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Langmuir 2006, 22, 4230-4236
Wetting of Pb on Oxidized Micropatterned Si Wafers Dominique Chatain,*,† Ce´cile Lesueur,† and Jean-Pierre Baland‡ Centre de Recherche en Matie` re Condense´ e et Nanosciences - CNRS, Laboratoire Propre du CNRS associe´ aux UniVersite´ s d’Aix-Marseille 2 et 3, Campus de Luminy, case 913, 13288 Marseille Cedex 9, France, and Centre de Recherche de Mode´ lisation Mole´ culaire, UniVersite´ de Mons-Hainaut, 20 place du parc, 7000 Mons, Belgium ReceiVed NoVember 10, 2005. In Final Form: February 21, 2006 The wetting of lead on silicon wafers with regularly patterned holes, and covered by native silica, has been investigated at 610 K under ultrahigh vacuum conditions. The advancing and receding macroscopic contact angles have been measured by slowly compressing and stretching a liquid lead bridge between two identically patterned substrates. These angles are shown to depend on the distribution of the holes in the wafers and the continuity of the triple line.
Introduction We describe an investigation of the wetting behavior of liquid lead on the surfaces of oxidized silicon wafers with regularly patterned holes. The experiments have been conducted under high vacuum (10-6 Pa) in order to prevent lead oxidization. Both the wetting hysteresis and the spreading of the liquid on the patterned substrates have been analyzed. This work has been conducted within the framework of studies of wetting on rough surfaces. Although most of the experiments performed in this field have dealt with wetting by water or organic liquids, the physical phenomena involved are identical. Wetting in the lead/silica system is characterized by a Young contact angle, θY, larger than π/2. When the substrate is sufficiently rough, the lead/silica interface becomes composite. So-called “composite wetting”1 occurs when capillary forces prevent the liquid from filling cracks and crevices in the surface. It should be stressed that the lack of penetration into surface depressions is not due to the pressure of the gas or vapor trapped in the holes because this is obviously very low under the vacuum conditions in which our experiments were performed. The first evidence of composite wetting was reported by Johnson and Dettre´2 in studies of the wetting by water of wax surfaces of increasing random roughness. They showed that the solid/liquid interface becomes composite above a roughness threshold corresponding to surface depressions with slopes exceeding π - θY (and with a width well below the capillary length of the liquid). This phenomenon was also observed in the wetting of rough fractal alumina surfaces by metals that exhibit a Young contact angle larger than π/2.3 The composite wetting phenomenon has recently been revisited4-11 in studies of regularly patterned surfaces described as super-repellant or ultrahydrophobic. This renewal of interest * Corresponding author. E-mail:
[email protected]. † Centre de Recherche en Matie ` re Condense´e et Nanosciences - CNRS. ‡ Universite ´ de Mons-Hainaut. (1) Johnson, R. E., Jr.; Dettre´, R. H. Surf. Colloid Sci. 1969, 2, 82. (2) Dettre´, R. H.; Johnson, R. E., Jr. AdV. Chem. Ser. 1964, 43, 136. (3) De Jonghe, V.; Chatain, D.; Rivollet, I.; Eustathopoulos, N. J. Chim. Phys. Fr. 1990, 87, 1623. (4) Onda, T.; Shibuishi, T. S.; Satoh, N.; Tsujii, N. K. Langmuir 1996, 12, 2125. (5) Nakae, H.; Inui, R.; Hirata, Y.; Saito, H. Acta Materialia 1998, 46, 2313. (6) Chen, W.; Fadeev, Y.; Hsieh, M. C: O ˆ ner, D.; Youngblood, J.; McCarthy, T. J. Langmuir 1999, 15, 3395. (7) Bico, J.; Marzolin, C.; Que´re´, D. Europhys. Lett. 1999, 47, 220. (8) O ¨ ner, D.; McCarthy, T. J. Langmuir 2000, 16, 7777. (9) Patankar, N. A. Langmuir 2003, 19, 1249. (10) Lafuma, A.; Quere´, D. Nat. Mater. 2003, 2, 457. (11) He, B.; Pantakar, N. A.; Lee, J. Langmuir 2003, 19, 4999.
