Fast Transport of Water Droplets over a Thermo ... - ACS Publications

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Fast Transport of Water Droplets over a Thermo-Switchable Surface Using Rewritable Wettability Gradient Theneyur Narayanaswamy Banuprasad,† Thamarasseril Vijayan Vinay,† Cherumannil Karumuthil Subash,† Soney Varghese,† Sajan D. George,‡ and Subramanyan Namboodiri Varanakkottu*,† †

School of Nano Science and Technology, National Institute of Technology, Calicut 673 601, India Centre for Applied Nanosciences, Department of Atomic and Molecular Physics, Manipal University, Karnataka 576 104, India

‡

S Supporting Information *

ABSTRACT: In spite of the reported temperature dependent tunability in wettability of poly(N-isopropylacrylamide) (PNIPAAm) surfaces for below and above lower critical solution temperature (32 °C), the transport of water droplets is inhibited by the large contact angle hysteresis. Herein, for the first time, we report on-demand, fast, and reconfigurable droplet manipulation over a PNIPAAm grafted structured polymer surface using temperature-induced wettability gradient. Our study reveals that the PNIPAAm grafted on intrinsically superhydrophobic surfaces exhibit hydrophilic nature with high contact angle hysteresis below 30 °C and superhydrophobic nature with ultralow contact angle hysteresis above 36 °C. The transition region between 30 and 36 °C is characterized by a large change in water contact angle (∼100°) with a concomitant change in contact angle hysteresis. By utilizing this “transport zone” wherein driving forces overcome the frictional forces, we demonstrate macroscopic transport of water drops with a maximum transport velocity of approximately 40 cm/s. The theoretical calculations on the force measurements concur with dominating behavior of driving forces across the transport zone. The tunability in transport velocity by varying the temperature gradient along the surface or the inclination angle of the surface (maximum angle of 15° with a reduced velocity 0.4 mm/s) is also elucidated. In addition, as a practical application, coalescence of water droplets is demonstrated by using the temperature controlled wettability gradient. The presented results are expected to provide new insights on the design and fabrication of smart multifunctional surfaces for applications such as biochemical analysis, self-cleaning, and microfluidics. KEYWORDS: wettability gradient, PNIPAAm layer, uphill transport, droplet transport, stimuli-responsive, temperature gradient

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surface is modified by physical patterning,11−15 chemical patterning,16 or a combination of these two.17−19 A plethora of works has been reported which relies on permanent m odification of the surface for a specific purpose,11,12,16−18,20−25 which hinders its use where a reconfigurable system is required. A potential route to realize reconfigurable droplet manipulation platform is to impart stimuli-responsive properties to either the surface or the liquid.1,26 Depending on the responsive nature of the materials, external stimuli can be light, temperature, pH, magnetic field, or electric field.27 However, tuning the liquid properties often require addition of chemicals and/or physical contacts that could lead to undesirable sample contamination. On the other hand, light-controlled wettability modulation was employed for the droplet transport over a photosensitive solid surface,

INTRODUCTION Precise control over the motion of tiny amount of liquids over a solid surface has great importance both in fundamental as well as applied aspects.1,2 To mention a few are biochemical analysis,3,4 droplet microfluidics,2,5 heat transfer,6 self-cleaning,7 and drug delivery.8 Inspired by nature, intense efforts have been devoted for the design and fabrication of surfaces capable of controlled droplet manipulation.9,10 In spite of the progress achieved in many of these, there are still a lot of fundamental issues that need to be addressed in detail, such as robustness, reconfigurability, fabrication issues, slow response time, contact angle hysteresis (CAH), large area manipulation capability, and contamination of the liquid/solid surface. An ideal method is expected to perform reconfigurable droplet manipulation with fine control over the dynamics of the droplet under environmental friendly conditions, preferably in a stimulicontrolled manner. Strategies based on wettability gradient are commonly employed for the controlled droplet transport where the © 2017 American Chemical Society

Received: May 25, 2017 Accepted: July 27, 2017 Published: July 27, 2017 28046

DOI: 10.1021/acsami.7b07451 ACS Appl. Mater. Interfaces 2017, 9, 28046−28054

Research Article

ACS Applied Materials & Interfaces

Figure 1. Droplet manipulation over thermo-switchable surface using gradient forces. (a) Wettability switching behavior of PNIPAAm grafted surface both below and above its LCST. PNIPAAm exhibits LCST at 32 °C. (b) SEM image and temperature controlled wettability switching of PNIPAAm grafted flat smart surface and (c) textured smart surface. Textured smart surface exhibits ultralow contact angle hysteresis above LCST and tunable wettability and hysteresis under temperature gradient. Scale bar is 50 μm. (d) Forces acting on a water drop placed over a surface having both temperature and wettability gradient.

