Article pubs.acs.org/Langmuir
Surfaces with Combined Microscale and Nanoscale Structures: A Route to Mechanically Stable Superhydrophobic Surfaces? Jonas Groten and Jürgen Rühe* Department of Microsystems Engineering - IMTEK, Chemistry and Physics of Interfaces, University of Freiburg, 79110 Freiburg, Germany ABSTRACT: Materials with superhydrophobic properties are usually generated by covering the surfaces with hydrophobic nanoscale rough features. A major problem, however, for any practical application of such strongly water-repellent surfaces is the mechanical fragility of the nanostructures. Even moderate forces caused by touching or rubbing the surfaces are frequently strong enough to destroy the nanostructures and lead to the loss of the superhydrophobic properties. In this article, we study the mechanical stability of superhydrophobic surfaces with three different topographies: nano- and microscale features and surfaces carrying a combination of both. The surfaces are generated by silicon etching and subsequent coating with a monolayer of a fluoropolymer (PFA). We perform controlled wear tests on the different surfaces and discuss the impact of wear on the wetting properties of the different surfaces.
■
INTRODUCTION In the last few decades, there has been strong interest in the generation of superhydrophobic (SH) surfaces. It is well established that extremely water-repellent surfaces can be obtained through a combination of hydrophobic materials with an appropriate topography, especially roughness caused by nanoscale features. Many practical applications for SH coatings such as self-cleaning windows, “lotus-effect” paints,1 selfcleaning coatings for solar panels,2 waterproof clothes,3−5 and anti-icing surfaces6,7 seem to be right at the doorstep. However, so far widespread practical applications of superhydrophobic surfaces are prevented by one prominent problem waiting to be solvedthe mechanical stability when such surfaces are exposed to real environmental conditions. In particular, the destruction of surfaces features by mechanical forces and the contamination of surfaces by compounds present in the atmosphere can lead to a rather fast loss of superhydrophobic properties.7 Nanoscale structures with high aspect ratio are intrinsically extremely fragile with respect to mechanical forces and can be destroyed even by rather low mechanical stress.6,8 Compared to nanostructures, microscale patterns show a much higher wear resistance. However, the structure size influences not only the mechanical behavior but also the wetting properties. Surfaces covered with micropatterned surfaces typically exhibit a rather large contact angle hysteresis (CAH) (i.e., difference between advancing and receding CA) because of the contact line pinning at edges of the microstructures. This hysteresis leads to higher roll-off angles (the angle to which a surface has to be tilted so that drops roll or slide off). When the contact line recedes on such surfaces, it has to dewet several microstructures in a concerted action. Low receding contact angles and thus high sliding angles reduce the © 2013 American Chemical Society
mobility of the water drops and thus hamper the desired selfcleaning effect.9−13 In several publications, it was shown that the presence of a hierarchical structure can help to increase the stability of the Cassie wetting state (to keep the air trapped inside the structures even under increased pressure),11,14,15 and in the last few years, several hierarchical structures have been also tested with respect to their mechanical properties. Lee et al.16 presented a mechanically durable superhydrophobic structure on a large-area substrate created by combining PMMA microspheres and a silicon grease. The latter is turned into a nanostructured ceramic through electron irradiation, which leads to a microporous structure with ceramic nanobumps. The obtained materials showed good mechanical stability in a standardized tape (ASTM D 3359-02) and pencil tests (ASTM D 3359-02). Deng et al.2 recently presented a transparent surface with tetraethoxysilane-modified porous silica spheres, which was not destroyed by double-sided tape or sand falling from a height of 30 cm. Promising results were also presented in studies where fluoroalkyl silane-modified particles embedded in a silicon rubber (PDMS) were coated on fabrics and created a durable coating.17 In this study, very good abrasion durability was shown by the Martindale method. In these studies, hierarchical structures were used and the wear-resistance was tested with various methods, but no comparison between different structure types (hierarchical, microstructures, and nanostructues) is presented. Bhushan’s group18,19 presented a surface with micropillars of epoxy resin that were additionally Received: November 21, 2012 Revised: January 22, 2013 Published: January 30, 2013 3765
dx.doi.org/10.1021/la304641q | Langmuir 2013, 29, 3765−3772
Langmuir
Article