has been boosted by the development of processes that allow the preparation of large area surfaces with the roughness of controlled shape and size on a scale well below the capillary length of the liquid. These surfaces can be prepared either by producing holes in silicon wafers using photolithographic techniques, by microcontact printing,12 or by auto-organizing monodisperse balls or wires.4,5 For studies of composite wetting, the surfaces are coated to obtain Young contact angles of the liquid that exceed π/2. The processing of silicon wafers generally produces holes with vertical walls. If the roughness features all have the same size, then all holes will be empty when the wetting is composite. In this case, the noncomposite-to-composite transition will be first order1,9,10 rather than continuous as in the case of the randomly rough surfaces examined previously.2,3 On a rough surface where the liquid/solid interface is continuous (or noncomposite), the macroscopic equilibrium contact angle, θEmacro, is given by the Wenzel equation13
cos θEmacro ) r cos θY
(1)
where r is the ratio of the actual to the geometric area of the surface and θY is the Young contact angle on a flat surface. The Young contact angle depends only on the energies of the three coexisting interfaces (liquid/vapor, solid/liquid, and solid/vapor). On heterogeneous substrates, the macroscopic equilibrium contact angle is given by the Cassie and Baxter equation14
cos θEmacro ) Σ(fi cos θYi)
(2)
where fi is the area fraction of the ith surface on which the Young contact angle is θYi. Although formulated for heterogeneous flat substrates, this equation can be used to estimate the equilibrium contact angle of a composite interface by setting the contact angle above the holes to π. Both equilibrium contact angles given by eqs 1 and 2 correspond to the minimum interfacial energy configuration of the liquid on the heterogeneous substrate. In addition to the change in the equilibrium contact angle, heterogeneities such as roughness and local changes in chemistry produce a scatter in the values of the measured macroscopic contact angles. If the size of surface defects is well below the capillary length of the liquid and the defects are randomly (12) Xia, Y.; Tien, J.; Qin, D.; Whitesides, G. M. Langmuir 1996, 12, 4033. (13) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988. (14) Cassie, A.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546.
10.1021/la053026i CCC: $33.50 © 2006 American Chemical Society Published on Web 03/29/2006
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Figure 1. Scanning electron microscope images of the substrates with circular features 30 µm in diameter: (left) 44% 3-µm-deep holes; (middle) 62% 30-µm-deep holes; (right) 53% 30-µm-deep posts. Table 1. Geometric Features of the Different Substrates
Figure 2. Actual distribution of the defects (holes or posts).
distributed on the surface, then the macroscopic contact angle will lie between a maximum advancing and a minimum receding contact angle, with the difference between these defining the wetting hysteresis. All other macroscopic contact angles measured within the hysteresis range are dependent on the location of the triple line on the different patches of surfaces i or at the edges between different patches. A simple picture of the different contact angles is given by Eick et al.15 for the case of a meniscus on a plate with saw teeth parallel to the triple line. In this article, we aim to compare the composite wetting behaviors of substrates where the same surface fraction of holes is distributed differently; the two typical substrates studied have circular holes (continuous solid surface) or circular posts (discontinuous solid surface). The topography of the first one is the negative of the second one. Experimental Section Substrates. The substrates were prepared at LAAS (Laboratoire d’Analyse et d’Architecture des Syste`mes, Toulouse, France) by reactive ion etching. Scanning electron microscope (SEM) images of the substrates are displayed in Figure 1. So-called defects, in the form of cylindrical posts or holes 30 ( 1.5 µm in diameter, were produced on the substrates by etching to depths of either 3 or 30 µm. The distribution of defects on the substrates was originally designed to be hexagonal so as to ensure a high symmetry of the liquid bridge triple lines. On inspection, the defects were found to be arranged on the vertexes of isosceles rather than equilateral triangles, as shown in Figure 2. Consequently, instead of three high-density rows of defects at 60°, the substrates display one high-density row and two symmetric rows at 63.5° with a linear density that is smaller by a factor of 0.87. This asymmetry has little impact on the shape of the triple lines, as will be shown later in the analysis of the experimental (15) Eick, J. D.; Good, R. J.; Neumann, A. W. J. Colloid Interface Sci. 1975, 53, 235.
