ARTICLE pubs.acs.org/Langmuir
Anisotropic Wetting on Checkerboard-Patterned Surfaces Xueyun Zhang,† Yuan Cai,‡ and Yongli Mi*,† †
Department of Chemical and Biomolecular Engineering and ‡Materials Characterization and Preparations Facility, Hong Kong University of Science & Technology, Clear Water Bay, Hong Kong ABSTRACT: A series of surfaces with microscale checkerboard patterns consisting of continuous central lines and discontinuous lateral lines were fabricated. The surface wetting properties of these checkerboard patterns were found to be anisotropic. The central continuous lines were found to have a strong influence on the dynamic wetting properties and moving trajectories of the water droplets. The droplets move more easily in the direction parallel to the central continuous lines and less easily in the direction perpendicular to the central continuous lines. Meanwhile, the droplets’ moving path tends to incline toward the central continuous lines from a tilting direction. When the microsurface was modified with a layer of nanowire, the surface wettability was found to be isotropic and superhydrophobic.
’ INTRODUCTION The surface wetability is an important property that is controlled by the surface energy and geometrical structure.15 If the surface morphology is isotropic, then the droplet is almost spherical. If the surface morphology is anisotropic, then the apparent contact angle varies around the contact line. Anisotropic wetting is quite common for naturally existing surfaces and is also widely produced in artificial fabrications.611 Surfaces with controlled anisotropic wetting can direct liquid flowing in the desired direction.1214 It was reported that the drop moving path could be controlled by creating a line on the superhydrophobic surface;15 by adjusting the tilt angle of the pattern substrate and the size of the water droplet, the sliding of a ball-like water droplet will follow the designed path precisely. Rosario et al.16 coated the nanowire surfaces with a monolayer containing photochromic spiropyran molecules, which is hydrophobic under visible light irradiation and hydrophilic under ultraviolet light irradiation. As a result, the advancing angle illuminated by UV light is lower than the receding contact angle irradiated by visible light, enabling the water drop to be moved solely under the influence of UVvisible light. Chaudhury et al.17 created a surface with gradient hydrophobicity over a distance of 1 cm by exposing the silicon wafer to react with alkylchlorosilane vapor through a diffusion-controlled process. The water drop was able to move upward from the hydrophobic to the hydrophilic end of the tilted silicon wafer surface. The potential applications have directed much research effort toward anisotropic wetting induced by surface chemistry or the topology difference.1828 The wetting properties, including the static contact angle, dynamic contact angles, and water drop shapes along various directions on the anisotropic surfaces, were theoretically and experimentally investigated. Various anisotropic surface structures were studied. Most of the investigated surface structures are parallel strips with height or chemistry r 2011 American Chemical Society
differences, including a periodic grooved structure19,24,27,29 as well as alternating and parallel hydrophobic/hydrophilic strips.23,30 Some other structures are also investigated, including arrays of square pillars31 and two-level hierarchical structures with a submicrometer-scale grating on the grooves.3234 The continuity of the three-phase contact line, which is determined by the topology of the roughness of the solid, is a critical parameter in determining the hydrophobicity.35 The typical topology that poses both continuous and discontinuous structures is the groove structure. It would be interesting to investigate the wetting anisotropy of the surface with regularly ordered alternating horizontal and vertical groove units as shown in Figure 1. Furthermore, with such a surface, the water drop will pass through with both continuous and discontinuous structures ! ! in certain directions (such as AB and BA ). Thus, the competition between the effects of the continuous and discontinuous contact lines can be compared. In the present study, we fabricated a series of surfaces consisting of alternating horizontal and vertical grooves units as shown in Figure 1. The structure is called checkerboard patterned because the horizontal and vertical groove units are analogous to the white and black units of the checkerboard. To our knowledge, the surface wettability of the checkerboard pattern has never been studied. Thus, the static and dynamic contact angles of a water drop along various orientations of the checkerboard pattern were studied in this article. Another phenomenon rarely investigated before is explained in detail here: morphology effects on the drop motion trajectories. Finally, the hierarchical structures (by growing a layer of nanowire on top of a microcheckerboard pattern surface) are also studied. Received: January 26, 2011 Revised: May 13, 2011 Published: July 06, 2011 9630
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Figure 1. (a) Schematic checkerboard pattern. (b) Optical images of the investigated surface with a checkerboard pattern. Dashed lines are the investigated directions of wetting measurements. (c) Magnification of the investigated checkerboard surfaces. The light parts are protruding. The dark parts are indented. The continuous pink lines are noted as the central lines, and the lines beside the central lines are the lateral lines. The ends of the ! lateral lines (marked by orange circles) are noted as free ends. Side views of a sessile drop (20 μL) on the checkerboard surface from the (d) AB ! ! direction, (e) AC direction, and (f) BD direction for contact angle measurements.
