Article pubs.acs.org/JPCC
Adhesion-Regulated Switchable Fluid Slippage on Superhydrophobic Surfaces Yang Wu,†,∥ Yahui Xue,‡ Xiaowei Pei,† Meirong Cai,† Huiling Duan,*,‡ Wilhelm T. S. Huck,*,§ Feng Zhou,*,† and Qunji Xue† †
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China ‡ State Key Laboratory for Turbulence and Complex System, CAPT, Department of Mechanics and Aerospace Engineering, College of Engineering, Peking University, Beijing 100871, China § Radboud University Nijmegen, Institute for Molecules and Materials, Heyendaalseweg 135, 6525 AJ Nijmegen, The Netherlands ∥ University of the Chinese Academy of Sciences, Beijing 100049, China ABSTRACT: Surface adhesion is regulated by sparsely grafting responsive hydrophilic polymer chains on superhydrophobic surfaces but without obviously changing the wettability. We study experimentally how adhesion of superhydrophobic surfaces affects liquid slip. The slip length of water on such surfaces decays quickly as the adhesive force increases. This intrinsic dependence is theoretically explained based on scaling descriptions for specific geometries. A slip length range of 87 μm can be achieved reversibly by changing the temperature below and above the low critical solution temperature (LCST) of the grafted temperature-sensitive polymer. The results shed light on the intrinsic mechanism of liquid slip on textured surfaces and have important implications in the design of smart microfluidic and biofluidic devices, in which the regulation of fluid flow is highly desirable.
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important role in governing the liquid slip on a solid surface.29 Thus, knowledge of the adhesive property of a textured surface is required to fully depict the dependence of liquid slip on wetting. However, until now, it was still not clear how surface adhesion affects liquid slip nor how a switchable liquid slip can be achieved. Wetting hysteresis as a measure of liquid adhesion significantly depends on the distribution of geometrical or chemical defects on the surface.25 Altering the shape or chemical constituents of those defects changes the adhesion. Here, we fabricated superhydrophobic surfaces with reversibly tunable adhesive properties by grafting temperature-sensitive poly(N-isopropylacrylamide) (pNIPAM) brushes from initiator/perfluorosilane (PFOTS)-modified anodized alumina substrates with irregular micro/nanoscale surface topography. We measured the slip length on the responsive superhydrophobic surfaces with switchable adhesion. It is shown that liquid slip on the solid surface exhibited intrinsic dependence on its adhesion. Furthermore, reversibly switchable liquid slip was realized by regulating the adhesive property of the superhydrophobic surface.
INTRODUCTION Liquid slip on hydrophobic surfaces enables drag reduction at solid−fluid interfaces and has been explored extensively because of potential applications in micro/nanofluidics, lubrication, and self-cleaning.1−4 Noticeable liquid slip on smooth hydrophobic surfaces has been reported, with slip lengths ranging from several nanometers to 1 μm.5−9 The largest slip lengths are attributed to gas adsorption or nanobubbles at solid surfaces.10−12 Slip lengths over 100 μm can only be obtained on superhydrophobic surfaces (i.e., wetting contact angle is higher than 150°)1 and are proposed to originate in the large contribution of the shear-free liquid−air area13−17 in the Cassie−Baxter state.18 Both wetting contact angle and slip length are measures of wettability of a solid surface. Effort has been made to seek a universal relationship between them.19−22 Though recent molecular dynamic simulations have shown that slip length on smooth hydrophobic surfaces is a quasi-monotonically increasing function of the wetting contact angle,22 it is not meaningful to use wetting contact angle as a fluid slip gauge when designing slipping superhydrophobic surfaces.23 The dependence of slip length on the size and topography of surface textures has been revealed by molecular dynamics simulations20 and experiments.16,24 Surface textures lead to wetting hysteresis that represents the adhesive property at a solid−liquid interface.25−28 Liquid adhesion at a solid surface is closely related to the molecular interactions between the liquid and solid surfaces.28 The latter also plays an © 2014 American Chemical Society
Received: November 11, 2013 Revised: December 13, 2013 Published: January 6, 2014 2564
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Figure 1. FE-SEM images of the aluminum sheet after (a) etching under low voltage, (b) anodizing under high voltage, and grafting pNIAPM chains from the initiator/PFOTS at (c) 2:40, (d) 3:40, (e) 4:40, and (f) 5:40, respectively.