substrates
fraction of solid surface (%)
fraction of holes/posts (%)
43 µm largest hole spacing 37 µm largest hole spacing 37 µm largest post spacing 43 µm largest post spacing
56 ( 2 38 ( 5 53 ( 4 38 ( 4
44 ( 2 62 ( 5 53 ( 4 38 ( 4
results. Table 1 gives some of the measured geometric characteristics of the roughness of the substrates. Apparatus. The wetting experiments were conducted in a previously described ultrahigh vacuum (UHV) chamber16 that was modified to perform liquid bridge experiments. A schematic of the apparatus is shown in Figure 3. The bold numbers in Figure 3 and those given in parentheses below refer to different parts of the device. The chamber is fitted with two windows (1) on opposite sides for imaging sessile drops or liquid bridges. Several “manipulators” allow the motion of the substrates and of the lead syringe (3). An additional horizontal window (2) located at the top of the chamber helps to adjust and verify the position of the various mobile components in the chamber. A syringe containing 99.9999% purity lead is used for the in-situ preparation of oxide-free drops. Liquid droplets, extracted from this syringe (3) as described in ref 16, are deposited on a horizontal lower substrate (4) located below the optical horizontal axis and into the vertical axis of the chamber (Figure 3a). After lead deposition, the lead syringe is withdrawn, as shown in Figure 3b, and a second horizontal upper substrate (5) is brought into the vertical optical axis and positioned above the horizontal axis (Figure 3c). The two substrates face each other. They are each connected by tantalum straps to heating stages that can be wired either in parallel or in series. The liquid bridge (Figure 3c) is formed from a sessile drop (Figure 3b) sitting on the lower substrate, which is brought into contact with the upper substrate by a vertical translation of the axis (6). The triple lines on the top and bottom substrates may be advanced onto fresh surfaces of the substrates or receded back to positions previously occupied by the liquid/solid interface; this is achieved by compressing or stretching the liquid bridge, as described in ref 17, by vertical translation of the axis (6) at speeds of either 130 or 65 µm/min. The temperature is measured using both a chromel-alumel thermocouple (7) in contact with the back of the bottom heating stage and a pyrometer (8) located above the horizontal window (2) and focused either on the bottom substrate (sessile drop configuration) or on the back of the top heating stage (liquid bridge configuration). The melting point of lead is used for temperature calibration. The thermocouple and the pyrometer indicate the same temperature difference from the lead melting point within an accuracy better than 1 K. Movies of the magnified sessile drop and liquid bridge are recorded during the formation, compression, and stretching of the liquid bridge by a video recorder connected to a CCD camera fitted with a zoom. The NIH Image software18 is used to grab a series of images from the tape and extract the coordinates of the liquid profile by edge detection. Dimensional calibration is performed by measuring a vertical displacement of the bottom substrate at a known translation speed. (16) Serre, C.; Wynblatt, P.; Chatain, D. Surf. Sci. 1998, 415, 336. (17) De Jonghe, V.; Chatain, D. Acta Metall. Mater. 1995, 43, 1505.
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Figure 3. Sketch of the UHV experimental device for wetting experiments in different configurations: (a) formation of the drop (1 and 2, windows; 3, syringe; 4, bottom substrate; 5, top substrate; 6, translating axis), (b) sessile drop (7, thermocouple; 8, pyrometer), and (c) liquid bridge.
Figure 4. (a) Macroscopic contact angle and (b) radius at the top right foot of a 44 mm3 liquid bridge on a substrate with 44% 30-µm-deep holes. Threshold images of the liquid bridge along compression (second column) and stretching (third column). The coordinates of the profile of the liquid in the sessile drop configuration are used to calculate its surface energy.16 When a liquid bridge is examined, the data extracted from the images are the locations of the feet of the two contact lines and the macroscopic contact angles at the four feet of the bridge as a function of its height. The macroscopic contact angles are determined from a fourth-order polynomial fit of the profile shapes. During an experiment, several volumes of liquid can be examined. The experiments are performed at 610 K (i.e., 10 K above the melting point of lead).