applying a temperature-induced wettability gradient is seldom explored because of its inherent high CAH.35 Recently, researchers have demonstrated directional spreading over a flat surface coated with PNIPAAm, where large CAH hindered the motion of the drops over the surfaces.35 However, ondemand macroscopic transport of droplets over the surface necessitates controlled tunability in WCA and CAH. Though in situ growth of PNIPAAm onto the rough surface may reduce the CAH under Cassie−Baxter condition, the droplet transport at higher speed with an external stimulus such as temperature remains a challenge.33,36 In a recent study, water droplet manipulation over spindle knot structure coated with PNIPAAm gel was demonstrated.37 However, the motion is limited to a few tens of micrometers with a very low velocity (about 0.1 μm/s) that is also under a combined action of physical gradient surface and high humidity and surface temperature. Moreover, this method is applicable only when the droplet base length is of the order of a few tens of micrometers, which limits its applicability for the manipulation of large drops (μL).

however high CAH limits its applicability to specific type of liquids.28,29 Even though this offers a remote control over the liquid manipulation, slow response time still remains a challenge.26,28,30 To mention a few, Ichimura et al. demonstrated light-controlled manipulation of oil drops, not applicable to water drops, also with a low velocity of the order of a few μm/s.28 In another instance, Monteleone et al. demonstrated water droplet manipulation over a polymer surface decorated with TiO2 nanorods with a velocity of about 1 mm/s.31 To realize this, the surface needs prolonged exposure to UV light (90 min), and the full reversal of the surface wettability requires dark treatment for several days. Another potential strategy that has been employed for wettability tailoring is to utilize the temperature dependent change in intrinsic properties of thermo-switchable materials.32 Poly(N-isopropylacrylamide) (PNIPAAm) is a well-known thermo-switchable polymer33,34 that exhibits temperature dependent water contact angle (WCA). PNIPAAm is reported to exhibit a lower critical solution temperature (LCST) at around 32 °C.34 Though PNIPAAm is extensively used for realizing smart wetting properties, droplet manipulation by 28047


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Figure 2. Surface wettability characterization of smart surfaces. (a) WCA of the fabricated surfaces at different temperatures. (b) Tilt angle of the surfaces at different surface temperatures. (c) Graph showing the WCA and tilt angle of both the flat and the structured (pitch = 50 μm) surfaces. Solid symbols (blue) and open symbols (black) represent the tilt angle and WCA, respectively. The data shown is the average of seven measurements with standard deviation as error bar. (d) Forces acting on the drop placed over the textured smart surface (pitch = 50 μm) having a temperature gradient of 1 °C/mm.

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RESULTS AND DISCUSSION A thin layer of PNIPAAm was grown on polydimethylsiloxane (PDMS) surfaces using benzophenone-initiated, UV-mediated graft polymerization (Experimental Section).39 Here, we used two types of PDMS surfaces for PNIPAAm grafting: flat as well as micropatterned surfaces (pitch 20 and 50 μm, Supporting Information, Figure S1). Herein, the PDMS patterned surfaces are made by following the approach explained elsewhere.40−42 The presence of PNIPAAm layer (thickness −0.8 ± 0.15 μm) was verified using Fourier transform infrared (FTIR) analysis (Supporting Information, Figure S2). Temperature dependent behavior of PNIPAAm molecules both below and above LCST (≈32 °C) is schematically shown in Figure 1a. The PNIPAAm grafted surfaces exhibit hydrophilicity below its LCST, whereas they exhibit hydrophobicity above the LCST (Figures 1b and c). However, a flat surface grafted with PNIPAAm has a high contact angle hysteresis (32 ± 1.3°), and the droplet resides onto the surface even above LCST, as shown in Figure 1b. A droplet on such a surface requires high tilt angle for the movement (38.35 ± 1.3°). On the other hand, a textured surface grafted with PNIPAAm (structured smart surface) exhibits a high WCA above the LCST (164.3 ± 3.6°) with an ultralow CAH (1−2°), as shown in Figure 1c. Both the surfaces exhibit similar behavior below the LCST. The major aim of this work is to realize temperaturecontrolled water droplet transport over the structured smart surface using wettability gradient forces. Here, the temperature gradient across the substrate between T2 and T1 (Figure 1d)

