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Feb 23, 2016 - material for bioinspired adhesives. Three different stick−slip patterns were observed on these surfaces (flat or micro- structured) d...
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Stick−Slip Friction of PDMS Surfaces for Bioinspired Adhesives Longjian Xue,*,†,‡ Jonathan T. Pham,‡ Jagoba Iturri,‡ and Aránzazu del Campo‡,§ †

School of Power and Mechanical Engineering, Wuhan University, South Donghu Road 8, 430072 Wuhan, China Max-Planck-Institut für Polymerforschung, Ackermannweg 10, 55128 Mainz, Germany § INM−Leibniz Institute for New Materials, Campus D2 2, 66123 Saarbrücken, Germany ‡

S Supporting Information *

ABSTRACT: Friction plays an important role in the adhesion of many climbing organisms, such as the gecko. During the shearing between two surfaces, periodic stick−slip behavior is often observed and may be critical to the adhesion of gecko setae and gecko-inspired adhesives. Here, we investigate the influence of short oligomers and pendent chains on the stick− slip friction of polydimethylsiloxane (PDMS), a commonly used material for bioinspired adhesives. Three different stick−slip patterns were observed on these surfaces (flat or microstructured) depending on the presence or absence of oligomers and their ability to diffuse out of the material. After washing samples to remove any untethered oligomeric chains, or after oxygen plasma treatment to convert the surface to a thin layer of silica, we decouple the contributions of stiffness, oligomers, and pendant chains to the stick−slip behavior. The stick phase is mainly controlled by the stiffness while the amount of untethered oligomers and pendant chains available at the contact interface defines the slip phase. A large amount of oligomers and pendant chains resulted in a large slip time, dominating the period of stick−slip motion.

1. INTRODUCTION Climbing organisms combine a number of mechanisms to adhere to surfaces, from hierarchical structural design to capillary driven fluid secretion. Of these organisms, geckos are of particular interest because they are known as the heaviest animal to use van der Waals forces for their locomotion on variable surfaces.1 Most commonly, adhesion of gecko is considered as dry adhesion, where nanoscale fibrillar features on the gecko’s toe pad form numerous contact points directly with the counterpart surface. Inspired by such observations, many synthetic mimics of similar micro- and nanostructures have been created with the aim of replicating the remarkable adhesive properties of a gecko’s foot.2,3 To create these bioinspired structures, polydimethylsiloxane (PDMS, Sylgard 184) is often used as a model material because it is simple to use, easy to control, and commercially available.4,5 Depending on the mixing ratio and curing temperature/time, the available elastic modulus ranges from several tens of kPa to a few MPa. It has been demonstrated that the elastic modulus of the material has an important role in determining the adhesion performance of biological and artificial systems.6−9 Since shearing has been discovered to be a prerequisite for a gecko to gain its fabulous adhesive abilities,10−12 continuous attention has been paid to the friction of bioinspired adhesives. A wide range of friction forces have been realized on variable systems. Moreover, anisotropic frictions have been demonstrated on asymmetric fibrillary adhesives.13−17 Stick−slip frictional behavior, on the other hand, is often not considered in view of the added complexity. During climbing, however, the slip phase of © 2016 American Chemical Society

stick−slip motion can lead to detrimental performance if it occurs at nonoptimal times. It has been shown that stick−slip friction can be reduced/eliminated by structuring the surface,17−20 by controlling the preload and the shearing velocity.21 Using tilted PDMS microflaps, Das et al.21 reported that stick− slip friction is strongly affected by the roughness of the counterpart surface, the loading force and the shearing velocity. In contrast, we investigate the stick−slip behavior of pillar-less PDMS surfaces under different processing conditions that lead to different compositions and stiffness. Although there are numerous studies related to the adhesion and friction of PDMS surfaces and structures; how un-crosslinked molecules in PDMS affect the adhesion and friction has not been widely considered.22 For commonly used commercial PDMS, a significant fraction of unreacted molecules (i.e., oligomers or monomers) and dangling ends are left in the material if not washed away.23 Moreover, using the mixing ratio of 10:1 (prepolymer/cross-linker) recommend by the manufacturer can still result in the elastic modulus fluctuating within a small range.24 In most cases, the presence of these residual oligomers or monomers is neglected. However, residual PDMS oligomers can be transferred to the counterpart surface upon contact.25,26 Arzt and co-workers27 have shown specifically the delivery of PDMS oligomer to the contact interface can reduce the performance of the structured adhesive during contacting/ detaching cycles. Received: February 9, 2016 Published: February 23, 2016 2428