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determined automatically by using the Laplace−Young fitting algorithm. The advancing and receding contact angles were obtained by approaching and departing sample surfaces from settled sessile droplets until the contact lines shifted, respectively. The adhesive force was measured using a highsensitivity microbalance system (Dataphysics DCAT11, Germany). Water droplets of 5 μL suspended on a metal ring were approached and retracted from the sample surface at a constant speed at a given temperature. The droplet started to move away from the sample surface once contacted, and the balance force would gradually increase and reach the maximum before the droplet broke away from the surface. The peak data recorded in the force−distance curve were taken as the break point adhesive force. Surface Slip Measurement. Boundary slippage effects were measured on a rheometer (HAAKE, RS6000, Germany). For this experiment, the plate-and-plate model was applied. A modifying anodized aluminum sheet was the first plate, and a standard smooth stainless plate was considered the no-slip plate. Temperature was controlled by the Peltier plate of the rheometer (with error less than 0.10 °C), and the distance (H) between the clamp and test surface was measured by a rheometer precisely. The fixed volume test liquid (distilled water) was injected into the gap by precise syringe. The clamp was driven by a given torque and rotated with a certain angular velocity. If the slips exist on surfaces, the clamp will obtain different shear stress, which is recorded by the controlling computer.
EXPERIMENTAL SECTION Materials. N-Isopropylacrylamide (NIPAM, 99%, J&K), CuCl 2 ·2H 2 O and oxalic acid (AR, Tianjin, China), N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA, 98%, J&K), 1H,1H,2H,2H-perfluorooctyltrichlorosilane (PFOTS, 97%, Aldrich), and an aluminum sheet (purity 99.99%, Grikin Advanced Materials Co., Ltd.) were used. Chlorosilane initiator was synthesized according to a previous report.30 Preparation of a Responsive Superhydrophobic Surface. The rough anodized aluminum sheet was prepared by an unconventional anodization process.31 After anodizing, the fresh anodized aluminum sheet was activated with an oxygen plasma chamber and was decorated with PFOTS and chlorosilane initiator. Typically, the sheet was immersed in the solution of 50 mL of toluene containing the chlorosilane initiator and PFOTS in different molar ratios for 12 h. The samples were taken out of the mixed solution, washed with hexane and ultrapure water, and then blown dry with N2. Last, the initiator/perfluorosilane (PFOTS)-modified surface conducted surface-initiated atom transfer radical polymerization (SI-ATRP) to prepare the thermal-sensitive superhydrophobic surface as reported in ref 32. Morphology and Element Analysis. Scanning electron microscope (SEM) images were obtained on a JSM-6701F field emission scanning electron microscope (FE-SEM) at 5−10 kV. X-ray photoelectron spectra (XPS) were obtained on a multifunctional XPS/AES system (model PHI- 5072, Physical Electronics, Inc., Eden Prairie, MN, USA). The binding energy of C 1s (284.8 eV) was used as the reference Wetting State and Adhesive Force Measurement. The contact angle, advancing angle (θadv), and receding angle (θrec) were measured with the DSA-100 optical contact angle meter (Kruss Company, Ltd., Germany) at 20 and 50 °C. An amount of 5 μL of deionized water was dropped on the samples using an automatic dispense controller, and the contact angles were
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RESULTS AND DISCUSSION As shown in Figure 1a, the ultrapure aluminum sheet had a microterrace structure after etching under low voltage in slate solution, and innumerable nanowires (about 100 nm) emerged on the microterrace structure after anodizing under high voltage. The special dual structures on the sheet led to an excellent hydrophobicity after silanization.31 After decorating with initiator/PFOTS, the initiator was specially chosen for 2565
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Figure 2. (a) XPS full spectra of (1) the fresh anodized alumina, (2) after initiator/PFOTS (3:40) modification, and (3) grafted pNIPAM by ATRP. (b) The fine spectra of the C element of (1) the fresh anodized alumina, (2) after initiator/PFOTS (3:40) modification, and (3) grafted pNIPAM by ATRP.