180 sccm C4F8 was applied for 8.5 s in the passivation cycle, and 130 sccm SF6 plus 13 sccm O2 was applied for 7 s in the etching step. After a total of 349 cycles, the obtained silicon spikes forming the nanograss were slender and sharp, with an opening angle of around 10°. The plasma was generated with an 800 W coil and 16 W platen power at 13.56 MHz at a pressure of 28 mTorr. Helium back side cooling at 8.0 Torr was applied. Surface Modification. To render the surfaces superhydrophobic, a benzophenone-based silane (4-(3-chlorodimethylsilyl)propyloxy benzophenone) was first synthesized and immobilized on the surface according to published procedures.25,26 The benzophenone silane was immobilized on the SiO2 surface of a silicon wafer at room temperature from a dilute (∼10−3 M) toluene solution using Et3N as a catalyst and acid scavenger. The solutions with the substrates were left to stand overnight, and afterwards the samples were cleaned by rinsing extensively with toluene and methanol. Poly(3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,10-heptadecafluorodecyl acrylate) (PFA) was prepared as described previously.25,27,28 A 50- to 100nm-thick film of the polymer was then deposited onto the surface by dip coating. The samples were vertically withdrawn from a 10 mg/mL solution of PFA in 1,1,2-trichlorotrifluoroethane (TCTFE; Fluka) at a speed of 60 mm/min. The samples were exposed to 254 nm UV light for 5 min (Stratalinker from Oxygene). During irradiation, the benzophenone groups on the substrate were activated and established covalent bonds to polymer molecules in the coating. Any noncovalently bound polymer was removed by rinsing the sample with approximately 50 mL of TCTFE. As reported, this procedure results in thin, covalently attached polymer films with thicknesses on the order of 10−15 nm.25,29 A schematic depiction of the process flow for the preparation of the different structures (micro, nano, and dual scale) can be found in Figure 1. Wear Testing. The wear tests were performed with an SRV tribometer (Optimol Instruments Prüftechnik GmbH). A steel ring (diameter 2 cm, thickness 1.5 mm) was pressed on a surface with a force perpendicular to the surface plane. To create shear stress, the ring was oscillated for 30 s on the surface at a frequency of 20 Hz and a maximal displacement of 4 mm. Parallel alignment of the ring to the substrate is guaranteed in the used setup because the ring is attached to the tribometer using a ball junction. Whenever force is applied, the ring is automatically (self-) aligned to the surface plane. SEM Images. SEM images were recorded using a DSM 962 from Zeiss at an acceleration voltage of 5 kV. Contact Angles. Contact angles (CAs) were measured using an OCA20 measurement system from Dataphysics GmbH, Germany. Static contact angles were determined using drops of deionized water having a volume of 9 μL. It was technically not possible to place smaller drops on the superhydrophobic surfaces because of the low interaction of water with the surface, which causes only very low adhesion. On the other side, if a small water drop was dropped from a small distance above, then immediate roll-off was observed. Advancing/receding CAs were recorded at a dosing/withdrawal speed of 0.1 μL/s. Roll-off angles were measured with an OCA20 contact angle system from Dataphysics equipped with a rotatable sample stage.
spray coated with hydrophobized 10 nm large SiO2 particles or with multiwalled carbon nanotubes. In both cases, they revealed the good durability of the SH properties in water jet experiments. The surfaces were placed in water jets with a pressure of 1 kPa, and the CA was measured after the treatment. To test the mechanical stability, Bushan et al. performed a friction test with an AFM tip (with a load of 100 nN up to 10 μN) and with a ball on a flat tribometer (with a load of 10 mN). They could show that the hierarchical rough surfaces have a lower friction coefficient and suffer less from wear compared to nanorough surfaces and microstructured surfaces having only one length scale. The wetting properties, however, could not be measured after the friction tests in these studies because the regions rubbed in the wear test were much too small for wetting measurements. Although the studies described above show promising results with respect to mechanical stability, the structures in the studies performed so far were always built from a combination of different materials. This way, it is not possible to distinguish whether the origin of the stability is a result of the hierarchical topography, an effect of the material properties, or a combination of both. Xiu et al.20 used microscale and nanoscale structures from a single material by combining pyramids from KOH-etched silicon with a superimposed nanostructure, but only very moderate shear stress was applied and the dependence of the microstructure sizes and densities was not investigated. In this study, we present polymer-modified silicon surfaces with precisely defined (1) microscale and (2) nanoscale structures and (3) surfaces carrying both types of structures. In the first case, standard photolithography is used to generate microposts of varying size and distance. In the second case, the surfaces are covered with silicon needles with a high aspect ratio also known as black silicon or silicon nanograss, which are created by a combination of passivation and anisotropic etching. In the third case, the nanograss structures are superimposed onto the microstructure to give a dual-scale roughness. All structures are coated with a fluoropolymer monolayer that results, especially in the case of structures with nanoscale roughness, in superhydrophobic surfaces with contact angles (CA) close to 180° and almost no contact angle hysteresis (CAH). The wetting properties of surfaces carrying dual-scale structures are compared to those of the microstructure and the pure nanostructure both before and after strong mechanical stress both under a simple load and strong shearing of the surfaces. Advantages and limitations for the concept of dual-scale roughness are then discussed with respect to the topography independently of material aspects.