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abbreviations. For example, s-PDMS samples that underwent this acetone-washing step are then named ws-PDMS. To modify the surface chemistry of PDMS surfaces, they were treated for 10 s with an oxidative Plasma Activate Statuo 10 USB (plasma technology GmbH, Rottenburg, Germany) with a power of 100 W at a pressure of 0.1 mbar. The oxygen plasma-treated PDMS samples, whose surfaces are hydrophilic, are then indicated with a prefix “o”, such as osPDMS, oe-PDMS, ows-PDMS, and so forth. Friction tests on oxygen plasma-treated PDMS surfaces were carried out immediately after plasma treatment. Friction measurements were performed in ambient conditions using a custom-built device described in our previous work.17,28 In brief, a spherical ruby probe with diameter of 5 mm was brought into contact with the sample surface and a normal loading force was applied, which was kept constant during the lateral shearing (Figure 1a). The sample moved at a velocity of 100 μm/s over a distance of 500 μm, forward and backward, while the forces were simultaneously recorded.

Here we focused on the mechanism of stick−slip behavior of PDMS surfaces with varying processing conditions used in our past reports.4,5,17 This includes changing the curing time, washing the samples in organic solvent, and oxygen plasma treatment. By varying these processing conditions, we found that PDMS oligomers and pendant chains migrate to the contact interface and alter the stick−slip pattern. While the stick phase is mainly controlled by the apparent elastic modulus of the PDMS, the slip phase is strongly influenced by the amount of untethered oligomers and pendant chains, which is associated with both the curing time and the washing in organic solvent.

2. EXPERIMENTAL SECTION PDMS elastomer kits (Sylgard 184) were purchased from Dow Corning (Midland, MI). Acetone (Analytical grade) was obtained from SigmaAldrich. In this study, all PDMS samples were prepared by mixing the prepolymer and cross-link agent in a ratio of 10:1, but cured and processed with different conditions. After proper mixing, the PDMS precursor was degassed in a desiccator for 20 min in order to remove the air bubbles. After pouring onto a silicon wafer, the PDMS precursor was allowed to relax for 15 min and then thermally cured in an oven. The thickness of the prepared PDMS sheet was controlled by the volume to be ∼1 mm. The following naming conventions will be used throughout the rest of this study (Table 1). A PDMS sheet prepared by curing at 90

Table 1. Definition of Short Names of PDMS Samples Treated in Various Conditionsa short name

curing

s-PDMS os-PDMS ws-PDMS ows-PDMS e-PDMS oe-PDMS owe-PDMS p-PDMS ep-PDMS op-PDMS oep-PDMS

90 °C, 1 h 90 °C, 1 h 90 °C, 1 h 90 °C, 1 h 90 °C, 1 h + 65 °C, 14 h 90 °C, 1 h + 65 °C, 14 h 90 °C, 1 h + 65 °C, 14 h 90 °C, 1 h 90 °C, 1 h + 65 °C, 14 h 90 °C, 1 h 90 °C, 1 h + 65 °C, 14 h

oxygen plasma

acetone washing

Figure 1. (a) Schematic of the friction test with a ruby sphere (5 mm diameter) as the probe; the sample (blue) amount on the sensor for later force is moved in the lateral direction (indicated by the red arrow) with a specified normal loading. (b) Representative friction curve on flat PDMS cured at 90 °C for 1 h at a normal load of 1 mN and shear speed of 100 μm/s. The red arrows indicate the static friction force in the direction of trace and retrace. F denotes the friction force in the direction of trace and retrace.

yes yes

yes yes

yes yes

yes

Surface microstructures were characterized by white light confocal microscopy (μsurf, Nanofocus AG, Oberhausen, Germany), optical microscopy, and atomic force microscopy (AFM). AFM measurements were performed on a JPK Nanowizard 3 (JPK Instruments, Germany) instrument. AFM probes had quadratic pyramidal N doped Si tips (Nanosensors) with a frequency of 160−220 kHz and a spring constant of 27−71 N/m. Spring constants for the cantilevers were determined at the start of each experiment. Topographical imaging was performed in tapping mode (scan rate < 1 Hz, 512 pixels) over 5 μm2 surfaces and in dry conditions. Both images and force spectroscopy plots were subsequently analyzed by JPK Data Processing software, giving the root mean square (RMS) roughness of the surface. The elastic modulus was obtained by fitting a Hertz/Sneddon model curve (ν = 0.5). The water contact angles were measured on a KRÜ SS DSA10-MK2 (KRÜ SS GmbH, Germany) drop shape analysis system with 3 μL of deionized water as the probe fluid. The average value of the water contact angle was obtained by measuring the same sample at three different locations. Tensile testing was carried out with a Zwick Roell Z005 Universal Testing Machine equipped with a 50 N load cell. Crosshead velocity was 20 mm min−1. Samples were cut into a dogbone shape with a gauge length of 12 mm and width of 2 mm.