relatively small as it is dominated by the hydrophobic PFOTS background monolayer. However, θrec decreases significantly with the increase of the polymer density, especially below the LCST when water droplets adhere to the hydrophilic chains. The adhesive force was obtained from a high-sensitivity microbalance system (Dataphysics DCAT11, Germany) following a previously reported approach.33 Figure 3 shows the corresponding relationship of the adhesive force as a function of the ratio of initiator/PFTOS at different temperatures. The PFOTS-only modified surface has the lowest adhesive force of only 2 and 5 μN at 20 and 50 °C, respectively, and the adhesive force increases with the density of the grafted polymer chains at 20 °C (LCST), the change is not obvious. These are related to the collapsed and stretched structures of the polymer chains as shown in Scheme 1(a) and (b), respectively. The wetting hysteresis on a substrate also reflects the surface adhesion property. A strong affinity of water on a solid surface generally leads to both significant adhesion and wetting hysteresis. Thus, the adhesive force F can also be estimated from θadv and θrec by the following equation
growing the pNIPAM molecular chains. Figure 1c−f showed the FE-SEM of the grafted pNIPAM sheet from the different initiator/PFOTS ratios, respectively. As shown above in the figures, pNIPAM chains were grafted at some sites for the low ratios of the initiator surface. With the increase of initiator ratio, the pNIPAM chains were covered uniformly (Figure 1e), and for the high ratios of initiator (Figure 1f) surface the aluminum sheet was covered by thick pNIPAM chains. The surface compositions of the modified anodized aluminum sheets were measured by XPS. Figure 2a shows the XPS full spectra of (1) the fresh anodized alumina, (2) after initiator/PFOTS (3:40) was modified, and (3) grafted pNIPAM. After modification with the initiator and PFOTS, a strong signal for F1s at 689.0 eV and the C1s signal split into two peaks (Figure 2b), which were attributed to C−C(C−H) and C−F. After grafting of the polymer chains, the N1s signal emerges at 400.1 eV. With the decrease of initiator ratios, the N element content decreased and the F element content increased, respectively, which proved the decrease of pNIPAM chains on the prepared surface (Table 1). Table 1. XPS Atomic Concentration Responsive Surface with Different Ratios
F ∝ γL(cos θrec − cos θadv)
where γ and L are the surface tension and the arc length of the three-phase contact line, respectively. The term on the righthand side of eq 1 represents the force needed to slide a droplet on a solid surface. From Table 2, it is seen that the contact angle hysteresis increases with the ratio of initiator/PFTOS, especially when the temperature is lower than the LCST. Thus, according to eq 1, the adhesive force increases with the wetting hysteresis. Therefore, the two approaches, i.e., the wetting state and adhesive force measurements, are consistent with each other. They both show that the adhesive force increases with the polymer grafting density at the stretched state (LCST). The magnitude of F represents the liquid−solid interaction strength, which has been elevated by increasing the hydrophilic polymer brushes. The grafted pNIPAM chains are uniformly and sparsely distributed on the PFOTS-modified superhydrophobic surfaces and cause strong pinning of wetting contact lines, which contributes to adhesion. Joanny and de Gennes34 elucidated the case of strong pinning on dilute defects, and by assuming diskshaped defects, Quéré derived the relationship between the adhesive force F (i.e., eq 1) and the defect density ϕs, that is25
concentration (%) molar ratio
C
O
N
F
Al
5:40 4:40 3:40 2:40 PFOTS-only
51.47 45.26 41.81 36.95 38.56
15.41 14.72 13.62 9.54 18.05
5.39 4.04 3.87 2.01 0.00
23.72 32.15 38.47 51.18 35.74
3.96 3.83 2.23 0.33 7.65
(1)