Table 1. Physical Dimensions and Protrusion Area Fraction, f, of the Samples Used in This Study area
line
gap
feature
fraction, f
width, a (μm)
width, b (μm)
period, a + b (μm)
0.36
1.8
3.2
5
0.54
2.7
2.3
5
0.64 0.72
3.2 3.6
1.8 1.4
5 5
’ EXPERIMENTAL DETAILS Sample Preparation. Surface structures consisting of a checkerboard pattern with various line widths and spaces were fabricated on silicon wafer substrates. The basic unit of the checkerboard pattern consists of grooves with a fixed period of 5 μm and a length of 10 μm as schematically shown in Figure 1a. The ratio of the protruding area to the whole projected area is defined as the protrusion area fraction, f. The widths of the protruding lines, a, and the gap between protruding lines, b, were varied in order to obtain samples with various protrusion area fractions. The physical dimensions of the samples with various protrusion area fractions used in this study are shown in Table 1. The desired pattern was first formed on the photoresist layer deposited on a silicon wafer by photolithography. The pattern in the photoresist layer was then transferred to the underlying silicon by etching with an Auto Etch 490 etcher (Lam Research, Fremont, CA, U.S.A.) for different durations to achieve the different feature heights. Structures used in this study have a height of around 8 μm. Figure 1b,c shows a typical fabricated checkerboard pattern, which contains central continuous lines and lateral discontinuous lines. The central continuous lines of the checkerboard surfaces are marked with pink dashed lines in Figure 1c. The lateral discontinuous lines are the short lines beside the central lines. The ends of the lateral lines connecting the space are noted as free ends of the lateral lines that are labeled with orange circles in Figure 1c. Because the surface pattern is not isotropic, the wetting properties of the checkerboard-patterned surfaces were investigated in several directions as specified by the dashed lines in Figure 1b. The symbol i Bj is used to define the direction that s s f f points from i to j as shown in Figure 1b. The directions of DB and BD are aligned with respect to the central continuous lines of the checkers s f f and CA are perpendicular to the board pattern. The directions of AC
! ! central continuous lines. The directions of AB and BA are oriented 45° with respect to the central continuous lines. We also fabricated surfaces with hierarchical structure by growing a layer of nanowires on the top of a fabricated microscaled checkerboard surface by chemical vapor deposition.36 The nanowires cover the whole sample surface, including both the protruding lines and the indented grooves. Figure 2 shows a typical fabricated hierarchical pattern. Before studying the surface wettability, we first made the surfaces hydrophobic. The examined specimens were silanized with perfluorooctyltrichlorosilane (C8H4Cl3F13Si) in the vapor phase as described in our previous work.29,37 The silicon substrate was first put into a model PDC-32G plasma cleaner (Harrick Scientific, U.S.A.) for about 15 min to remove organic contaminants. Then, the substrate was put into a desiccator containing C8H4Cl3F13Si and was vacuum pumped to allow the trichlorosilane groups of the C8H4Cl3F13Si molecules to react with the silicon surfaces. Apparent Contact Angle Measurement. The apparent contact angles of 20 μL sessile water drops on the prepared surfaces under static conditions were measured under a laboratory atmosphere at room temperature using a contact angle meter (goniometer G10, Kr€uss GmbH, Hamburg, Germany). The mean value was calculated from at least 10 individual measurements. Contact angles measured from the side views of the water drop from the i to j direction were noted as θij, as illustrated in Figure 1. Drop Motion and Shape Investigation. An apparatus was designed and constructed to monitor the drop shape and motion. The setup is shown schematically in Figure 3. The water drop was put on the examined surface that was attached to a fixed plate. Camera I, a microscope, and a light source were placed perpendicularly to the drop sliding direction to record the side view of the drop. Camera II was positioned perpendicularly to the drop plane in order to record the top view of the drop motion. Camera I, a microscope, a light source, the fixed plate, and camera II were attached to a rotatable platform that can be tilted to different angles. After the water drop was put on the examined surface, the platform was slowly tilted from a horizontal level until the drop began to move down. Therefore, the top view and side view (perpendicular to the sliding direction) of the drop can be record simultaneously by two cameras, from the droplet beginning to be tilted until the droplet slides down the specimen surface. When the horizontal plane is tilted, the drop will be deformed by gravity (Figure 3b). At some critical tilted angle, the drop will spontaneously 9631
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Figure 2. SEM images of a fabricated hierarchical surface at different magnifications.