yes yes

s = Standard curing condition (90 °C, 1 h), e = extra curing step (65 °C,14h) after the standard curing condition (90 °C, 1 h), o = treatment with oxygen plasma, w = washing with acetone, and p = patterned surface with standard curing condition (90 °C, 1 h). The combination of prefixes means the multiple treatments.

a

°C for 1 h will be referred to as standard PDMS (s-PDMS). PDMS cured at 90 °C for 1 h followed by heating at 65 °C overnight (14 h) is called extra-cured PDMS (e-PDMS). The curing conditions were chosen to follow the same line of the previous works.4,5,17 For patterned PDMS surfaces (p-PDMS), the PDMS precursor was poured on a mask prepared by standard photolithography.28 The hexagonal cell has a circumcircle diameter of 20 μm, a wall thickness of 3 μm, and a height of 5 μm. In order to remove un-cross-linked monomers or oligomers in cured PDMS, the samples were washed by immersing in acetone and shaking for 1 h, replacing with fresh acetone and repeated at least three times. The washed PDMS was then dried in high vacuum at room temperature. The washing procedure was monitored by weighing the washed PDMS and was considered to be complete after no reduction in weight was measured. The remaining solid was then considered as the cross-linked PDMS network. The residual PDMS washed away is mainly composed of untethered oligomeric chains or monomers. The samples where the washing step was implemented are indicated with a prefix “w” in their

3. RESULTS AND DISCUSSION Stick−Slip Behavior of PDMS with Different Curing Process. Figure 1b shows representative friction force data recorded on an s-PDMS surface with a constant normal loading force of 1 mN. A single test includes shearing of the PDMS in both forward and reverse directions (trace and retrace). The 2429

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fit to a cosine function (red line in Figure 3b), which indicates an inertia controlled slip behavior.29 This type of stick−slip will be referred to as stick−slip pattern I. Increasing the normal loading force from 1 mN to 7 mN decreased tslip from 0.25 to 0.19 s, while tstick slightly increased from 0.18 to 0.19 s (Figure 3c). tstick and tslip together contributed to a decrease in tp of 0.05 s, meaning the stick−slip period is dominated by the slip rather than stick phase on s-PDMS. e-PDMS, which is PDMS with extra curing time, also showed stick−slip behavior, but with a different stick−slip pattern than that observed on s-PDMS (Figure 3d, e). It should be mentioned that the extra curing did not change the surface topography giving an RMS roughness of ∼1 nm (Figure 2b). The slip phase begins with a rapid drop in force followed by a deceleration before the next round of a stick phase. This feature of the slip phase observed on e-PDMS is similar to the reported overdamped stick−slip29 and will be referred to as stick−slip pattern II. It suggests a larger resistance during the slip phase than pattern I slip. Different from the linear increase in friction force in an overdamped stick−slip,29 the stick phases here have a sinusoidal-like increase in the friction force (Figure 3e). Similar to s-PDMS, the increasing of loading force also resulted in a drop of tp (Figure 3f). Contribution of Stiffness to Stick Phase. Comparing the phases of stick and slip on e-PDMS to s-PDMS, a larger tstick and a smaller tslip was found on e-PDMS, which together contributed to a smaller tp (Figure 3f). In other words, the stick phase dominates the stick−slip period on e-PDMS, especially at large loading forces. Since sliding does not occur during the stick phase, resistance from the PDMS network deformation increases during the stick phase. A larger tstick means a larger resistance, which is also indicated by Δf, defined by the difference between fs and f k. Normalized by the corresponding friction force F, Δf/F is much larger on e-PDMS than on s-PDMS (Figure 3c, f). With extra curing, the elastic modulus increased to 2.28 ± 0.04 MPa, which is a subtle but significant increase compared to that of s-PDMS (1.42 ± 0.13 MPa), a 60% increase. This increase in elastic modulus suggests an enhanced cross-linking. This is supported by the difference in weight loss, 4.4 wt % from s-PDMS and 4.2 wt % from e-PDMS, after washing with acetone. The increase in elastic modulus and decrease in weight loss by washing suggest that the extra curing at 65 °C for 14 h may cause the fusion between pendant chains (Figure 4) increasing cross-linking and thus the elastic modulus (stiffness) of PDMS. This is consistent with the fact that a stiffer material can generate a larger resistive force during shearing.18,30 In order to confirm the contribution of stiffness to the stick phase, we constructed a honeycomb-like structure on top of the surface (p-PDMS, Figure 5a). We use the curing procedure identical to s-PDMS, such that it is reasonable to assume pPDMS has the same surface chemistry as s-PDMS. Therefore, the presence of the honeycomb-like pattern on the surface greatly reduces the effective elastic modulus compared to s-PDMS while keeping the surface chemistry constant (Table 2).31 When sliding on p-PDMS in the direction against the edge of holes (arrow in Figure 5a), no obvious stick−slip was detected (Figure 5b). The effective elastic modulus is decreased to 1.25 ± 0.11 from 1.42 ± 0.13 MPa (Table 2), which is governed by the deformation of the thin hexagonal walls. This result is consistent with reports that patterns on the surface of a material can suppress or eliminate stick−slip motion during the sliding, likely due to a decrease in the lateral surface stiffness.18,20,32−34 While it would be reasonable to assume that this is a simple effect of