The thermal-sensitive polymer has a lower critical solution temperature (LCST) of 32 °C. Below the LCST, the polymer chains are solvated and stretched, whereas above the LCST, pNIPAM undergoes a collapse transition (see Scheme 1). The polymer brushes are hydrophilic and work as chemical defects at the superhydrophobic surface. By changing the molar ratio of initiator to PFOTS, the grafting amount of those polymer brushes was altered, and accordingly, the surface wetting and adhesive properties changed. Table 2 shows the advancing angle θadv, receding angle θrec, and the contact angle hysteresis (θadv − θrec) for different surfaces above and below the lower critical solution temperature (LCST, 32 °C) of pNIPAM. As shown in Table 2, at both temperatures, the change in θadv is 2566
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Scheme 1. Schematic of Superhydrophobic Surfaces with Grafted Temperature-Sensitive pNIPAM Chains: (a) Above the LCST of pNIPAM, Intramolecular Hydrogen Bonds Are Formed and the Polymer Chains Collapse, Making the Surface with Low Adhesion and Hysteresis and (b) Below the LCST of pNIPAM, the Intermolecular Hydrogen Bonding Is Formed and the Polymer Chains Are Stretched, Resulting in High Adhesion and Hysteresis
The slip length on a solid surface also strongly depends on the liquid−solid interaction strength29 and can be measured by the fluid pressure drop as a function of the flow rate in narrow channels,35,36 by measuring the force−distance curves using atomic force microscopy37,38 or by using microparticle image velocimetry (μPIV).39,40 The value of slip length is influenced by surface roughness,41 fluid viscosity,15 contact angle,20 and the shear rate.8 Here, we used a commercial rheometer (HAAKE, RS6000, Germany) with a plate-and-plate arrangement to record the torques M over the test samples and a referenced stainless steel plate at a constant angular velocity.15,16,42 For circular Couette flow, by assuming nonslip and Navier slip boundary conditions at the upper plate and lower sample surfaces, respectively, and surfaces separated by a gap H, the slip length λ can be calculated according to the following mathematical relationship15,42
Table 2. Advancing Angle (θadv), Receding Angle (θrec), and Contact Angle Hysteresis for Surfaces with Different Molar Ratios of Initiator/PFOTS at 20 and 50 °C at 50 °C molar ratio
5:40
4:40
3:40
2:40
PFOTS
θadv/deg θrec/deg hysteresis/deg
157.7 124.7 33.0
158.5 126.3 32.2 at 20 °C
160.6 146.0 14.6
162.3 150.0 12.3
162.7 156.5 6.2
molar ratio
5:40
4:40
3:40
2:40
PFOTS
θadv/deg θrec/deg hysteresis/deg
152.7 0 152.7
153.7 87.3 66.4
156.3 102.3 54.0
159.3 127.0 32.3
164.5 160.0 4.5
λ /H = M nonslip/Mslip − 1
(3)
where Mnonslip and Mslip are torques over the referenced nonslip steel plate and test samples, respectively. To relate liquid slip with adhesion, we plot the measured slip length as a function of the adhesive force on surfaces with different initiator/PFOTS ratios at 20 and 50 °C in Figure 4. It is clearly seen that as the adhesive force increases the slip length decays quickly. Due to the collapsed structure of the polymer chains at 50 °C, further
Figure 3. Adhesive force of polymer brush grafted surfaces as a function of the initiator/PFOTS ratio at 20 and 50 °C. The solid and dashed lines are theoretical fits by eq 2 for the experimental results at 20 and 50 °C, respectively.
F = c·ϕs log(1/ϕs)
(2)
where ϕs is the number density of the defects (i.e., grafted pNIPAM chains) per unit area, and c indicates the interaction strength between the water molecules and the hydrophilic brushes. In our experiments, the adhesive force on pure PFOTS-modified superhydrophobic surfaces is quite small. Thus, F vanishes as ϕs → 0. For a good approximation, we choose ϕs = r/(r + 1), where r is the molar ratio of initiator/ PFOTS and fit the experimental results of the adhesive force as a function of r by eq 2 (c = 0.43 mN for 50 °C and c = 0.85 mN for 20 °C), as shown in Figure 3. At 20 °C when the stretched hydrophilic brushes cause a strong pinning effect, the general trend described by eq 2 is followed.