! Figure 4. Specimens on the tilted plane in tilting directions of (a) AB , ! ! ! ! ! (b) BA , (c) DB , (d) BD , (e) AC , and (f) CA . Dashed lines indicate the tilting directions. how much the drop moving path deviates from the tilting direction. Similar to the notion of the sliding angle, angle β was noted as βij when the specimen was in an i Bj tilting direction. Figure 3. (a) Schematic drawing of the setup for monitoring the drop motion. (b) Two-dimensional schematic drawing of a water drop on a tilted plane. (c) Photograph of the side view of a typical deformed drop captured by camera I. move as a whole (i.e., both the front and the rear contact lines of the drop move). At the moment of the drop moving as a whole, the front contact angle reaches the advancing angle, θA, and the rear contact angle reaches the receding contact angle, θR. The critical tilting angle is noted as sliding angle R. The front and the rear contact lines may not start moving simultaneously. In this study, we study the situation only after both the front and rear contact lines move. Figure 3c shows the side view of a typical deformed drop captured by camera I. In this study, the specimens with a checkerboard surface pattern were placed in various orientations when they were tilted, as shown in Figure 4af. When the specimen was placed next to a dashed-line arrow pointing from the i to j direction as shown in Figure 4af, the tilt direction is along the i Bj direction, the specimen is referred to as being in the i Bj tilting direction, and the sliding angle obtained is noted as Rij. The motions of the water drop on the checkerboard surfaces were monitored and recorded by cameral II (Figure 3) via the top view. The time sequence of the water drop on the surface can be illustrated by a series of images extracted from the recorded video with an interval of no longer than 1 s. The motion trajectory of the water drop can thus be obtained by identifying the center of the drop contour as the representative point of the water drop and expressing the location of the drops with pixel coordinates. For illustrative purposes, when the water drop ! moves on the checkerboard surface in the AB tilting direction as shown in Figure 4a, the typical images extracted from the video are shown in Figure 5a and the drop motion trajectory is shown in Figure 5b, where the tilting direction of the drop motion is set to be parallel to the y axis. The angle between the linearly fitted drop motion trajectory and the y axis is noted as the deviation angle, β, which is used to characterize
’ RESULTS AND DISCUSSION Checkerboard Surfaces. The wettabilities of a checkerboard surface with f = 0.54 were studied with the apparent contact angle data shown in Figure 6 and the dynamic contact angle data shown in Table 2. The surface wettability is found to be anisotropic. As shown s f in Figure 6, the contact angles of water droplets in the BD s f and DB (θBD and θDB) directions are higher than those in the other directions (θAC, θCA, θAD, and θDA). The dynamic contact angles in various tilting directions are also different. Table 2 lists the sliding angles, advancing angles, receding angles, and contact angle hysteresis in various tilting directions in ascending order of sliding angle (6 μL) of the water drops on the same checkerboard surface. The water droplets with two different volumes, 6 and 20 μL, show similar sliding behavior, which is expected because the droplet size of 6 μL is still much larger than the characteristic length of the pattern. The droplet size effect emerges only when the droplet scale is comparable to the pattern scale. Sliding s f angles are smallest in the DB tilting direction and largest in s f the AC tilting direction. This result is applied to all investigated checkerboard surfaces with an area fraction range from 0.36 to 0.73 as shown in Figure 7. The sliding angle is mainly determined by the contact angle hysteresis.3841 The smaller the contact angle hysteresis, the smaller the sliding angle. Our data shown in Table 2 is consistent with this trend. The contact angle hysteresis data follows the same ascending order of the sliding angle in various tilting directions. As shown in Table 2, when the specimen ! is in the DB tilting direction, the contact angle hysteresis and the ! sliding angle are the smallest. When the specimen is in the AC tilting direction, the contact angle hysteresis and the sliding angle are the largest. It is well known that the surface topological nature 9632
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Figure 5. (a) Sequential images of a moving water drop, captured by cameral II in Figure 3 (t0 < t1 < t2), on the examined checkerboard surface with a tilting direction parallel to the line and arrow. (b) Drop motion trajectory with the tilting direction parallel to the y axis.