■
■
EXPERIMENTAL PART
RESULTS Sample Preparation. Samples with three different topographies are prepared: (1) the pure nanograss (nanoscale structures, NS), (2) microposts (microscale structures MS), and (3) microposts with nanograss (dual-scale structures DS). In the latter case, two different situations were realized: (3a) one where the nanograss was created both on top and in between posts (i.e., on the floor of the substrate (DS-TF)) and (3b) microposts with nanograss only in between the posts (DSF). The surfaces that were covered with microposts were used only to allow a comparison to previously published work and will not discussed in much detail in the following text.
Silicon surfaces with a roughness in different length scales have been prepared by two different etching processes. The post patterns were transferred onto a photoresist (AZ1518) by standard lithography. Then a 1600 nm silicon oxide layer was grown on the wafer in the parts not covered by the photoresist, acting as an etch barrier for the subsequent etching process. The Bosch process,21 which consists of alternating cycles of passivation with C4F8 and reactive ion etching with SF6 and O2, leads to a post structure with nearly vertical sidewalls (ion-coupled plasma etcher (ICP) from ST Systems, U.K.): the height of the structures was 30 μm and can be controlled by the number of cycles. Nanoscale structures were also fabricated with reactive ion etching in the same machine, but the duration of the etching and passivation cycles was adapted to create an overpassivation according to the black silicon method as reported in the literature.22−24 Here, 3766
dx.doi.org/10.1021/la304641q | Langmuir 2013, 29, 3765−3772
Langmuir
Article
Table 1. List of the Dual-Scale Structures Studied and Characteristic Structure Sizesa structure name
post size s/μm
post distance d/μm
DS-F-9/9 DS-TF-9/9 DS-F-32/32 DS-TF-32/32 DS-F-32/8 DS-TF-32/8 DS-F-32/64 DS-TF-32/64
9
9
32
32
32
8
32
64
The post height was 30 μm in all cases. F denotes structures with nanograss only on the floor. TF denotes structures with nanograss both on top and on the floor of the pillars. a
properties of the micro- and nanostructured surfaces is elucidated, two different situations are interesting: one where a load is applied to the surface and one where the surfaces are sheared. Accordingly, the samples were tested with respect to normal and shear stress. In the normal stress test, a silicon wafer with an area of 1 cm2 was pressed on the surface with a defined applied load and released again. Shear stress was created by oscillating a steel ring with a diameter of 2 cm and a thickness of 1.5 mm in the surface plane while a certain load was applied (ring-on-flat tribometer). Obviously, the extent of wear will also depend on the duration of the application of the mechanical stress. However, because we are interested only in a comparison of the different surfaces and do not intend to study wear phenomena in silicon structures, the duration of the mechanical stress is kept constant throughout all experiments. Although the geometric area of the ring is well known, the real contact area is strongly reduced by the micro- and nanoscale patterns. Additionally, it might change during the experiment because of the rupture of surface structures, so the local pressure on the surface remains unknown Advancing and receding CAs were measured after both tests on the NS surface to monitor the influence of the mechanical stress on the wetting behavior. In Figure 2, the CAs of the tested surfaces are presented for the two test situations. In tests where only a normal force was applied, both the advancing and receding CAs stay above 170°, even after a load of up to 1000 N was applied. This result shows that forces
Figure 1. Process flow for the preparation of surfaces with roughnesses having different length scales. A nanograss surface having nanoscale roughness features (NS) (left) and microposts combined with nanograss (middle DS-F and right DS-TF) are produced. The growth of nanograss on top of the posts can be controlled by the presence of a SiO2 layer on top of the posts.