Figure 4. Dependence of slip length on adhesive force: the solid squares and open circles are experimental results obtained on polymergrafted surfaces at 20 and 50 °C, respectively, and the red solid line is a theoretical fit at 20 °C by eq 2. The dashed blue line is obtained by shifting the fit at 20 °C by Δb = −13 μm, and blue dashed lines are theoretical fits by eqs 2 and 5. The values of the initiator/PFOTS ratio for each data point are marked except those clustered together at 50 °C. 2567
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increasing the initiator/PFOTS ratio does not change the adhesion or slip length much. Increasing the initiator density at 20 °C always decreases the slip length or adhesive force as a consequence of a high pinning effect by those stretched hydrophilic polymer chains. To explain this monotonically decreasing behavior of slip length as a function of adhesive force, we recall the scaling law of slip length for pillared geometry developed by Ybert et al.21 The dilute hydrophilic chains on superhydrophobic surfaces affect liquid slip. In the Cassie−Baxter state, assuming nonslip boundary conditions on the solid surfaces of the defects and infinite slip length on the other phase (e.g., the liquid−air interface) yields a scaling law of the slip length bideal in the limit of ϕs → 021 bideal =
α +β ϕs
Figure 5. Switchable slip length on polymer-grafted and pure PFOTSmodified superhydrophobic anodized aluminum sheets at different temperatures.
(4)
of slip length were obtained, while PFOTS-only surfaces exhibit a very small change and in opposite direction.
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where α and β are numerical factors, indicating the dependence on the underlying geometry of the surface. The applicability of eq 4 has been confirmed by the experiments of Lee et al.,16 especially for the case of low defect/pillar density. The finite slip length on the liquid−air interface or pure PFOTS-modified surface requires a modification to eq 4, and the effective slip length on polymer-grafted PFOTS-modified superhydrophobic surfaces can be obtained by21 1 1 1 = + beff (1 − ϕs)bp bideal
CONCLUSION In summary, we have utilized a model surface sparsely grafted with temperature-responsive hydrophilic polymer chains to study the fundamental phenomenon of liquid slip on superhydrophobic surfaces by a series of rheological experiments. Because of the low polymer density, the surface wettability was little affected, but the contact angle hysteresis and the adhesive force changed significantly as a function of the density of the polymer chains and the temperature. The slip length on those surfaces behaves as a monotonically decreasing function of the adhesive force. This intrinsic dependence was explained theoretically and agreed well with the experimental results. Finally, we realized a reversible control of liquid slip on such responsive superhydrophobic surfaces by changing below and above the LCST of the grafted polymer. The present work enables a better understanding of liquid slip on textured surfaces and benefits potential applications in intelligent fluidic devices needing regulation of fluid flow.
(5)
where bp is the slip length on pure PFOTS-modified sheets. As measured in the present work, bp = 143 μm at 20 °C and bp = 130 μm at 50 °C. Using ϕs as an internal variable, the dependence of the slip length on adhesive force is described by eqs 2 and 5. We plot the theoretical fits (α = 0.36 mm, β = −1.07 mm) to the experimental results at 20 °C by eqs 2 and 5 in Figure 4 and see excellent agreement with experimental data. Both results at 20 and 50 °C show a similar trend: increasing the adhesive force reduces the slip length. It is clear that the grafted PNIPAM chains increase the adhesive force but without obviously changing the advancing contact angle (see Table 2), which leads to a significant decrease of the slip length. The adhesive force shows the dominated effects on liquid slip at superhydrophobic surfaces over the wetting contact angle. The present systematic investigation sheds light on the influence of the surface textures on the liquid slip at superhydrophobic surfaces. The Cassie−Baxter equation predicts equal contact angles for textured surfaces with the same solid fraction.18 The fact that slip lengths obtained on such surfaces are different can be explained by the complicated influence of surface textures on the wetting hysteresis and thus adhesive force.26,34,43 Our finding facilitates the control of liquid slip on textured surfaces. As seen in Figures 3 and 4, at molar ratio 5:40, the responsive polymer chains show an influence on adhesion when collapsed at 50 °C (Scheme 1a), but when stretched at 20 °C (Scheme. 1b), they cause a high adhesive force. Therefore, by regulating the test temperature, an interesting switching slip length transition was achieved, as shown in Figure 5. The slip length was obtained of about 117 μm at 50 °C but only about 30 μm at 20 °C. After every slippage measurement, the simple treatment with tetrahydrofuran and ethanol was taken for the textured surface. The large and small slip length also can be obtained at 50 and 20 °C, respectively. So, the switchable cycles
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. *E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by NSFC (21125316, 11225208, 51335010, and 11172001), 973 projects (2013CB632300, 2011CB013101), and Key Research Program of CAS (KJZD-EW-M01).
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