Figure 6. Contact angles of water droplets on the checkerboard surfaces with f = 0.54 in various directions.
has a significant effect on the wetting properties and liquid movement.35,42 Upon observing the checkerboard surface pats s f f tern as shown in Figure 4, one will find that in the DB or BD direction the surface structure is parallel to the tilting direction of the water droplet with the least disruption along the drop moving s f s f path, whereas in the AC or CA direction the surface structure is perpendicular to the tilting direction with the most disruption s s f f along the drop moving path. The structures in the DB and AC directions of the checkerboard pattern are analogous to that of the groove surfaces in the parallel and perpendicular directions. The sliding angle result of the checkerboard surface is similar to that of the groove surface, which has a smaller sliding angle in the parallel direction and a larger sliding angle in the perpendicular direction.4345 It has been pointed out that the contact angle behavior (advancing, receding, and hysteresis) is determined by the interactions of the liquid and the solid at the three-phase contact line.46 The continuity of the contact line is important in determining the hydrophobicity.35 Comparing the surface with discontinuous morphology to the surface with continuous morphology, the latter is found to be with a smaller contact angle hysteresis.29,37 Thus, the continuity of the contact line contris f butes to the smaller contact angle hysteresis in the DB tilting s f direction as opposed to that in the AC tilting direction. The structures of the investigated checkerboard surfaces are more ! ! continuous in the DB and BD directions and more discontinuous in the other directions, which may be the reason for the ! ! smaller sliding angle in the tilting directions of DB and BD
because the energy barrier is smaller for the continuous structure than for the discontinuous structure.37 It is interesting that the ! sliding angles in the AB tilting direction are smaller than those in ! ! ! the BA tilting direction, such as for DB and BD tilting directions, shown in Figure 7. However, the sliding angles in ! ! the AC and CA tilting directions are nearly the same. The observed phenomena may be attributed to lateral short lines on the checkerboard patterns because the main structures, the ! ! central continuous lines, are the same for the AB ( DB ) and ! ! BA ( BD ) tilting directions. To move on the tilted checkerboard surface, the water drop has to pass through surface structures, including the protruding lines as well as the gaps between the protruding lines. The gaps are the more energy-resistant region ! ! compared to the protruding line region. In the AB ( DB ) tilting direction, the lateral lines help the water drop advance further toward the more energy resistant place (gap between lines) ! ! whereas in the BA ( BD ) tilting direction, the lateral lines help the water drop to reach the central continuous line (i.e., a less ! ! energy resistant place). Therefore, lateral lines in the AB ( DB ) tilting direction are more effective in helping the drop to move ! ! than those in the BA ( BD ) tilting direction, which in turn ! ! reasonably explains the smaller sliding angle in the AB ( DB ) ! ! tilting direction, in! comparison ! to that in the BA ( BD ) tilting direction. In the AC and CA tilting directions, the lateral line directions are not very different, which explains why the hydro! ! phobicities of the AC and CA tilting directions are almost the same. Unlike the sliding behavior, the apparent contact angles in the ! ! ij direction are nearly the same as those in the ji direction (as shown in Figure 6) for the checkerboard surface with f = 0.54 and in Figure 8 for all