. Then, SiO2 layer was etched away by NaBF4 (F ) to yield the silicon nanostructures (NanoSi). Finally, the Ag particles are removed by dipping in HCl-HNO3-H20 solution overnight at RT. Safety considerations The mixture H2SO4/H2O2 (piranha) solution is a strong oxidant. It reacts violently with organic materials. It can cause severe skin burns. It must be handled with extreme care in a well-ventilated fume hood while wearing appropriate chemical safety protection. HF is a hazardous acid which can result in serious tissue damage if burns were not appropriately treated. Etching of silicon should be performed in a well-ventilated fume hood with appropriate safety considerations: face shield and double layered nitrile loves. 2.3. Silicon nanowires synthesis via the vapor–liquid–solid (VLS) mechanism (SiNW-A, SiNW-B, lP-SiNW-A and lP-SiNW-B) The silicon nanowires are synthesized using the vapor–liquid– solid (VLS) growth mechanism as previously described [8,15,16,30–33]. The SiNW synthesis was achieved on two different surfaces: – On flat silicon (Siflat), leading to surfaces SiNW-A and SiNW-B. – On silicon micropillars, leading to surfaces lP-SiNW-A and lPSiNW-B. Briefly, a 40 Å Au layer was thermally evaporated on Si/SiO2 (300 nm) and then placed in a chemical vapor deposition (CVD) furnace. The temperature is raised up to 500 °C. At this temperature, the Au layer is dewetted forming gold nanoparticles. These gold nanoparticles serve as catalyst for the SiNW growth. Then, a silane gas (SiH4) is injected at 40 sccm under a furnace pressure of 0.4 T during either 10 or 60 min. The SiH4 dissociates to give Si and H2 preferentially on the Au droplets, leading to the formation of AuSi liquid eutectic droplets. When, the AuSi droplets are oversaturated by Si, the silicon precipitates and forms a Si crystal at the solid–liquid interface.
2.5. Scanning electron microscopy SEM images were obtained using an electron microscope ULTRA 55 (Zeiss) equipped with a thermal field emission emitter, three different detectors (EsB detector with filter grid, high efficiency In-lens SE detector, Everhart-Thornley Secondary Electron Detector) and an energy dispersive X-ray analysis device (EDX analysis). 2.6. Wetting properties Surfaces have been characterized using a Drop Shape Analysis System (Krüss GmbH – DSA100). It comprises an automated tilting support integrating a light source, a CDD camera and an automatic dosing system. It is to be noted that the rotation axis of the tilting table and the optical axis (corresponding to CDD camera) are the same. Thus on the images and video taken while the surface stays horizontal, the droplet shape is deformed. A ± 1° error is assumed on contact angle computation and each result is averaged on 5 measurements. The wetting properties are characterized through the following steps: (i) A drop of 4 lL is deposited onto the surface, static apparent contact angle (CA) is measured. (ii) Then the tilting table rotates from 0° to 90° while drop deformation and contact angle variation are recorded. (iii) Advancing and receding angles are measured just before contact line depining during tilting. The difference between advancing and receding angles gives the contact angle hysteresis (Dh). 3. Results 3.1. Fabrication of different interfaces Here, we have investigated 6 different structured silicon surfaces presenting different morphologies. Fig. 2 displays the SEM image of silicon l-pillars (lP-Si) with 10 lm5 lm10 lm (diameter*pitch*height) prepared by microfabrication techniques i.e. optical lithography followed by DRIE step. Dimensions of the l-pillars were chosen from a previous study presenting a Us value (solid area fraction) of 0.54 [30]. It has to be noted that a wavy side wall structure is observed and due to the successive steps of passivation/etching occurring during the DRIE process. Then, these l-pillars were subjected to a metal-assisted electroless etching (AgNO3/NaBF4) to produce silicon interfaces with a double scaled micro- and nanostructuration (lP-NanoSi). Fig. 3 shows SEM images of lP-NanoSi. We can easily see that the lP
2.4. Surface functionalization All the interfaces were UV/O3 treated for 30 min at room temperature to remove any organic contaminant and to generate surface silanol groups. Then the activated surfaces were directly dipped into a freshly 1H, 1H, 2H, 2H-perfluorodecyltrichlorosilane (PFTS, ABCR, GmbH, Germany) solution (10 3 M) in hexane for 4 h at room temperature in a dry nitrogen purged glove box. The resulting surfaces were rinsed first in hexane and then twice in dichloromethane, twice in ethanol and then dried under a gentle nitrogen flow.
Fig. 2. SEM image of lP-Si surface evidencing the wavy side wall structure.
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Fig. 3. SEM images of lP-NanoSi interface tilted view (A), zooms at the bottom (B) and top (C) of the sample.