In contrast to the above works, herein we demonstrate fast and reconfigurable macroscopic transport of μL sized water drops over a PNIPAAm grafted textured surface by temperature controlled wettability gradient. Experiments reveal that the fabricated material exhibits three wetting modes: superhydrophobic with ultralow CAH (above 36 °C), hydrophilic with high CAH (below 30 °C), and a transition regime with temperature dependent WCA and CAH (between 30 and 36 °C), where a large change in WCA of about 100° is obtained. The term water contact angle (WCA) in this paper refers to the apparent water contact angle measured as we employ hierarchical micro- and nanoscale surfaces.38 We applied a temperature gradient along the surface in the intermediate regime and realized fast macroscopic transport of water drops with a transport velocity of approximately 40 cm/s. The force analysis matches well with the experimental results. It is also demonstrated that the transport velocity could be tuned either by varying the temperature gradient along the surface or by inclination angle of the surface. Further, we demonstrate a simple practical application where we show the merging of two droplets using temperature controlled wettability gradient. The material returns to its temperature dependent uniform wetting regime (superhydrophobic or hydrophilic) upon removing the temperature gradient. This work opens up new vistas to realize rewritable wettability patterns controlled by temperature for microfluidic applications such as macroscopic transport and directed merging of droplets. 28048


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Figure 3. Droplet transport over textured smart surface. (a) Schematic of the experimental setup for the droplet manipulation. (b) Macroscopic transport of a water drop using wettability gradient force. (c) Distance−time graph corresponds to the drop motion shown in panel b. Scale bar is 1 mm.

surface or chemical interaction between the surface and the liquid. Experimentally, we characterize the CAH of the surface by taking the difference between the advancing contact angle (θA) and the receding contact angle (θR). Hysteresis force can be described as46

results in a varied degree thermo-switching of the PNIPAAm layer, creating a nonlinear wettability gradient over the surface. Consequently, a droplet placed on such a surface experiences mainly three type of forces: force due to Marangoni stresses (FMarangoni), force due to wettability gradient (Fwettability), and the resistive force due to contact angle hysteresis of the surface (Fhysteresis). A nonuniform temperature distribution at the surface results in the surface tension gradient along the base radius of the droplet (R). Marangoni force acting on a drop due to surface tension gradient of dγ/dx can be expressed as43 FMarangoni = πR2

dγ dx

Fhysteresis = πRγ(cos θR − cos θA)

Thus, for a droplet to be transported along the gradient surface, the gradient driven forces should overcome the hysteresis force, i.e, Fnet > 0, where Fnet is the vector sum of the gradient and hysteresis forces. To realize this condition, we need a reconfigurable surface with large variation in WCA, keeping the CAH low. Figure 2 shows the WCA and tilt angle (TA) measurements on the flat as well as patterned PNIPAAm grafted PDMS surface. Here, the tilt angle is measured as the angle at which the droplet starts to move upon tilting the base substrate.47 It is pertinent to note that, as the temperature changes from room temperature (28 °C) to 40 °C, the WCA of water droplet (8 μL) changes from 63 ± 1 to 94 ± 2° for the flat surface (Figure 2a) with a reproducibility for more than 50 cycles. A sharp variation in WCA is observed at around 32 °C, which corresponds to the LCST of the PNIPAAm molecules. Even above the LCST, the CAH in this case is so high (≈32 ± 1.3°) that a tilt angle of 39° is required for the droplet movement (Figure 2b). The lowered value of WCA below LCST compared to that of pristine PDMS confirms that the water droplet responds to the thermoresponsive polymer, not to the PDMS substrate. The magnitude of the temperature-controlled hydrophilic to superhydrophobic transition is determined by the combined

(1)

where γ represents the surface tension of the drop at a fixed temperature. In the case of water, the surface tension exhibits an inverse relation with temperature (temperature coefficient ≈ −0.15 mN/m·K);44 thus, FMarangoni drives the drop toward the colder side of the substrate. The wettability gradient force due to the spatial variation in WCA along the length of the substrate can be written as35,45 Fwettability = πR2γ

dcos θ dx

(3)

(2)