The structures were produced by the Bosch process employing alternating cycles of reactive ion etching and passivation, with the post geometries previously defined in a photolithographic step. The nanograss is produced by a maskfree etching process. However, to fabricate surfaces with nanograss in locally defined areas SiO2 can be used as an etch barrier. The remaining SiO2 layer on top of the microposts after microfabrication thus results in nanograss only in between the microstructures (DS-F). If an additional SiO2 stripping step is performed after the microfabrication, then the nanograss can be fabricated on top of the whole microstructure (DS-TF). A scheme of the process flow for the fabrication of the three different topographies is shown in Figure 1. After the etching process, the structures were hydrophobized by the photochemical attachment of a PFA monolayer. To this end, first a self-assembled monolayer (SAM) of a benzophenone silane was generated from a solution in toluene.26 Onto these monolayers rather thick layers (between 50 and 100 nm) of PFA are deposited by dip coating. Illumination with 254 nm UV light for 5 min leads to the covalent attachment of polymer molecules contacting the surface through the reaction of the benzophenone units in the SAM with a C−H bond in the backbone of the PFA. During the extraction of the samples in TCTFE, all polymer except the covalently attached monolayer can be removed. Surfaces with Nanoscale Roughness Only. When the influence of mechanical stress on the wear and the wetting
Figure 2. Contact angles of NS surfaces after the application of stress under static and shearing conditions. In static experiments, a 1 cm2 silicon wafer was pressed onto the NS surface (gray −▲−, advancing CA; gray −▼−, receding CA). In shear experiments, a ring with a surface area of 0.87 cm2 was oscillated on the surface (−▲−, advancing CA; −▼−, receding CA). 20 Hz, 30 s shear time. 3767
dx.doi.org/10.1021/la304641q | Langmuir 2013, 29, 3765−3772
Langmuir
Article
The NS surfaces without microposts before the wear test (Figure 4a1) and after a wear test with a load of 1 N (Figure 4a2) look almost identical. Accordingly, the CAs of these two surfaces are also almost identical (Figure 4e−h, black line). However, after a force of 20 N is applied, a strong change in the topography of the NS surface is clearly visible (Figure 4a3) and the nanoscale needle structure is almost completely lost. Figure 4b−d shows the wear behavior of the dual-scale structures (DS-TF) with nanograss on top of the posts. On such structures, the needles on top of the micrometer-sized posts are affected by already shearing with 1 N load. This can be well understood because the stress on needles on top of the microstructures is higher compared to that on the flat surfaces. In samples with needles on posts, the mechanical contact is possible only in areas where posts are located. Thus, the stress in structures DS-TF-9/9 and DS-TF-32/32 is higher by a factor of 4 and that in structure DS-TF-32/64 is higher by a factor of 9 (Figure 4d). It is also clearly visible that the microscale posts withstand the shear stress from the wear test at these loads. However, when the loads become higher (e.g., with a load of 20 N), the 9-μm-wide (DS-TF-9/9) and the 32-μm-wide posts (DS-TF-32/64) cannot withstand the shear stress, the posts break, and spots originating from the rupture of the microposts are visible in between the nanograss carpet (Figure 4b3,d3). In contrast to this, the 32 μm posts of DS-TF-32/32, which have a shorter distance between posts and thus experience a lower shear stress at the same load, withstand the applied force. In this case, only the needles on top of the posts are affected (Figure 4c3). In addition to the nanograss in between the posts, additional material that originates from ruptured structures during the wear test is present on the surfaces. The formed debris partly (Figure 4b3) or sometimes fully (Figure 4d3) covers the spots created by the ruptured posts. Before the wear tests, all surfaces show perfect SH behavior with advancing and receding CAs close to 180° (Figure 4e−h, 0 N). This is not surprising because the NS surface is an SH surface without a measurable CAH24 and the addition of the microposts only further reduces the possible liquid−solid contact area. Pinning, which is generally present on microstructured surfaces, is not present because the water drops are in contact with the nanoscale needle tips only before the wear test. For all of the post-structured geometries, the advancing CA stays above 170° for all loads up to 20 N (Figure 4e) whereas the receding CA drops immediately to values of between 120 and 150° (Figure 4f) even for a low force of 1 N. Although the receding CA of DS-TF-32/64 does not further decrease and stays at around 140°, the receding CA of the structure DS-TF-9/9 decreases with increasing load down to 100°. In addition, in the latter case rather large variations of the CA in different locations are observed. It is interesting that most structures show a lower receding CA after a wear test with 1 N load compared to samples after exposure to higher forces of 2 or 4 N. The high advancing CA even after strong wear on the nanograss on top of the pillars is due to fact that drops have to span the gap to the next post row in order to advance.9,12 This behavior is also observed when the mechanical stress has led to breaking off of the posts from the surface as is the case for DSTF-32/64 and DS-TF-9/9 after shearing with 20 N. The nanograss in between the destroyed posts here acts in the same way as the air in between a post structure wetted in the Cassie state. The drop can advance only over this region when the advancing CA of the nanograss is met. The fact that the
parallel to the axis of the needles do not impair their mechanical stability and that silicon needles are very stable with respect to an exposure to normal forces. Thus, in the following text only experiments under shearing conditions will be discussed. The steel ring oscillating on the surface did not change the wetting behavior for small loads up to 8 N. When a load of 20 N is applied during shearing, the receding CA drops to around 20° and the advancing CA stays above 160° so that a large contact angle hysteresis is observed. However, after the surface with a load of 50 N is sheared, the advancing CA drops to 20° and the sample becomes quite strongly hydrophilic, demonstrating the low shear stability of the nanograss surfaces. Post Structures with Nanograss on Top. If post structures are coated with a hydrophobic material, then water drops usually stay in the Cassie state on top of the posts. In this wetting state, the advancing CA is strongly influenced by the post distance because a drop has to reach the subsequent row of posts in order to advance on the surface. This temporary pinning leads to very high advancing CAs.9,10,30 The receding motion of drops on post surfaces is influenced by the pinning of the contact line at the edges of a post row. In the receding motion, the drop usually dewets from multiple posts in a concerted event. This behavior leads to a small jump of the receding CA during depinning. In these jumps, it increases by 1 to 2°. Figure 3 shows the receding CA whereas the volume of a drop on structure DS-TF-32/64 is reduced at a speed of 0.1 μL/s.