negative friction force is associated with the force in the reverse direction. In each direction, the curve starts with a sharp increase in the force (static friction) while the sample is laterally deformed. In this region, sliding does not yet occur and the slope is related to the lateral stiffness of the material. Overcoming the static friction force (indicated by the red arrows) leads to sliding between the probe and sample (kinetic friction). The difference between the kinetic friction force and the zero force offset defines the friction force F in the corresponding direction. On a homogeneous surface, friction forces in the trace and retrace directions are identical. The oscillatory fluctuations in the kinetic friction indicate stick−slip motion during shearing. Since the surfaces of the sPDMS and ruby sphere are smooth with RMS roughness of ∼0.5 and ∼3.7 nm (Figure 2), respectively, the stick−slip pattern is

Figure 2. Three-dimensional AFM images of (a) s-PDMS, (b) e-PDMS, and (c) ruby sphere.

not the result of surface roughness of the contacting surfaces. The increase of normal loading force within our test range (1−7 mN) does not eliminate the stick−slip behavior on s-PDMS (Figure 3a). A stick−slip period tp, can be divided into stick (tstick) and slip (tslip) phases (Figure 3b), where tp = tstick + tslip. The stick time, tstick, is defined as the time period between the position where the stick phase begins to the position where a peak force value of the stick phase fs is measured. After the peak force, the slip phase starts and ends at the position where the force is equal to the kinetic friction force f k; this is the slip time tslip. Therefore, the friction force F ≈ (f k + fs)/2.29 The detection of f k also defines the beginning of next stick−slip period. The slip phase is 2430

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Figure 3. Representative stick−slip motions on (a) s-PDMS and (d) e-PDMS at normal loads of 1, 4, and 7 mN. Detailed stick−slip patterns on (b) sPDMS (pattern I) and (e) e-PDMS (pattern II). In (b) and (e), tstick, tslip, and tp define the time of the stick phase, the slip phase, and the period of stick− slip, respectively. fs and f k are the peak force of stick phase and the kinetic friction force, respectively, whose difference was defined as Δf. (c) Dependence of tstick, tp, and Δf/F of (c) s-PDMS and (f) e-PDMS on normal loading force. The gap between tstick and tp defines the value of tslip. Each data point in panel (c) and (f) represents mean values of 10 stick−slip periods. The error bars indicate standard deviations.