Here, dcos θ/dx represents the spatial gradient of wettability. Fwettability drives the drop toward the more wettable side (colder side) of the surface. Thus, in the present case, both these gradient forces act in the same direction and initiate the selfpropulsion of the drop toward the colder side of the surface. The resistive force mainly comes from the contact angle hysteresis (Fhysteresis) which opposes the motion of the drop.46 This force arises due to the surface inhomogeneities of the 28049


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below this temperature, which could slow the motion of the drops. Assuming a linear temperature distribution, a temperature gradient of 1 °C/mm was obtained (Supporting Information, Figure S6). Please note, the wettability gradient is nonlinear because the temperature−WCA response of PNIPAAm is nonlinear (Figure 2a). We placed a water drop (8 μL) at the hot end and recorded its dynamics using a fast camera (at 440 fps). The advancing side of the drop starts to move, sensing the wettability gradient (Supporting Movie 2). Because the CAH is very low at the hot side, the receding edge of the drop easily detaches from the substrate, resulting in a macroscopic translation of the drop toward the colder side. The time−distance graph of the drop motion is shown in Figure 3c. The center of the droplet baseline was chosen as the tracking point. When the drop reaches the middle of the surface, the motion of the drop accelerates. A maximum instantaneous velocity of about 40 cm/s is observed when the drop moves in the second half of the gradient surface. The nonlinear velocity of the droplet motion is attributed to the nonlinear wettability behavior of PNIPAAm with temperature (Figure 2a). Wettability gradient is low in the first half (2.5°/mm between the temperatures 36 and 34 °C), while it increases drastically to 10°/mm in the second half (between the temperatures 34 and 32 °C). An average velocity of about 8 cm/s is achieved for a transport distance of 2.9 mm along the surface. The observed velocity is about two orders higher than that reported using temperature-gradient driven Marangoni flows.45,48 Upon removing the temperature gradient, the surface goes back to uniform wetting regime where the drop remains in its equilibrium contact angle (hydrophilic if T < LCST and superhydrophobic if T > LCST). We performed similar experiments with both acidic (1 M HCl) and saline (1 M NaCl) droplets, and the results are similar to those for pure water droplets (Supporting Information, Figure S7) For reconfigurable microfluidic operations, it is desired to have a fine control over the transport velocity of the drops. Here, we propose two ways to tune the droplet velocity: (1) by varying the steepness of the temperature gradient and (2) by varying the inclination angle of the surface (uphill motion). To uncover the influence of steepness of the temperature gradient on droplet transport, we performed experiments with four different temperature gradients, keeping the cold side at 32 °C. We selected 32 °C as lower cut off temperature because CAH drastically increases below this temperature, which could hinder the droplet motion. Details of the experimental parameters and corresponding results are shown in Table 1 and Figure 4a. The data shown in Table 1 is the mean of four individual measurements with the standard deviation as error bar. Time t = 0 s corresponds to the moment where the droplet touches the hot side (Thot).

effect of PNIPAAm switching and the surface texture. The textured surfaces before PNIPAAm grafting exhibit superhydrophobicity at all temperatures (Supporting Information, Figure S3), while textured surfaces grafted with PNIPAAm (with interpattern spacing of 50 and 20 μm) exhibit temperature dependent wettability switching. In contrast to the flat case, here, an increase in temperature above LCST leads to superhydrophobicity with water rolling off nature (ultralow CAH with a tilt angle ∼1−2°). As shown in Figure 2a, a maximum change of WCA of about 100° is achieved between 30 and 36 °C. A maximum slope of 25 °C is observed between temperatures 30 and 36 °C. The experimental observation indicates that below LCST, the behavior on the patterned surfaces are determined by PNIPAAm layer, whereas the synergic influence of roughness (Cassie−Baxter state)36 as well as water repelling behavior of PNIPAAm determine the rolling off nature of the water droplet above its LCST (Figure 2b). It is worthwhile to mention here that though patterned PDMS surfaces without PNIPAAm grafting exhibit water rolling off nature, it precludes the possibility of controlling the motion by an external stimulus such as temperature (Supporting Movie 1). In addition to the large variation in the WCA from hydrophilic to superhydrophobic by tuning the temperature of the substrate to switch the PNIPAAm molecules, the present study demonstrates the hitherto unreported control over CAH of the PNIPAAm surface by the temperature (Figure 2b). PNIPAAm grafted surfaces with a pitch of 50 μm showed a better performance (lower TA and larger WCA change) in comparison with that of 20 μm. Above the LCST, the grafted surfaces exhibit Cassie−Baxter behavior,33 where the projected solid−liquid contact area determines the superhydrophobic behavior. The projected contact area fraction in the case of surfaces with a pitch of 50 μm is approximately 0.2, while that of 20 μm is approximately 0.5. In short, lower projected contact area leads to a larger fraction of trapped air bubbles, which enhances the WCA and decreases the CAH. This could explain why surfaces having a pitch of 50 μm show better performance. Owing to the better performance, for the droplet transport experiments, we selected the PNIPAAm grafted surfaces with a pitch of 50 μm (Figure 2c). First, we analyzed the forces acting on a water drop placed at the smart surface (pitch = 50 μm) having both temperature and wettability gradient (Supporting Information, Force Analysis, Supporting Figures S4 and S5). Assuming that the surface remains horizontal, we neglect the influence of gravity. Figure 2d shows the forces acting on the drop on a textured surface with a temperature gradient of 1 °C/mm. It is evident that the total force acting on the drop is maximum at 32 °C and then decreases. For a droplet to move along the surface, the net force should be positive. We define a transport zone between the temperatures 31 and 36 °C where the net force is greater than zero. This means, in this particular case, the droplet will stop at the position where temperature is 31 °C. Above 36 °C, the WCA variation with temperature is negligible, so the net force is nearly zero. It is clear from Figure 2d that the dominating force for the droplet transport is wettability gradient force, and the influence of Marangoni force is weak. To experimentally realize droplet transport as predicted by the force analysis, a setup shown in Figure 3a was implemented (Experimental Section). We used a smart textured surface 4 mm in length and applied 32 °C on one end and 36 °C on the other, as shown in Figure 3a. We selected 32 °C as the lower cut off because the total force starts to decrease drastically