Figure 3. Receding CA when water is withdrawn from a 9 μL drop on a post surface (DS-TF-32/64) at a speed of 0.1 μL/s. Events where the contact line jumps are marked by arrows.
First, the CA decreases without a movement of the contact line until the CA has reached a critical value. At this value, multiple posts are dewetted and the CA jumps back to a higher value (as indicated by arrows in Figure 3). The CAs directly before these dewetting events were taken as the true receding CA and an average of these events from different positions on the sample were taken as the receding CA of the surface. Fluctuations of the advancing CA are not observable during the addition of liquid because the wetting of new posts occurs one by one and the high CA necessary to reach a new post top can also be maintained after a new post is wetted. Figure 4a−d shows SEM images of the surfaces DS-TF-9/9, DS-TF-32/32, and DS-TF-32/64 as well as the NS surface before and after the wear tests. The corresponding advancing and receding CAs for these surfaces are shown in Figure 4e,f. The contact angle hysteresis (CAH) is depicted in Figure 4g, and the sliding angles are depicted in Figure 4h. 3768
dx.doi.org/10.1021/la304641q | Langmuir 2013, 29, 3765−3772
Langmuir
Article
Figure 4. SEM images of the nanoscale rough NS surface (a) and dual-scale rough structures DS-TF (b−d). Surfaces before the wear test (a1−d1), after the wear test with 1 N (a2−d2), and after a wear test with 20 N (a3−d3). The corresponding advancing (e) and receding CAs (f), the contact angle hysteresis (g), and the roll-off angles (h) are measured as a function of the load applied in the wear test.
receding CA shows lower values for a test force of 1 N than for the same experiment, where the surface is even more strongly sheared, can be explained by a detailed look at the SEM images in Figure 5. When a load of 1 N is applied in the wear tests, only the needle tips are affected by the mechanical stress (Figure 5a), whereas at higher loads complete needles break off (Figure 5b). In the first case, the breaking off of the tips leads to the generation of small hydrophilic spots that can pin the water contact line. In the latter case, however, when the complete needles break, a very rough carpet of disordered material is left behind. In this case, the needle sidewalls, which also carry a hydrophobic coating, are located at the surface so that the overall surface remains rough and hydrophobic and the
Figure 5. Detail of the silicon needles on the micropost tops of DSTF-9/9 after a wear test with loads of 1 (a) and 2 N (b).
3769
dx.doi.org/10.1021/la304641q | Langmuir 2013, 29, 3765−3772
Langmuir
Article
receding CA remains rather high (and even higher than after a wear test with 1 N in the case of DS-TF-32/64). The standard deviations of the measurements, especially for the receding CAs, increase strongly after wear test with higher loads. This originates from quite strong variations of the local chemistry and topography because the receding CA strongly depends on pinning effects at hydrophilic spots or post edges. For superhydrophobic surfaces, the roll-off angle is an important value. The roll-off angle depends on the contact angle hysteresis (and thus to a large extent on the receding contact angle in the studied structures) and on the drop volume. In Figure 4h, the roll-off tilt angles for 9 μL drops are shown. For the NS surface, no roll off is observed once the substrate has been sheared with 20 N, whereas for all DS-TF surfaces the drops can roll off in the whole range of loads studied. The roll-off angle on post geometries is influenced by the pinning at the post edges and thus might depend on the direction that the sample is tilted with respect to the post structure. The tilt angles measured here are thus always measured in the direction parallel to the post structure. Structure DS-TF-32/64 shows the smallest roll-off angle, which is a direct consequence of the low receding CA. Again, some surfaces show a higher roll-off angle after a wear test with 1 N than after a wear test with higher loads, which is a direct consequence of the minimum in the receding angle as discussed above. It is interesting that the roll-off angle for the DS-TF-32/ 64 structure is less than 20° even after a wear test with 20 N, which shows that such surfaces still show acceptable waterrepellent properties even after very harsh mechanical treatment. A comparison of samples DS-TF-9/9 and DS-TF-32/64 shows that the decisive factor for the wetting properties of such strongly worn surfaces is the number of hydrophilic spots generated by the fracture of the pillars. Thus, there will be a trade off. Higher densities of pillar structures will increase the mechanical stability. However, if the stress is so high that the pillars are broken, higher densities of pillars will cause more hydrophilic spots and thus a stronger pinning of water on the newly generated surfaces. Comparison of Different Structure Types. In Figure 6, the wetting properties of structures with microstructures having nanograss only between the pillars and those having nanograss on top and in between the pillars are shown and compared to nanograss-only coated surfaces. Additionally, an image of structures DS-F-32/32 and DS-F-32/64 after a wear test with 8 N is displayed in Figure 7. Although the NS and the structures with black-silicon-coated micropillares (DS-TF-32/32 and DS-TF-32/64) show no CAH without mechanical stress, the structures having nanograss only on the floor between the micropillars (DS-F-32/32 and DS-F32/64) already show considerable hysteresis before mechanical stress is applied to the surface. After shearing with rather low loads (i.e., as low as 1 N), the structures with needles on top of the posts also start to show a significant CAH. At higher loads, the samples with nanograss on the bottom only and with nanograss on the bottom and on the top do not differ much and show a very low CAH compared to that of the NS structure.