Fig. 4. SEM images of the SiNW interfaces synthesized either on Siflat or on lP-Si substrates.
are now totally covered by silicon nanostructures of 500 nm in height. The metal-assisted electroless etching occurred not only on top and side walls of the lP-Si, but also on silicon surface between the pillars (Fig. 3B). The other surfaces were synthesized by CVD growth. By changing the growth parameters, different surface morphologies can be obtained [8,15,16,30–33]. Here, four different substrates coated with SiNWs are synthesized either on flat silicon (Siflat) or on Si-lP surfaces (see Section 2.3).
SiNWs have been synthesized on flat silicon surface (Siflat) following the protocol described in ‘‘materials and methods’’ section. A furnace pressure of 0.4 T and reaction times of 10 or 60 min have been used for SiNW-A and SiNW-B surfaces, respectively. SiNW-A consists of a layer of 7 lm-long NWs with diameters ranging from 80 to 150 nm. Their average orientation is about 80° with respect to the horizontal plane (Fig. 4) [31]. SiNW-B is composed of nanowires with diameters ranging from 80 to 150 nm and 45 lm in height. This surface presents two layers: an upper layer of straight
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nanowires (20 lm) and lower layer of tangled ones (25 lm) (Fig. 4) [8,15,16,30]. lP-SiNW-A and lP-SiNW-B surfaces consist of lP-Si, with same dimensions as described above, covered by either SiNW-A or SiNW-B (Fig. 4) [30]. For lP-SiNW-A, SiNWs are mostly localized on top of lpillars and between pillars and only a few wires were observed on the side wall, whereas lP-SiNW-B surface presents identical surface morphology than SiNW-B. Indeed, since the SiNW-B double layers are thick (45 lm of total thickness), they have completely covered the lP-Si surface, leading to their disappearance under this thick layer of wires (Fig. 4). 3.2. Wetting properties 3.2.1. Wavy side wall structure surface of lP-Si compared to double scale (micro and nano) surface (lP-NanoSi) Static Young angles on PFTS-coated flat silicon surface along with apparent static contact angles obtained on lP-Si and lPNanoSi surfaces for 9 different liquids of different surface tensions (Table S1 in Supplementary information (SI)) are displayed in Fig. S1 in SI. On the flat surface, Young angles are found to be larger than 90° for liquids with surface tension down to 34 mN/m, and lower than 90° for the others (21.6 < c < 34 mN/m, diamonds), which represents a good oleophobic character (hY 120° for water) (Fig. S1). The lP-Si surface coated with PFTS exhibits a superomniphobic character: apparent contact angles around 140° for Young angles below 90° (Fig. S1). However, this property is not achieved for all tested liquids since it is only observed for surface tensions down to 27.5 mN/m (hexadecane) (Fig. S1, triangles). This phenomenon may not exist starting with a Young angle less than 90° if we assume a perfect and smooth pillar surface. Indeed, in that case, the structure should result in total wetting. However, a closer look at these structures points out a nonperfect edge, but rather undulations evocating a re-entrant structure (wavy side wall) (Fig. 2). Such structuration was explained by the successive steps of passivation/etching performed and occurring during the DRIE process as already shown by Zhao and co-workers [28,29]. Indeed, the
authors have shown that such silicon lpillars presented SCA values superior to 150° for both water and hexadecane due to the presence of these wavy structuration. In fact, by removing this side wall structuration, the SCA for hexadecane decreased to 120°. When static contact angle values obtained for lP-Si were compared to those measured on lP-NanoSi surface, we observed changes in the wetting properties. Indeed, lP-NanoSi displayed contact angles of 135° even for a liquid with a surface tension as low as 21.6 mN/m (n-octane) (Fig. S1, crosses). The results suggest that the combination of two roughness scales: silicon micropillars covered by nanostructures (lP-NanoSi) gives (i) the same behavior as for lP-Si for cL down to 27.4 mN/m, and (ii) the threshold in cL corresponding to a sharp decrease in CA is estimated to be lower than 20 mN/m, whereas this threshold is equal to 27.4 mN/m for lP-Si. In that case, it is clear that a multi-scale-structure improved the wettability behavior by increasing the CA whatever the liquid tested, preventing the static CA decrease for low cL liquids. Corresponding contact angle hysteresis for the different surfaces and liquids is reported in Fig. S2. While the planar surface exhibits values between 20° and 50°, large CAH values are measured on lP-Si surface, which can be explained by contact line pining and deformation at the microscale (Fig. S2). These effects should be inhibited by coupling these different structuration levels [42]. It has to be mentioned that for decane and octane with surface tensions of 23.8 and 21.6 mN/m, respectively, square shape droplets are observed, suggesting total impalement of the liquid inside the microstructuration with a huge CAH. The lP-NanoSi surface displays a quasi-null hysteresis for water and glycerol (Fig. S2, crosses), which is consistent with expected values. Indeed, energy barrier during displacement of the contact line on pillars is lowered by the nanostructuration. However, for liquids of lower surface tension, the CAH is comprised between 40° and 70°. In that case, the high interface area provided by this surface cannot counter-balance the force developed by liquids with surface tension lower than 60 mN/m. Figs. S3 and 5 display SEM images of polydimethylsiloxane (PDMS) droplet deposited on PFTS-modified lP-Si and lP-NanoSi
Fig. 5. SEM of a PDMS droplet deposited on PFTS-modified lP-NanoSi.