Table 1. Experimental Parameters and Results Corresponding to the Experiments with Different Temperature Gradientsa Thot (°C)

Tcold (°C)

TG (°C/mm)

VM (cm/s)

VA (cm/s)

36 35 34 33

32 32 32 32

1 0.75 0.5 0.25

43.1 ± 4.27 33.05 ± 6.06 23.4 ± 7.7 11.9 ± 2.82

9.15 ± 0.96 11.34 ± 0.49 14.4 ± 0.37 8.01 ± 0.31

a

TG, temperature gradient; VM and VA, maximum and average velocity, respectively. 28050


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Figure 4. Textured smart surface as a versatile droplet manipulation platform. (a) Distance traveled as a function of time of an 8 μL droplet placed at four different temperature gradients: 1 °C/mm (Thot = 36 °C, Tcold = 32 °C, black symbols), 0.75 °C/mm (Thot = 35 °C, Tcold = 32 °C, green symbols), 0.5 °C/mm (Thot = 34 °C, Tcold = 32 °C, blue symbols), and 0.25 °C/mm (Thot = 33 °C, Tcold = 32 °C, red symbols). In all cases, droplet hits the surface at t = 0 s at the position of Thot. (b) Uphill transport of a water drop placed at a temperature gradient of 1 °C/mm (Thot = 36 °C, Tcold = 32 °C). (c) Temperature-controlled targeted merging of water drops placed over the textured smart surface at a temperature gradient of 1 °C/mm (Thot = 36 °C, Tcold = 32 °C). Scale bar is 1 mm.

These observations confirm that the motion of the drop in the presence of the temperature gradient is due to the wettability gradient forces. Experiments performed with an inclination angle 20° or above resulted in a motion in the direction of the gravity, which indicates that wettability gradient is not sufficient to direct the drop upward. As a simple attempt to real world applications, we used the fabricated smart surface for targeted merging, as shown in Figure 4c. Temperature regime and the steepness of the temperature gradient were kept the same as in the case of uphill transport. We placed two drops: one on the cold side (A) and the other on the hot side (B). The droplet placed at the cold side spreads upon impact because of low WCA at this temperature. The droplet placed at the hot side (B) experiences the wettability gradient and moves toward the other drop, eventually merging (Supporting Movie 4). To avoid the confusion with the gravity driven motion, we performed these experiments at an inclination angle of about 2°, and the droplet motion was observed in the uphill direction (against gravity). An average velocity of about 8 cm/s (for the drop B) is obtained in this case.