Figure 6. Comparison of advancing and receding CAs of NS structures with two different dual-scale structures DS-TF (green, red) and DS-F (violet). (a) DS-TF-32/32 and DS-F-32/32. (b) DS-TF-32/64 and DS-F-32/64. The black lines in a and b show the CAs of the NS surface for comparison.
Figure 7. SEM images of structures DS-F-32/32 and DS-F-32/64 after a wear test with a load of 8 N.
present on the surface, then even moderate shear forces lead to the breakage of the needles on the NS surface, which in turn leads to small hydrophilic spots. However, if the wear is not too strong, then the surface remains covered by an irregular carpet of broken silicon needles. Because the sidewalls of the needles are also covered with a fluorinated polymer, the surface covered with the carpet of broken needles shows almost perfect superhydrophobicity as well. However, with increasing wear (i.e., with increasing load in the friction experiment (at constant duration of the experiment) or with increasing time of exposure to the shear stress), the nanoscale needles are worn away, which leads to a dramatic drop in the receding CA (Figure 4f) and thus a completely sticky surface (Figure 4h). When structures that carry both micro- and nanoscale rough features are strongly mechanically challenged, even after shearing with very
■
DISCUSSION Surfaces decorated with perfluorinated nanoscale rough features on top show superhydrophobic properties with no measurable contact angle hysteresis, very high contact angles, and low rolloff angles (Figures 4h and 6a,b). If only nanostructures are 3770
dx.doi.org/10.1021/la304641q | Langmuir 2013, 29, 3765−3772
Langmuir
Article
introduced, the wear resistance improves. This, however, comes at the price of increased contact angle hysteresis so that there is a trade-off between the different requirements. It is obvious that post structures with aspect ratios larger than 1 are not ideal candidates for the generation of wear-resistant surfaces. Probably other microscale geometries such as pyramids20 or rounded shapes might further improve the wear resistance of the microstructures and allow the retention of the superhydrophobic properties under much stronger shearing conditions. Posts were chosen here only to facilitate comparison to experiments described in the literature because the details of contact line pinning depend strongly on the particularities of the geometry of the surface features. Even though the use of structures with two different length scales improves the properties significantly, it is obvious that a hydrophilic bulk material with a hydrophobic coating will always cause problems when some of the structures break because this will open hydrophilic spots, as also pointed out by Verho et al.7 Therefore, it might be advantageous to use an intrinsically hydrophobic bulk material instead. However, this is not trivial because hydrophobic materials usually have rather weak mechanical properties. The origin of this is that in hydrophobic materials the molecules are typically held together mostly by comparably weak forces. Here the challenge would be to find a hydrophobic material that has sufficient mechanical properties and a high wear resistance. Another parameter that needs to be addressed is the prevention of chemical degradation or contamination of the surfaces. One way to overcome these problems is to generate surfaces that can be renewed or can renew themselves after damage.17,31,32
high loads (20 N), SH properties are retained. All dual-scale surfaces show a high advancing CA and a receding CA above 90°, and the drops can roll off when the surfaces are tilted. The value for the receding CA and hence the details of the wetting behavior (i.e., the extent of the superhydrophobicity) depend on the size and the distance of the microstructure. On the one hand, the high value of the receding CA of structure DS-TF32/64 shows that it is advantageous to have a large distance between the microstructures because this lowers the pinning sites for the contact line. On the other hand, this also increases the mechanical stress on a single post and thus increases the probability that this structure is broken. Thus, the optimal density of the microstructures to be generated will depend strongly on the maximum stress to which the surface is to be exposed. It is interesting that surfaces with a fractured microstructure after the wear test can actually show a lower CAH than surfaces where the microstructure is still intact. The high standard deviation in the measurement of the receding CA, however, which was measured from 20 to 30 different receding events at different locations, shows that the receding CA depends on the details of the local surface topography and chemistry. The large variance of the measured values arises from local differences in the topography after destruction. Hydrophilic spots from broken microstructures might be completely covered with nanoscale material from abrasion as seen in Figure 4d3 whereas the hydrophilic spots remained open on structure DS-TF-9/9 in Figure 4b3. In summary, this means that for any practical application of superhydrophobic surfaces the details of how the mechanical stress impacts the micro- and nanostructures are crucial to the final wetting properties of the surfaces.