Results and Discussion Contact Angle of Pb on Native SiO2. The equilibrium contact angle of Pb on a smooth Si wafer covered with a native layer of SiO2 (about 3 nm thick) was first measured. To be as close as possible to the equilibrium conditions, the contact angle was measured after spontaneous spreading, either of sessile drops or of the liquid bridge on the top substrate. Three volumes of liquid
(10, 27, and 49 mm3) and two bridge cycles were investigated on the same substrates. A value of 110 ( 3° was found at 610 K. This angle can be compared with the value of 116.5° obtained on plain silica at 1198 K by Nikolopoulos and Ondracek.19 Native silica appears to be better wetted than plain silica. Liquid Bridge between Substrates Patterned with Holes. This section begins with a detailed description of a compressionstretching cycle of a 44 mm3 liquid bridge between two substrates containing 44% 30-µm-deep holes. The pictures displayed in Figure 4 are shadows corresponding to a vertical section of the bridge in an equatorial plane. For the four feet of the bridge, the macroscopic contact angles and the position of the triple lines (18) Rasban, W. NIH Image Program; National Institutes of Health: Bethesda, MD, 1992. Also available from the Internet by anomymous ftp from zippy.nimh.nih.gov. (19) Nikolopoulos, P.; Ondracek, G. Verbundwerkstoffe; Deutsche Gessellschaft fu¨r Metallkunde: Stuttgart, Germany, 1981; p 391.
Wetting of Pb on Oxidized Micropatterned Si Wafers
along the horizontal axis referring to a fixed point (chosen arbitrarily as the center of the original sessile drop diameter) are measured. Hysteresis Loop. The diagrams in Figure 4 display the contact angles and displacements of the triple line at the top right foot of the bridge as a function of its height along a compressionstretching cycle. Figure 4 also shows a sequence of images of the liquid bridge along the compression-stretching cycle. Once the liquid bridge has formed, the general trend during compression is as follows: first, the macroscopic contact angle, θM, increases, but the triple line remains at approximately the same location; when θM has reached a plateau value, any further compression mainly results in an advance of the contact line. When the liquid bridge is stretched, θM first rapidly decreases without movement of the contact line. Once θM has reached a plateau value, the contact line recedes until the bridge detaches from the upper plate. Spontaneous Spreading on the Top Substrate. As soon the top of the drop contacts the top substrate, the liquid spontaneously spreads on it and stops when the macroscopic angle has reached 134 ( 2°. (This is an average value of the right and left top angles taken from six experiments.) This value is close to the equilibrium angle of 130°, calculated from eq 2, considering that 44% of the surface has a local contact angle of π. It can be concluded that during the spontaneous spreading the macroscopic angle almost reaches the equilibrium value. Stick-Slip Motion on AdVancing and Receding. The advancing contact angle at the plateau value oscillates between 136 ( 2 and 148 ( 4° on the upper substrate (triangles with apex down in Figure 4a) and between 131 ( 3 and 138 ( 3° on the lower substrate. In parallel, a stick-slip motion of the triple lines takes place (triangles with apex down in Figure 4b): the triple line as seen in Figure 4b periodically stops and then advances. Figure 4a and b shows that the contact angle generally drops when the triple line jumps; however, this is not a strict rule. The reason is that the triple line motion is not centered on the symmetry axis of the bridge and it can move in any direction. This has been clearly shown by Cubaud and Fermigier20,21 in their study of the advance of a drop of a water-glycerol mixture on a chemically patterned silicon substrate. These advancing angles are larger than the equilibrium contact angle: the highest advancing angle is the upper bound of the hysteresis loop, and the lower ones approach the equilibrium value because they are formed when the triple line spontaneously jumps. The 10° difference observed between the contact angles on the upper and lower substrates could be due to gravity, which helps the lower triple line jump over the defects. The receding contact angle (triangles with apex up in Figures 4a) oscillates on both the upper and lower substrates in the same range between 110 ( 3 and 121 ( 2°. A stick-slip motion of the triple line also takes place during receding motion (triangles with apex up in Figure 4b). The lowest receding angles approach the Young contact angle of 110 ( 3° measured on the smooth substrate, but they are never lower than this value. This provides evidence that the liquid does not enter the holes of the substrate. Additional evidence is given in Figure 5, which is a SEM image of a part of the triple line of a solidified drop of lead after it has advanced and receded over the surface (during liquid bridge cycles). Some of the holes that were covered by the liquid are filled, but most of them are empty. Figure 5 shows that the triple line lies on the solid surface and forms kinks to avoid the holes. Because the minimum receding angle is very close to the Young (20) Cubaud, T.; Fermigier, M. Europhys. Lett. 2001, 55, 239. (21) Cubaud, T.; Fermigier, M. J. Colloid Interface Sci. 2004, 269, 171.