other investigated checkerboard surfaces with various surface fractions. The effects of the protrusion area fraction on the apparent contact angle are shown in Figure 8. The solid line in Figure 8 is the theoretical expectation by the CassieBaxter equation (cos θC = f cos θ0 (1 f))47 assuming θ0 = 107°, which is the apparent contact angle measured for the flat substrate. The apparent contact angles in ! ! the DB and BD directions show nearly no dependence of the protrusion area fraction within the investigated range, but they decrease with increasing protrusion area fraction in the other directions as shown in Figure 8. The almost nonexistent depen! dence of the contact angle on the area fraction in the DB and ! BD directions on the checkerboard surface was also found on the DSS surface,37 which is caused by pinning to metastable states because of larger contact angle hysteresis for the discontinuous contact line comparing to that for the continuous contact line.48 9633
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Table 2. Water Contact Angle Data for a Silane-Modified Checkerboard Surface (f = 0.54) on a Tilted Plane in Various Tilting Directions sliding angle R (deg) tilting direction
6 μL
20 μL
advancing angle θA (deg)
receding angle θR (deg)
contact angle hystersis θhys (deg)
s f
DB
27.5 ( 0.3
13.5 ( 0.6
124.5 ( 0.9
114.0 ( 1.9
10.4 ( 2.8
BD
36.4 ( 1.9
17.7 ( 0.8
133.2 ( 1.0
117.9 ( 0.7
15.3 ( 1.7
AB s f BA
37.7 ( 0.5 49.3 ( 0.3
18.8 ( 0.3 24.1 ( 0.6
134.1 ( 3.9 135.2 ( 0.7
117.9 ( 1.7 113.1 ( 1.4
16.2 ( 5.5 22.1 ( 2.1
CA
59.6 ( 0.3
27 ( 0.3
147 ( 1.2
114.3 ( 0.5
32.7 ( 1.7
AC
59.8 ( 0.3
26.5 ( 0.6
148.2 ( 1.1
112.5 ( 1.2
35.6 ( 2.4
s f s f
s f s f
Figure 7. Sliding angles of water droplets with 20 μL volume on the checkerboard surfaces with various area fractions in various tilting directions.
Figure 9. Representative motion trajectories of water droplets on the ! ! tilted checkerboard surfaces in tilting directions of (a) AB , (b) BA , ! ! ! ! (c) DB , (d) BD , (e) AC , and (f) CA . The directions shown here are the same as those shown in Figure 4.
Figure 8. Contact angles of water droplets on the checkerboard surfaces with various area fractions in various directions. The solid line is the theoretical predication by the CassieBaxter equation.
The linear dependence of the contact angle on the area fraction in ! ! ! ! the AC , CA , AB , and BA directions on the checkerboard surface is similar to that found on the continuous CSS surfaces.37 The CassieBaxter predications are in between the contact angles ! ! of DB ( BD ), and the other directions cannot fit any set of the
data in any direction. The sliding angles increase with increasing protrusion area fraction as shown in Figure 7, which is consistent with previous reports.37,49 The observed dependence between the sliding angle and the area fraction can be qualitatively explained by the contact angle result shown in Figure 8. The droplet retention force is estimated by Fr = mg sin R = kγR(cos θR cos θA),50,51 where m is the mass of the liquid drop, g is gravitational acceleration, k is a prefactor depending on the contact line shape, γ is the droplet surface tension, and 2R is the drop width. For a droplet with a certain volume, a decrease in the contact angle indicates an increase in the contact width of the water drop on the surface, which in turn causes an increase in the sliding angle according to the retention force equation above. Thus, the dependence between the sliding angle and the protrusion area fraction in Figure 7 can be explained by the contact angle result in Figure 8. 9634
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Figure 10. Deviation angles of water droplets on the checkerboard surfaces with f = 0.54 in various tilting directions.