■
■
CONCLUSIONS In this study, nanoscale structures (“silicon nanograss”) were combined with microscale features to study the influence of the wear resistance of the surfaces on the wetting properties. If surfaces, which carry only nanoscale roughness features, were challenged with forces perpendicular to the surface (without shear) or were sheared with very small loads, then strongly superhydrophobic properties were retained and were more or less identical to the initial values. This behavior, however, changed drastically when slightly higher shear stress was applied. Even rather small shear forces led to the partial or complete destruction of the nanostructures and the complete loss of the hydrophobic properties. The wear can be so strong that even rather hydrophilic surfaces are obtained. To improve the rather weak mechanical stability, the nanostructures were combined with regular post structures on the micrometer scale. Such structures showed under no-load conditions the same SH wetting properties as the pure nanograss; therefore, compared to the pure post surfaces without any nanograss, significantly less pinning was observed. When such structures are exposed to shearing stress, the microstructures carry much of the load, and even upon quite strong mechanical stress, the extremely hydrophobic properties are conserved. The dual-scale rough structure can thus combine two desired properties−excellent superhydrophobicity in the beginning (without much pinning at the microscale posts) and a better mechanical wear resistance compared to that of surfaces with only nanoscale structures. How the optimal surface structure would look, however, depends on what the exact expectations are concerning contact angle hysteresis and wear resistance. As more and larger microstructures are
AUTHOR INFORMATION
Corresponding Author
*Tel: +49 761 203 7160. Fax: +49 761 203 7162. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS We thank Tobias Amann from Fraunhofer IWM for the possibility to perform the wear tests and Prof. Dr. Reinecke, IMTEK, for the access to the SEM.
■
ABBREVIATIONS SH superhydrophobic; CA contact angle; CAH contact angle hysteresis; NS nanoscale rough surfaces (silicon nanograss); PFA poly(3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,10-heptadecafluorodecylacrylate); TCTFE 1,1,2-trichloro trifluoroethane; SEM scanning electron microscope
■
REFERENCES
(1) Solga, A.; Cerman, Z.; Striffler, B. F.; Spaeth, M.; Barthlott, W. The dream of staying clean: lotus and biomimetic surfaces. Bioinspiration Biomimetics 2007, 2, S126. (2) Deng, X.; Mammen, L.; Zhao, Y.; Lellig, P.; Müllen, K.; Li, C.; Butt, H.-J.; Vollmer, D. Transparent, thermally stable and mechanically robust superhydrophobic surfaces made from porous silica capsules. Adv. Mater. 2011, 23, 2962−2965. (3) Zhu, X.; Zhang, Z.; Wang, K.; Yang, J.; Xu, X.; Men, X.; Zhou, X. A facile route to mechanically durable responsive surfaces with reversible wettability switching. New J. Chem. 2012, 36, 1280−1284. 3771
dx.doi.org/10.1021/la304641q | Langmuir 2013, 29, 3765−3772
Langmuir
Article
(4) Zhou, H.; Wang, H.; Niu, H.; Gestos, A.; Wang, X.; Lin, T. Fluoroalkyl silane modified silicone rubber/nanoparticle composite: a super durable, robust superhydrophobic fabric coating. Adv. Mater. 2012, 24, 2409−2412. (5) Zimmermann, J.; Reifler, F. A.; Fortunato, G.; Gerhardt, L.-C.; Seeger, S. A simple, one-step approach to durable and robust superhydrophobic textiles. Adv. Funct. Mater. 2008, 18, 3662−3669. (6) Cao, L.; Jones, A. K.; Sikka, V. K.; Wu, J.; Gao, D. Anti-icing superhydrophobic coatings. Langmuir 2009, 25, 12444−12448. (7) Verho, T.; Bower, C.; Andrew, P.; Franssila, S.; Ikkala, O.; Ras, R. H. A. Mechanically durable superhydrophobic surfaces. Adv. Mater. 