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Figure 5. Kinks of the receding triple line on a substrate with discontinuous holes.
Figure 6. Diameter (left and right open symbols) and center (closed symbols) of the top diameter of the liquid bridge along compression (triangles with apex down) and stretching (triangles with apex up).
contact angle measured on an oxidized wafer without holes, the wandering of the triple line on this substrate composed of 44% holes does not significantly affect the shape of the drop. This is consistent with previous observations of the wetting of tin on chemically patterned Si/SiO2 substrates,17 where it was determined that the minimum receding contact angle on a surface with distributed defects of a high contact angle is the same as on the defect-free surface even if the defects occupy 33% of the surface area. The macroscopic minimum receding contact angle is thus clearly related to the location of the triple line rather than to the “compositional area” of the surface (eq 2). This conclusion prevails because the contact line can follow the topographical corrugation on the surface. However, the closer the holes, the larger the cooperative sticking effect of a row of holes. The stick-slip motion on receding is related to the length of triple line attached to the edges of the holes. Indeed, as the triple line “sweep” back rows of holes, this length increases to a maximum when the “macroscopic” triple line lie about above the diameter of the holes. This creates a barrier to receding. Unsticking proceeds by a mechanism of avalanche as described by Cubaud and Fermigier for chemically patterned surfaces.20,21 Global Displacement of the Bridge. Figure 6 displays the displacements of the two edges and the center of the contact diameter of the liquid bridge on the top substrate. Zero on the x-axis is chosen as the center of the sessile drop diameter from which the liquid bridge was formed. The figure shows that the triple lines expand or contract asymmetrically. Thus the center of the bridge moves during the compression-stretching cycle, indicating that the bridge undergoes global displacement. This is because the triple lines expand or contract erratically by an
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Chatain et al. Table 2. Results for Substrates with Holes
angle/sample
θcalcd EQ
θspont
θtop A
bottom θtop R - θR
θbottom A
44% holes 30 µm
130
134 ( 2
(148 ( 4) - (136 ( 2)
(138 ( 3) - (131 ( 3)
(110 ( 3) - (121 ( 3)
44% holes 3 µm
130
131 ( 2
(148 ( 4) - (139 ( 3)
(140 ( 3) - (133 ( 3)
(111 ( 3) - (120 ( 2)
62% holes 30 µm
139
142 ( 3
(153 ( 4) - (142 ( 3)
(143 ( 2) - (138 ( 2)
(116 ( 2) - (125 ( 2)
avalanche process from one or a series of contiguous defects.20,21 As the defects are not regularly distributed along the triple line, the detachment features do not have the same probability of occurrence. Other Substrates with Holes. The results obtained on three types of substrates are summarized in Table 2. The results for each type are an average of the 2 best compression-stretching cycles for three different volumes of liquid. The experiments performed on substrates that are 44% holes give the same results, whatever the volume of the liquid bridge or the depth of the holes. Holes as shallow as 3 µm are not filled, which means that the liquid surface above the holes is quite flat. For the substrates that are 62% holes, the equilibrium angle calculated with eq 2 is 139°. The spontaneous macroscopic angle on the top substrate is 142(3°, consistent with this equilibrium angle. The advancing angle on the top substrate fluctuates between 142(3° and 153(4°, and on the bottom substrate between 138(2° and 143(2°. As in the case of the 44% hole substrate, these advancing angles are larger than the equilibrium angle, but are closer to it on the bottom substrate, probably because of gravity effects. The receding contact angles oscillate between 116(2 and 125(2°. Again the lower angles are close to the Young contact angle on the smooth surface, but significantly higher. The larger surface fraction occupied by the holes increases the minimum receding angle because, as shown in Figure 7, the triple line cannot avoid to lie along the edges of the holes. Liquid Bridge between Substrates Patterned with Posts. In this section we will