Although the effect of the surface topology on the drop motion was studied,16,17,52 the influence on the drop motion trajectory by the surface topology is seldom addressed. In this study, the drop motion trajectories on the checkerboard surfaces along various orientations were monitored by camera II shown in Figure 3. The drop motion trajectories on the checkerboard surfaces represented by the red lines along various tilting direc! tions are shown in Figure 9. We first note that, in the DB and ! BD tilting directions, the drop moves nearly parallel to the tilting directions. The deviation angles between the tilting direction and the drop moving path are nearly zero as shown in Figure 10. ! ! Other than in the DB and BD directions, the paths of water droplets deviate from the tilting directions. The deviation is ! the largest when the drops move in the AB tilting direction (Figure 10). Correlating the drop moving trajectory with the surface topology, we find that central continuous lines (Figure 1c) on the surfaces tend to direct the drop movement toward a ! central continuous direction. For drop motions in the DB and ! BD tilting directions, the central continuous lines are almost parallel to the tilting direction, which results in nearly zero degree ! deviation angles (Figures 9 and 10). For drop motions in the AB ! and BA tilting directions, the central continuous lines are 45° with respect to the tilting direction and the drops exhibit around a 20 to 30° deviation from the tilting direction respect to the ! with ! central lines. For drop motions in the AC and CA directions, the central continuous lines are 90° with respect to the tilting direction and should not induce any deviation with respect to the drop moving path. However, in such cases, the lateral lines will be significant to the motion path of the droplets. As shown in Figure 9, ! there is around ! a 15° deviation for the moving path from the AC and CA tilting directions. The role of the lateral short lines is evident from the result, showing that the deviation ! ! angle in the AB direction is larger than that in the BA direction. ! In the AB direction, half of the free ends (Figure 1c) of the lateral short lines are pointing to the tilting direction, whereas i ! the BA direction, half of the free ends of the lateral short lines are pointing away from the tilting direction. The remaining half of ! ! the free ends in both the AB and BA directions are 90° with respect to the tilting direction, and their effects on the drop moving path are the same. The free ends of the lateral short lines pointing to the tilting direction lead to a larger deviation angle of the drop moving path than those pointing against the tilting
Figure 11. Sequential photographs recording the detaching behavior of water droplet on a fixed hierarchical surface shown in Figure 2. (a) A water drop suspended on a syringe tip. (b) The shape of the water drop when lifted from the sample surface. (c) The water drop keeps hanging on the tip after lifting. (d) The shape of the water drop just before detaching from the tip when the volume of the water droplet reaches a critical amount. The orange circle highlights the separated point when the water drop starts to detach from the syringe tip. (e) The water drop rolls on the surface when detaching from the tip. (f) The drop rolls out of observation range.
direction. This effect is similar to the result that the sliding angle ! ! in the AB tilting direction is smaller than that in the BA tilting direction. As can be seen, both the central continuous lines and the lateral short lines play important roles in dynamic surface wettability. The central continuous lines dominate the role when they are not vertical to the tilting direction. This finding would be valuable in guiding the fabrication of liquid motion control devices. Hierarchical Superhydrophobic Surfaces. We also fabricated surfaces with hierarchical structures by growing a layer of nanowire on top of the microcheckerboard patterns by chemical vapor deposition and then modified the surfaces with fluorosilane as shown in Figure 2. The hierarchical structure with microand nanostructures are found to be superhydrophobic, as expected from recent studies.53,54 However, the surface wetting properties are found to be isotropic rather than anisotropic, as is its topology. The contact angles of the hierarchical surfaces are so high that they cannot be accurately measured by the sessile drop method. Therefore, the water drop is suspended on the syringe tip and lowered to touch a fixed sample surface, and then the water drop is lifted from the surface, as demonstrated by the dynamic process in Figure 11ac. At the lifting points (Figure 11b), the droplet shows a spherical shape because of the small affinity of water for the surface. The water drop keeps 9635
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the wetting is anisotropic as a result of the anisotropic surface topologies. Both central continuous lines and lateral short lines of the checkerboard patterns influence the surface wetting properties. The drop is easiest and hardest to move in the directions parallel and perpendicular to the central lines, respectively. The water drop moving trajectories are found to deviate from the tilting direction. They are mainly inclined toward the direction of the central lines of the checkerboard pattern and are also influenced by the pointing directions of the free ends of the lateral short lines. The influence of the central lines on the drop moving path is stronger, and the influence of the lateral short lines is weaker. The hierarchical structures of micro- and nanoscales were also studied by modifying the microscaled checkerboard surfaces with a layer of nanowires. The wettability of hierarchical surfaces is found to be isotropic and superhydrophobic. The anisotropic topological nature of the hierarchical structure cannot be distinguished by water drops. The result can guide the design of liquid control devices. Figure 12. Shapes of the water droplet when (a, b) moved by the syringe tip on a microscale checkerboard surface with a contact angle of 123° and (c, d) moved by the syringe tip on the superhydrophobic hierarchical surface.