increase in contact area: the walls in the honeycomb pattern tilted in the direction of shearing offering a larger contact area with the probe under a large normal loading force. The bottom of the honeycomb pattern can also contribute to the total contact area with a large normal loading force (Supporting Information Video 1). Therefore, samples can be finely tuned (Supporting Information Table 1) by controlling both the curing time and surface patterns that contribute to the lateral stiffness. Different from tstick, tslip appears to have no clear correlation with variations in the lateral surface stiffness. With the two different curing procedures, relatively modest but different surface energies were measured by water contact angles of 115.4° ± 0.3° and 109.0° ± 1.0° for s-PDMS and e-PDMS, respectively. To decouple the effects of elastic modulus (stiffness) and surface chemistry, we used oxygen plasma to minimize the difference in surface chemistry of the surfaces.36 The surfaces were treated with oxygen plasma for a short time of 10 s, and used immediately after treatment. The treatment with oxygen plasma converts the PDMS top surface into a smooth layer of silica with the thickness of few nanometers;37,38 and the RMS roughness of the silica layer is even smaller than that before the treatment (Figure S1). The presence of silica layer effectively eliminated the difference in surface energy caused by different curing procedures. Surprisingly, a new pattern of stick−slip (pattern III) was found on all oxygen plasma treated samples (Figures 6 and S2). After a sharp increase in friction force during the stick phase, the jumps from the peak to the valley are nearly instantaneous, leading to a highly stick-dominated stick−slip period. The stick−slip motions are much more irregular as compared with other samples. We assume this is a result of the deformation and cracking of the silica layer on the PDMS surface under mechanical stress.39 The more surprising fact is that tslip ≈ 0.02 s is almost identical in all four cases (Figures 6c, f and S2), while the tstick were still following the order of elastic modulus. This suggests that the slip phase is mainly governed by the surface composition rather than the stiffness of the material. While the difference in surface energies of as-cured hydrophobic PDMS (s-PDMS and e-PDMS) is negligible, the amount

Figure 4. Schematic drawing showing (a) the presence of pendant chains (free black line linked to the dot) and oligomers (red) in PDMS network (black), and (b) the extra curing at 65 °C for 14 h increased the cross-linking of PDMS by the fusing of pendant chains.

contact area, extra curing of the p-PDMS (thus called ep-PDMS) surprisingly recovered the stick−slip behavior with the same contact area, as illustrated in Figure 5c, d. This demonstrates that observation of stick−slip behavior in patterned PDMS surfaces is in fact a function of the elastic modulus and not the contact area, which can be supported by other findings.35 A close look at a stick−slip event (inset in Figure 5c) showed combined features of s-PDMS and e-PDMS: first, both the stick and slip phases showed similar qualitative features as that of s-PDMS (Figure 3b); second, tstick is larger than tslip representing the feature of ePDMS (Figure 3f). Increasing normal loading from 1 to 7 mN, tslip are considered constant (the gap between tp and tstick), while tstick increased from 0.04 to 0.06 s (Figure 5d). Different from that on a flat surface, both tp and tstick increased with increasing normal loading. The modest increase in tstick (and tp) is rationalized by an 2431

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Figure 5. (a) Three-dimensional image of honeycomb-like p-PDMS pattern. The hexagonal cell has a circumcircle diameter of 20 μm and the wall thickness is 3 μm. The height of the pattern is 5 μm. The blue arrow indicates the direction of friction measurement. (b) Typical friction curve on pPDMS cured at 90 °C for 1 h at a shear speed of 100 μm/s with different normal loading forces. (c) Typical friction curve on ep-PDMS at a shear speed of 100 μm/s with different normal loading forces. The inset shows the detailed pattern of stick−slip motion. (d) Dpendence of tstick, tp, and Δf/F of epPDMS on normal loading force. The gap between tstick and tp defines the value of tslip. Each data point in panel (d) represents mean values of 10 stick−slip periods. The error bars indicate standard deviations.

tstick stayed constant while tslip increased slightly, which resulted in the increase of tp. During contact of s-PDMS with the probe, the un-cross-linked molecules are likely transferred to the probe25−27 and may establish an equilibrium state after ∼30 cycles, or 300 s of shearing. These moveable oligomers at the contact interface may align along the shearing directions facilitating the slip motion, resulting in the increase of tslip after ∼300 s of constant shearing at 100 μm/s. After washing away the removable PDMS residuals in s-PDMS (ws-PDMS), we repeated the friction tests (Figure 7c). Surprisingly, the stick−slip pattern (pattern II) displayed the same features as that found on e-PDMS (Figure 3e). Furthermore, there is a critical time of shearing at ∼300 s where the slip and stick times plateau (between cycles of 5 and 30) (Figure 7d). As compared to that on s-PDMS (Figure 7b), tstick increased to more than 0.3 s during the plateau period. The washing step does not influence the cross-linked network of the PDMS, suggesting that unreacted PDMS residuals contribute to the stick phase but play a role as lubricant. However, tslip also started to increase after 300 s shearing. Since PDMS oligomers were removed by the washing step, we propose that pendent chains also contribute to the slip phase. These pendent chains are the polymer molecules where one end is chemically bonded to the PDMS network while the other end is free (Figure 2). The presence of abundant PDMS residuals (4.4%) in s-PDMS may shield the influence of pendant chains near the surface. As such, the influence of pendant chains is only observed when the oligomeric chains are removed. Pendant chains cannot be washed away and are always present in the system; thus, their influence on stick and slip phases cannot be eliminated.