emphasize the differences between the wetting behavior of a liquid bridge between two substrates with holes and two substrates with posts. The compression-stretching cycle of a 54 mm3 bridge between two substrates with 30 µm high post covering 38% of the surface is described in detail. The results obtained on three types of substrates are summarized in Table 3. The results for each sample are an average of the two best compression-stretching cycles of three different volumes of liquid. Hysteresis Loop. The diagrams of Figure 8 display the contact angles and displacements of the triple line at the top right foot of the bridge as a function of its height, during a compressionstretching cycle. Figure 8 also shows a sequence of images of the liquid bridge during the compression-stretching cycle. The hysteresis loop is easily recognized, but is “smoother” than the one presented in Figure 4 for the substrates with holes. There is no more distinct stick-slip motion of the triple line.
Figure 7. Triple lines on a substrate with 62% holes.
Table 3. Results for Substrates with Posts angle/sample
θcalcd EQ
38% posts 30 µm
139
38% posts 3 µm 53% posts 30 µm
θtop R θbottom R
θspont
θtop A
θbottom A
163 ( 5
155 ( 2
153 ( 1
129 ( 2 125 ( 3
139
160 ( 2
155 ( 1
153 ( 1
130 ( 2 125 ( 2
131
161 ( 2
156 ( 1
154 ( 1
124 ( 3 122 ( 2
Spreading on the Top Substrate. When the drop contacts the upper substrate, there is no spreading, but the top of drop flattens as it is compressed. The macroscopic contact angle is very large. The value of this angle in Figure 8a is close to 170°; the average values of 163(5° given in Table 3 are underestimated because the boundaries of the triple line on the images cannot be precisely located. Indeed, the distance between the surface of the drop and the surface of the solid are too close to be resolved on the images (due to thresholding and the resolution of the CCD detector). The absence of spreading results from the discontinuity of the solid surface. Because of the absence of gravity forces on the top substrate, the very first contact of the liquid occurs on a single post. Contact with the six surrounding posts only occurs when further compression of the drop forces contact of the liquid surface with the surfaces of the posts. On the lower substrate, gravity forces can change the liquid surface curvature so as to achieve contact with surrounding posts. This is clearly observed in calculations with the Evolver software22 of the surface shape of a drop sitting on substrates with posts.23 Triple Line Motions on AdVancing and Receding. The advancing contact angle on the plateau has a constant value of 155(2° for the upper substrate and 153(1° on the bottom substrate. Both of these values are larger than the equilibrium contact angle of 139° calculated with eq 2. These advancing angles are smaller than the initial advancing angle on the upper substrate because they are measured for large contact areas between the liquid and the substrates, where the fraction of posts has the average value given in Table 1 (for small contact area, the area fraction of hole seen by the triple line is much larger than the average value). In Figure 8b, the motion of the macroscopic triple lines appears to be continuous. Actually, the triple line itself is discontinuous, as it is consists of circles at the edges of the posts. As explained above, macroscopic triple line advance requires compression to force the liquid to touch new posts, and the only spontaneous spreading occurs over distances equal to the post diameter. The receding contact angles of 129(2° and 125(3° on the upper and lower substrates, respectively, are close to each other. They are higher than the Young contact angle on the posts, which is the signature of a “composite interface” (as seen in Figure 9a). These angles are also higher than the receding angles found previously for the substrates with discontinuous holes at the same area fraction (see the last line of Table 2). This could mean that the fraction of triple line on silica is longer on a sample with discontinuous holes. (22) Brakke, K. A. Surface EVolVer Program; available by anonymous file transfer protocol (ftp) from geom.umn.edu (128.101.25.31) as pub/evolver.tar.Z. (23) Chatain, D.; Lewis, D.; Baland, J. P.; Carter, W. C. Langmuir 2006, 22, 4237.