hanging on the syringe tip and cannot be dispensed on the sample until enough volume is provided (Figure 11d). The orange circle in Figure 11d highlights the separated point when the water drop starts to detach from the syringe tip. The droplet rolls off the horizontal surface spontaneously once detached from the tip as shown in Figure 11df. It is reported that the contact angle hysteresis is more important in characterizing the hydrophobicity than the maximum achievable contact angle. The contact angle hysteresis, which is the difference between advancing and receding contact angles, can be indirectly reflected by moving the droplet on the sample surface. Here, we use the syringe tip to drag the water drop along the sample surface and allow a visualization of adhesion between the droplet and sample surface. For comparison, we also use a sample with the microscaled checkerboard with a contact angle of 123°. For the microstructured sample (Figure 12a,b), the water droplet undergoes an obvious deformation under the drag of the syringe tip as shown in Figure 12b. This implies that the microstructure surface has a high adhesion to water and has a high contact angle hysteresis. However, for the sample with hierarchical structure, no distortion is observed in the shape of the water droplet during the dragging process ((Figure 12c,d). These observations, together with the spontaneous “rolling off” of the water droplet in the sessile drop method, indicate that the surface has both advancing and receding contact angles close to 180° in the Cassie state. The isotropic wetting properties in the hierarchical surfaces indicate that the surface anisotropic topology nature may not be distinguished or even vanish when the surface is modified to be superhydrophobic. This phenomenon has seldom before been observed in this field.
’ CONCLUSIONS We fabricated a series of microscaled checkerboard patterns and investigated their wetting properties. Static and dynamic contact angles and drop moving trajectories of the water drops on the checkerboard surfaces with various protrusion area fractions in various directions were investigated. It is found that
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT The financial support of the RGC (research grant RGC 617607) is greatly acknowledged. ’ REFERENCES (1) Herminghaus, S. Europhys. Lett. 2000, 52, 165. (2) Zhu, L.; Feng, Y. Y.; Ye, X. Y.; Zhou, Z. Y. Sens. Actuators, A 2006, 595. (3) Lee, H. J.; Michielsen, S. J. Polym. Sci., Part B: Polym. Phys. 2007, 45, 253. (4) Roach, P.; Shirtcliffe, N. J.; Newton, M. I. Soft Matter 2008, 4, 224. (5) Zhang, X.; Shi, F.; Niu, J.; Jiang, Y. G.; Wang, Z. Q. J. Mater. Chem. 2008, 18, 621. (6) Zheng, Y. M.; Bai, H.; Huang, Z. B.; Tian, X. L.; Nie, F. Q.; Zhao, Y.; Zhai, J.; Jiang, L. Nature 2010, 463, 640. (7) Kwak, M. K.; Jeong, H. E.; Kim, T. I.; Yoon, H.; Suh, K. Y. Soft Matter 2010, 6, 1849. (8) Feng, L.; Li, S. H.; Li, Y. S.; Li, H. J.; Zhang, L. J.; Zhai, J.; Song, Y. L.; Liu, B. Q.; Jiang, L.; Zhu, D. B. Adv. Mater. 2002, 14, 1857. (9) Zheng, Y. M.; Gao, X. F.; Jiang, L. Soft Matter 2007, 3, 178. (10) Whitney, H. M.; Kolle, M.; Andrew, P.; Chittka, L.; Steiner, U.; Glover, B. J. Science 2009, 323, 130. (11) Liu, Y. Y.; Chen, X. Q.; Xin, J. H. Bioinspiration Biomimetics 2008, 3, 046007. (12) Mumm, F.; van Helvoort, A. T. J.; Sikorski, P. ACS Nano 2009, 3, 2647. (13) Tsougeni, K.; Papageorgiou, D.; Tserepi, A.; Gogolides, E. Lab Chip 2010, 10, 462. (14) Zhao, B.; Moore, J. S.; Beebe, D. J. Science 2001, 291, 1023. (15) Xu, Q. F.; Wang, J. N.; Smith, I. H.; Sanderson, K. D. Appl. Phys. Lett. 2008, 93, 233112. (16) Rosario, R.; Gust, D.; Garcia, A. A.; Hayes, M.; Taraci, J. L.; Clement, T.; Dailey, J. W.; Picraux, S. T. J. Phys. Chem. B 2004, 108, 12640. (17) Chaudhury, M. K.; Whitesides, G. M. Science 1992, 256, 1539. (18) Naidich, Y. V.; Voitovich, R. P.; Zabuga, V. V. J. Colloid Interface Sci. 1995, 174, 104. (19) Zhao, Y.; Lu, Q. H.; Li, M.; Li, X. Langmuir 2007, 23, 6212. 9636
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dx.doi.org/10.1021/la200342w |Langmuir 2011, 27, 9630–9637