2011, 23, 673−678. (8) Wu, Y.; Bekke, M.; Inoue, Y.; Sugimura, H.; Kitaguchi, H.; Liu, C.; Takai, O. Mechanical durability of ultra-water-repellent thin film by microwave plasma-enhanced CVD. Thin Solid Films 2004, 457, 122− 127. (9) Dorrer, C.; Rühe, J. Advancing and receding motion of droplets on ultrahydrophobic post surfaces. Langmuir 2006, 22, 7652−7657. (10) Gao, L.; McCarthy, T. J. Contact angle hysteresis explained. Langmuir 2006, 22, 6234−6237. (11) Gao, L.; McCarthy, T. J. The “lotus effect” explained: two reasons why two length scales of topography are important. Langmuir 2006, 22, 2966−2967. (12) Dorrer, C. Wetting of Tailor-Made, Micro and Nanostructuered Surfaces. Ph.D. Thesis, University of Freiburg, 2007. (13) Ö ner, D.; McCarthy, T. J. Ultrahydrophobic surfaces. Effects of topography length scales on wettability. Langmuir 2000, 16, 7777− 7782. (14) Zhao, H.; Park, K.-C.; Law, K.-Y. Effect of surface texturing on superoleophobicity, contact angle hysteresis, and “robustness. Langmuir 2012, 28, 14925−14934. (15) Verho, T.; Korhonen, J. T.; Sainiemi, L.; Jokinen, V.; Bower, C.; Franze, K.; Franssila, S.; Andrew, P.; Ikkala, O.; Ras, R. H. A. Reversible switching between superhydrophobic states on a hierarchically structured surface. Proc. Natl. Acad. Sci. U.S.A. 2012, 109, 10210− 10213. (16) Lee, E. J.; Kim, J. J.; Cho, S. O. Fabrication of porous hierarchical polymer/ceramic composites by electron irradiation of organic/inorganic polymers: route to a highly durable, large-area superhydrophobic coating. Langmuir 2010, 26, 3024−3030. (17) Wang, H.; Xue, Y.; Ding, J.; Feng, L.; Wang, X.; Lin, T. Durable, self-healing superhydrophobic and superoleophobic surfaces from fluorinated-decyl polyhedral oligomeric silsesquioxane and hydrolyzed fluorinated alkyl silane. Angew. Chem., Int. Ed. 2011, 50, 11433−11436. (18) Jung, Y. C.; Bhushan, B. Mechanically durable carbon nanotubecomposite hierarchical structures with superhydrophobicity, selfcleaning, and low-drag. ACS Nano 2009, 3, 4155−4163. (19) Ebert, D.; Bhushan, B. Durable lotus-effect surfaces with hierarchical structure using micro- and nanosized hydrophobic silica particles. J. Colloid Interface Sci. 2009, 368, 584−591. (20) Xiu, Y.; Liu, Y.; Hess, D. W.; Wong, C. P. Mechanically robust superhydrophobicity on hierarchically structured Si surfaces. Nanotechnology 2010, 21, 155705. (21) Wu, B.; Kumar, A.; Pamarthy, S. High aspect ratio silicon etch: a review. J. Appl. Phys. 2010, 108, 051101. (22) Groten, J.; Bunte, C.; Rühe, J. Light-induced switching of surfaces at wetting transitions through photoisomerization of polymer monolayers. Langmuir 2012, 28, 15038−15046. (23) Jansen, H.; Boer, D. M.; Legtenberg, R.; Elwenspoek, M. The black silicon method: a universal method for determining the parameter setting of a fluorine-based reactive ion etcher in deep silicon trench etching with profile control. J. Micromech. Microeng. 1995, 5, 115−120. (24) Dorrer, C.; Rühe, J. Wetting of silicon nanograss: from superhydrophilic to superhydrophobic surfaces. Adv. Mater. 2008, 20, 159−163. (25) Jeyaprakash, J. D.; Samuel, S.; Rühe, J. A facile photochemical surface modification technique for the generation of microstructured fluorinated surfaces. Langmuir 2004, 20, 10080−10085.
(26) Prucker, O.; Naumann, C. A.; Rühe, J.; Knoll, W.; Frank, C. W. Photochemical attachment of polymer films to solid surfaces via monolayers of benzophenone derivatives. J. Am. Chem. Soc. 1999, 121, 8766−8770. (27) Jung, D.-H.; Park, I. J.; Choi, Y. K.; Lee, S.-B.; Park, H. S.; Rühe, J. Perfluorinated polymer monolayers on porous silica for materials with super liquid repellent properties. Langmuir 2002, 18, 6133−6139. (28) Mock, U.; Michel, T.; Tropea, C.; Roisman, I.; Rühe, J. Drop impact on chemically structured arrays. J. Phys.: Condens. Matter 2005, 17, S595−S605. (29) Samuel, J.; Ruther, P.; Frerichs, H.; Lehmann, M.; Paul, O.; Ruhe, J. A simple route towards the reduction of surface conductivity in gas sensor devices. Sens. Actuators, B 2005, 110, 218−224. (30) Dorrer, C.; Ruhe, J. Some thoughts on superhydrophobic wetting. Soft Matter 2009, 5, 51−61. (31) Liu, Q.; Wang, X.; Yu, B.; Zhou, F.; Xue, Q. Self-healing surface hydrophobicity by consecutive release of hydrophobic molecules from mesoporous silica. Langmuir 2012, 28, 5845−5849. (32) Ionov, L.; Synytska, A. Self-healing superhydrophobic materials. Phys. Chem. Chem. Phys. 2012, 14, 10497−10502.
3772
dx.doi.org/10.1021/la304641q | Langmuir 2013, 29, 3765−3772