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Figure 8. (a) Macroscopic contact angle and (b) radius at the top right foot of a 54 mm3 liquid bridge on a substrate with 38% 30-µm-deep posts. Threshold images of the liquid bridge along compression (second column) and stretching (third column).
Figure 9. (a) Part of the triple line on a substrate composed of 38% posts showing the limited length of pinning of the macroscopic triple line of the drop on a dense row of posts. (b) Ghost of the location of the solid-liquid interface showing the length of triple line stuck on dense rows of posts. The triple line is almost circular.
Global Displacement of the Bridge. As in the case of a substrate with circular holes, the liquid bridge globally moves when the triple lines expand or contract (Figure 10). This means that the triple lines move in an “erratic” manner because the posts are not regularly distributed along a circular triple line. It should also be stressed that the bridge is much more “mobile” on surfaces with posts than on surfaces with holes, with the same area fraction of solid (as for the substrates that are 62% holes and 38% posts). This behavior is related to the discontinuity of the triple line on the surfaces with posts and is in agreement with the absence of the stick-slip motion of the triple line on posts when it advances and recedes. Recently, the sliding ability of a liquid drop on a solid with posts has been related to the transition from a noncomposite to a composite interface.24 Our experiments clearly
show that the continuity of the triple line, which requires the connectivity of the solid, is another issue that was noticed in refs 6 and 8. Other Substrates with Holes. The results of the other experiments are summarized in Table 3. The experiments performed on substrates that are 38% posts give the same results, whatever the volume of the liquid bridge or the height of the posts. Rows of holes (between the posts) as shallow as 3 µm are not filled. The liquid surface above the long rows of holes between the posts is quite flat, as in the case of substrates with holes. For the substrates that are 53% posts, the (24) Yoshimitsu, Z.; Nakajima, A.; Watanabe, T.; Hashimoto, K. Langmuir 2002, 18, 5818.
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Summary
Figure 10. Diameter (left and right open symbols) and center (closed symbols) of the top diameter of the liquid bridge during compression (triangles with apex down) and stretching (triangles with apex up).
equilibrium angle is 131°. The advancing angles are the same as those measured on the surface that is 38% posts, but the receding contact angles are lower. This is consistent with a previous paper17 that showed that the receding contact line sticks on the posts because they have the lowest Young contact angle. The values of the advancing angles seem to be related to the way that the surface of the liquid can touch the posts.
Composite wetting (hysteresis) and sliding behaviors of a liquid lead bridge between two silica substrates with posts or holes have been investigated. The substrates were prepared such that the relief of the substrate with holes is the “negative” of the relief of the substrate with posts. A detailed analysis of the advancing and receding of the triple lines of a liquid bridge gives information on the formation and motion of a triple line and on the wetting hysteresis. Most of the differences observed between the two types of substrates are related to the continuity/discontinuity of the macroscopic triple lines when they are on a surface with holes or posts, respectively. The use of a metallic liquid allows observations at high magnification of the frozen bridge feet by scanning electron microscopy in order to support the analysis of the wetting experiments. In a companion paper, calculations of the equilibrium shape of liquid drops using the Evolver software24 shed light on some of the experimental observations described in this article. Acknowledgment. D.C. acknowledges Paul Wynblatt from Carnegie Mellon University for fruitful discussions. D.C. and C.L. acknowledge financial support from the CEA of Cadarache. LA053026I