Table 2. Elastic Modulus Measured with AFM of PDMS Samples before and after Acetone Washing E (MPa) s-PDMS e-PDMS p-PDMS a

before washing

after washing

1.42 ± 0.13 2.28 ± 0.04 1.25 ± 0.11a

1.78 ± 0.08 3.51 ± 0.52 1.57 ± 0.08a

Calculated based on indentation test.

of un-cross-linked molecules within the materials is different because the longer curing time allows more molecules to connect to the network. The network percent by weight of s-PDMS reaches 95.6%, which is slightly enhanced to 95.8% with extra curing at 65 °C overnight. This demonstrates that the extra curing reduces the amount of residual PDMS oligomers (i.e., unreacted molecules). We then proposed a hypothesis that these moveable residual PDMS oligomers dominate the slip phase due to the increased mobility of molecules at the surface. In order to verify the contribution of moveable chains to the stick−slip motion, we conducted the cyclic tests shown in Figure 7. The friction tests were continuously carried out on s-PDMS and wsPDMS for 100 cycles with a 1 mN normal loading and a constant shear rate of 100 μm/s (Figure S3). The stick−slip motions on sPDMS for different repeating cycles are summarized in Figure 7a. The pattern of stick−slip stayed constant during the friction cycles. The evolutions of tp, tstick, and tslip are summarized in Figure 7b. In general, tslip is larger than tstick, giving a slip dominated stick−slip period. It is interesting to find that the time periods for the stick and slip phases after the first drop are relatively constant until the 30th cycle (300 s). Afterward, the 2432

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Figure 6. Representative stick−slip motions on (a) os-PDMS and (d) op-PDMS at normal loads of 1, 4, and 7 mN. Detailed stick−slip patterns (pattern III) on (b) os-PDMS and (e) op-PDMS. (c, f) Dependence of tstick, tp, and Δf/F of (c) os-PDMS and (f) op-PDMS on normal loading force. The gap between tstick and tp defines the value of tslip. Each data point in panel (c) and (f) represents mean values of 10 stick−slip periods. The error bars indicate standard deviations.

Figure 7. (a) Representative stick−slip patterns of different repeating cycles on s-PDMS. The numbers on the right side of the curves indicate the cycle number in the repeating measurements. (b) Dependence of tstick, tslip, and tp of s-PDMS on the measurement cycles. (c) Representative stick−slip patterns of different repeating cycles on ws-PDMS. The numbers on the right side of the curves indicate the cycle number in the repeating friction measurements. (d) Dependence of tstick, tslip, and tp of ws-PDMS on the measurement cycles. Each data point in panel (b) and (d) represents mean values of 10 stick−slip periods. The error bars indicate standard deviations.

exposed to air.37 For example, the exposure of os-PDMS to air for 10 h resulted in an increase of the water contact angle from 9.3° to 40.9° (Figure 8). The recovering of hydrophobicity is mainly considered to be a result of the transportation of oligomers to the surface, as well as rotation of polymer backbones in PDMS

Decoupling the Contributions of Free Oligomers and Pendant Chains. It is well-known that the top surface of PDMS forms a silica-rich layer upon oxygen-plasma treatment, rendering the surface hydrophilic.37,38 It is also well-known that the hydrophilicity is gradually lost when the surface is 2433

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dependence in the stick phase, a feature of pattern I. Further shearing on ows-PDMS (e.g., 100 cycles of shearing), however, recovered the pattern I stick−slip behavior. This is unexpected because the oligomers are in principle removed by the washing step. Therefore, the number of pendant chains near the surface, when cured at 90 °C for 1 h, is sufficient to modify the frictional properties by repeated shearing. When such pendent chains are combined with the existence of residual oligomers, the conversion from pattern III to pattern I can be completed in a much shorter time (Figure 9a). The combination of extra curing at 65 °C overnight and washing with acetone can keep owe-PDMS in a more hydrophilic state after a 10 h recovery period in air with a water contact angle of ∼20° compared to ∼30° of ows-PDMS. Shearing of owePDMS showed pattern III stick−slip, which persisted over the course of 100 cycles (Figure 9c). This shows that the decrease of pendant chains at the surface by a longer curing process is sufficient to retain pattern III stick−slip behavior. In turn, this demonstrates that when the number of free pendant chain ends is below a certain value, their effect on the stick−slip friction can be neglected.