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interfaces. PDMS is a silicon-based organic polymer with a surface tension of 27 mN/m. After thermal curing (60 °C), the droplet became solid. There is no doubt about the Wenzel state of the PDMS droplet on lP-Si interface (Fig. S3). Indeed, we can easily see that the micropillars are totally surrounded by PDMS. In the contrary, a Cassie–Baxter state is observed on lP-NanoSi interface. The PDMS did not fill the micropillars inter-spacing but rested on top of the nanostructured silicon lP (Fig. 5). The PDMS droplet is in contact only with the top of the posts of the lP-NanoSi surface, and forms microcapillary bridges between the main liquid drop and the top of the posts (Fig. 5A and B). This liquid bridge along the contact line has already been observed for ionic liquids on staggered rhombus (Si/SiOx) and for water/ethanol mixtures on PDMS–Si3N4 microstructures [43–45]. This suggests that superomniphobic interfaces can display a Cassie–Baxter state even with a high hysteresis for low surface tension liquids. SEM images in Fig. 5C and D display a close-view of the interaction between PDMS and nanostructures located on top of the micropillars. We can see that the PDMS is not impaled inside the silicon nanostructures. These SEM images show clearly the role of a double micro/nanostructured surface compared to a simple microstructuration. 3.2.2. Wetting properties of surfaces made by CVD grown silicon nanowires (SiNW-A, SiNW-B, lP-SiNW-A and lP-SiNW-B) We investigated 4 other surfaces consisting of SiNWs, synthesized by CVD process, either on flat silicon (Siflat) or on silicon micropillars (lP-Si) substrates (Fig. 4). Their wetting properties have been assessed in terms of static contact angles and contact angle hysteresis. Figs. 6 and S4 display the static CA values of SiNW-A, lP-SiNWA, SiNW-B, lP-SiNW-B and lP-NanoSi (for comparison) surfaces. For all these surfaces, the CA decreased from 160° (for water) to 140° for hexadecane (27.4 mN/m). It has to be noted that lPSiNW-A showed slightly higher CA values than SiNW-A. The CA decreased down to 98°, 110°, 125°, 126° and 135° for SiNW-A, lPSiNW-A, SiNW-B, lP-SiNW-B and lP-NanoSi, respectively for octane (c = 21.63 mN/m). Surprisingly, the best results, in terms of static contact angles, were obtained for the lP-NanoSi interface followed by those corresponding to SiNW-B and lP-SiNW-B due to their identical surface morphologies as already mentioned above. Concerning the SiNW-A and lP-SiNW-A, we can notice that the presence of micropillars improved only slightly the static contact angles especially for hexane and octane, the two liquids with the lowest surface tensions. Interestingly, in Fig. S5, all interfaces exhibited lower CAH than lP-NanoSi, one which has the highest CAH (higher than 50°) for most liquids except for water and glycerol (CAH 0°). The
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lP-NanoSi’ CAH increased brutally at a surface tension of 50 mN/ m to a value of 49° and then, slowly increased until a CAH value >60° (23.8 mN/m). For the SiNW-based interfaces, very low CAH values were measured for liquids of surface tension down to 34 mN/m. However, for liquids of surface tension lower than 34 mN/m, some differences, in terms of CAH values, appeared related to the surface morphologies. Indeed, the CAH increased brutally to 38 and 29° for SiNW-A and lP-SiNW-A, respectively for c = 32 mN/m whereas the CAH remained quite reasonable: 18° and 13° for SiNW-B and lP-SiNW-B, respectively. Then, the CAH for SiNW-A increased to a value >60° at 27.4 mN/m whereas the lP-SiNW-A displayed a CAH value >60° only at 23.8 mN/m with a CAH of 49° at 27.4 mN/m. Here, the micropillars seemed to slightly improve the CAH for liquids of c < 34 mN/m. Except for SiNW-A, all CAH presented values >60° at c = 23.8 mN/m. However, the SiNW-B and lP-SiNW-B CAH were 15° and 17°, respectively for c = 27.4 mN/m and increased for decane and octane to values >60°. In Fig. 7 are represented the advancing and receding contact angles measured by tilting experiments for SiNW-A, lP-SiNW-A and SiNW-B for liquids with c P 21.6 mN/m. We can see that for the advancing CA, all surfaces displayed comparable values 155° for c ranging from 72.2 to 27 mN/m. For liquids of c < 27 mN/m, lPNanoSi still presented a CA around 150° even for n-octane. The advancing CA of lP-SiNW-A and SiNW-B surfaces started to decrease at 23.8 mN/m and 21.6 mN/m, respectively. However, from a close look at the receding CA, we can deduce that lP-NanoSi gave the worst results as soon as c reached 50 mN/m (left part of the dashed line in Fig. 7), leading to higher CAH values. Concerning the lP-SiNW-A surface, the receding CA followed those corresponding to the advancing CA. However, for