Figure 8. Water contact angles of os-PDMS and oe-PDMS just after oxygen plasma treatment, and os-PDMS, ows-PDMS, oe-PDMS, and owe-PDMS exposed to air for 10 h. Each data point in a bar represents measurements of three different locations on the sample. The error bars indicate standard deviations.

exposing low-surface-energy groups (such as pendant chains) to the top surface.37−40 Removing of oligomers by both extra curing and washing therefore can postpone the hydrophobic recovery. But the efficiencies in prolonging the recovery of hydrophobicity vary with the processing conditions (Figure 8). The most efficient way to minimize this recovery is a combination of both extra curing and washing, which resulted in a water contact angle of ∼20° after an exposure to air for 10 h. The difference in the recovery of hydrophobicity of PDMS was used to decouple the contribution of free oligomers and pendant chains to the observed stick−slip behavior. os-PDMS and opPDMS showed pattern III stick−slip where the stick phase instantaneously jumped to the next period with a universal tslip of ∼0.02s (Figures 6 and S2). It is reasonable to assume that osPDMS possesses an abundance of oligomers or pendant chains under the surface that can migrate to the surface, such that 50 cycles of shearing can revert the stick−slip behavior from pattern III to pattern I (Figure 9a). This illustrates that the contacting

4. CONCLUSION In this study, we investigated the stick−slip behavior during friction of cross-linked PDMS with a counterpart surface. The mixture of prepolymer and cross-linker generate a cross-linked network together with pendant chains attaching to the network and untethered oligomeric chains. The curing procedure can greatly change the amount of these moveable chains, but unlikely able to eliminate them. When the cross-linked PDMS is brought into contact with a counterpart surface, the untethered chains and the pendant chains close to the surface can migrate to the contacting interface. During shearing, the amount of chains available at the interface defines the slip phase while the stick phase is mainly dominated by the elastic modulus (stiffness) of the material. Our results on the influence of moveable small molecules and pendent chains on friction may be a useful analogy for understanding the function of the lipids found on gecko’s setae. Furthermore, our results on stick−slip behavior of patterned surfaces offer information for the design of bioinspired adhesive surfaces as well as insight into understanding the friction of silicon rubbers, which are widely used in our daily life.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b00513. pPDMS with a normal loading force of 1 mN, moving at a speed of 100 μm/s over 500 μm to the right and then back (AVI) Shear stiffness calculated from the slope of the curve in the static friction part; AFM images of os-PDMS and oePDMS; representative stick−slip motions of oe-PDMS and oep-PDMS; dependence of friction force on the repeating cycles on s-PDMS (PDF)

Figure 9. Representative stick−slip period of repeated friction on (a) osPDMS, (b) ows-PDMS, and (c) owe-PDMS for repeating cycles. The curves were shifted in axes X and Y for comparison.

interface is again dominated by the oligomers or pendant chains after continuous shearing for ∼8 min. Further shearing did not change the stick−slip pattern, but slightly increased the tslip and thus also increased tp as found on s-PDMS (Figure 3). After a washing step with acetone followed by oxygen-plasma treatment (ows-PDMS), the conversion of the stick−slip pattern from an instantaneous to gradual slip phase was retarded (Figure 9b). Shearing for 50 cycles on ows-PDMS qualitatively resulted in a pattern between pattern III and pattern I. This is demonstrated by the dependence of force on time in the slip phase, a feature of pattern III (a sharp jump), and a sine

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 2434

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ACKNOWLEDGMENTS The authors thank Dr. Michael Kappl for fruitful discussion, and the Deutsche Forschung Gemeinschaft for financial support within the program SPP1420 “Biomimetic Materials Research: Functionality by Hierarchical Structuring of Materials” (Projects CA880/1, BU 1556/26). L.X. thanks the National Natural Science Foundation of China (51503156) for support.

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DOI: 10.1021/acs.langmuir.6b00513 Langmuir 2016, 32, 2428−2435