liquids with c = 32 mN/m, receding CA started to decrease to reach a value of 72° for n-octane with a CAH >20°. Finally, SiNW-B surface showed the same receding CA profile than lP-SiNW-A surface until c = 32 mN/m. However, below 32 mN/m, the receding CA did not drop sharply as for lP-SiNW-A surface, and exhibited a CAH 20°. Then for c < 27.4 mN/m, receding CA decreased to 80° and 70° for n-decane and n-octane, respectively, giving CAH values >20°. From these results, we can conclude that the wetting behavior is dominated by the surface morphology. The surfaces composed of SiNW and especially SiNW-B surface exhibited the best wetting properties i.e. high static CA and lower CAH. Interestingly and although CAH value of 50° was observed for lP-SiNW-A surface for hexadecane (27 mN/m), SEM images showed that a droplet of PDMS (27 mN/m) is still not impaled inside the structure (Fig. 8). Here, we can also observe the formation of microbridges between each double scale feature. In Fig. 8C, we can see a closer view of interaction between the wires and the droplet. Thanks to their ductility, SiNW can resist the pressure applied by the liquid droplet without breaking. It has been shown that SiNW dimensions such as length, diameter, but also the presence of defects and the growth orientation have a strong influence on the ductile to brittle transition i.e. bending without breaking property [46]. However, that has been demonstrated for only one wire. Here, we have a cooperative effect due to the presence of numerous wires that have been recruited to repel the liquid droplet. Wires can then interact with the droplet either by the upper part of the wire or their side walls as shown in Figs. 8C and S6. 4. Discussion
Fig. 6. Static contact angles for lP-NanoSi, lP-SiNW-A and SiNW-B surfaces as a function of surface tension (c).
A comparison between the different surfaces suggests some clues toward the optimal strategy to reach both high contact angle and low hysteresis for most liquids, including those with low surface tension. Fig. 7 shows the advancing and receding contact angles of various liquids of different surface tensions, from which
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Fig. 7. Advancing and receding contact angles for liquids of various surface tensions on the different double scale structured interfaces: lP-NanoSi, lP-SiNW-A and SiNW-B. Left part from the dashed black line represents CAH values >20°.
some trends can be drawn. The threshold in cL below which the CAH sharply increases, i.e. CAH P20°, is a relevant indication of the quality of the superomniphobicity of a surface: it has to be as small as possible for a better omniphobic character. In particular, a comparison between typical double structured surfaces (lPNanoSi, lP-SiNW-A) suggests that the height of the small scale elements crucially influences the threshold in cL. Indeed, lP-NanoSi surface presents non-flexible 1 lm high Si nanostructures, seems to have the poorest features for omniphobicity with a threshold value of 50 mN/m.
However, as seen in Fig. 5, even when high CAH is observed for liquids with low surface tensions, the droplet is not impaled inside the micro- and nanostructuration. That proves that Cassie–Baxter state can still exist even at high CA hysteresis, resulting from the depinning mechanism when the surface is tilted. The lP-SiNW-A surface consisting of micropillars covered by flexible 6 lm high silicon nanowires exhibits much better results for low surface tension liquids, with a threshold at 32 mN/m. An explanation could be that lP-SiNW-A exposed globally re-entrant features composed by SiNWs covering the top of lP, as drawn in Fig. 9 (white dashed
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Fig. 8. SEM images of a PDMS droplet deposited on PFTS-modified lP-SiNW-A.
Fig. 9. Evidence of re-entrant geometry on lP-SiNW-A surface.
line). Moreover, the presence of SiNWs between the micropillars reinforces the omniphobic character of this interface (horizontal white dashed line). However, the best results are obtained for the SiNW-B surface, comprising a double layer of nanowires with a height of 20 lm for the upper layer: a threshold is found at 32 mN/m as well, except that the CAH does not increase very much and stays around 20° instead of increasing to a value of 50°. Therefore, instead of being completely stuck on a surface, a low surface tension droplet can still slide relatively easily on SiNW-B surfaces. This difference evidences that the best performance is obtained with a surface presenting the key design parameters with: (i) long nanowires structured as a double layer: a lower dense layer of entangled nanowires and an upper loose layer of straight nanowires, (ii) small pitch and (iii) structures with small diameter (