In the experiments corresponding to the data shown in Table 1, we used a patterned surface with 4 mm length (l) and a water drop with a volume of 8 μL. Here, a temperature gradient results in a corresponding wettability gradient. We define wettability gradient WCAG as (WCAhot − WCAcold)/l, and the same is extracted from Figure 2c. It is evident that the maximum velocity increases upon increasing the WCAG. A maximum velocity of about 40 cm/s is obtained when WCAG equals to 6.75°/mm (TG = 1 °C/mm), while it decreases to approximately 10 cm/s for the case of WCAG equal to 1.75°/ mm (TG = 0.25 °C/mm). To uncover the influence of the nonlinear wettability gradient on the droplet transport, we plot a distance−time graph for all the TG cases considered here (Figure 4a). A nonlinear behavior is observed in the cases where Thot is between 36 and 34 °C, and the possible underlying mechanism of this observation is discussed in Figure 3c. Strikingly, a nearly linear behavior is observed in the case where we keep Thot at 33 °C. This observation is attributed to the linear WCAG between the temperatures 33 and 32 °C, as evident from Figure 2c (fitted lines). In this case, VA is nearly equal to VM. Next, we challenged our fabricated smart surfaces for droplet transport against gravity (uphill motion). For this, we performed experiments by keeping the smart textured surface at different inclination angles, applying the temperature gradient along the uphill direction and keeping the hot side down. Temperature regime and the steepness of the temperature gradient were kept the same as in the case of the horizontal surface (Figure 3b). Time-lapse images of the droplet transport on a surface kept at an inclination angle of about 15° are shown in Figure 4b (Supporting Movie 3). As we can see, the droplet climbs but with a reduced velocity (0.37 mm/s) compared to that of the horizontal situation. The reduced velocity is attributed to the influence of gravity. As control experiments, we carried out experiments at fixed temperatures both below and above the LCST, keeping the inclination angle fixed at 15° (Figure 2b). In the case below LCST, the droplet just spreads over the surface, while it rolled off in the direction of gravity in the case of above LCST (a direction opposite to that of wettability driven transport).

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CONCLUSION In summary, we demonstrate the potential of PNIPAAm grafted textured surface for reconfigurable microfluidic operations such as macroscopic transport and directed merging. The fabricated polymer surface exhibits temperature controlled WCA and CAH: superhydrophobic with rolling off nature (above 36 °C), hydrophilic with high CAH (below 30 °C), and a transport zone with tunable WCA and CAH. Force calculations in the transport zone reveal that the gradient forces acting on the drop overcome the hysteresis force. We explored this regime for realizing droplet manipulation platform controlled by temperature-induced wettability gradient. A maximum transport velocity of about 40 cm/s is achieved at a temperature gradient of 1 °C/mm. The droplet velocity could be tuned by varying either the temperature gradient or tilting angle. Furthermore, targeted merging of two droplets placed at different positions is realized by properly tuning the temper28051



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ature distribution. The presented results demonstrate the versatility of the fabricated soft smart surface for reconfigurable droplet manipulations, which we expect to find applications in microfluidics, biochemical analysis, and targeted drug delivery.

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Research Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b07451. Figure S1, SEM images of the surfaces before and after PNIPAAm grafting; Figure S2, FTIR spectra of PNIPAAm grafted sample; Figure S3, WCA vs T for uncoated superhydrophobic surfaces; Figures S4 and S5, force analysis; Figure S6, temperature mapping; Figure S7, droplet transport of acidic and saline droplets; Figure S8, schematic of PNIPAAm grafting (PDF) Supporting Video 1: Rolling nature of uncoated superhydrophobic surface (MPG) Supporting Video 2: Fast transport of water droplet under temperature gradient (corresponds to Figure 3b) (MPG) Supporting Video 3: Uphill transport of water droplets (corresponds to Figure 4b) (MPG) Supporting Video 4: Targeted merging of water droplets (corresponds to Figure 4c) (MPG)

EXPERIMENTAL SECTION

Fabrication of the Structured PDMS Surface. Poly(methyl methacrylate) (PMMA) master structures with the required interpattern spacing (50 and 20 μm) were prepared using laser patterning (pulse duration 50 fs, center wavelength 800 nm, repetition rate 5.1 MHz, scanning speed 1 mm/s, energy 200 nJ).37 A femtosecond laser was used here (Ti:sapphire femtosecond oscillator, Femtolasers, Austria). The replication of the master structures onto PDMS was done using soft-lithography approach.37 The components of Sylgard 184 PDMS were taken in the 10:1 ratio and mixed thoroughly. The air bubbles were removed by keeping the solution in the vacuum desiccator for an hour. Then, the solution was poured onto the PMMA master structure and treated at 75 °C for 45 min. The solidified PDMS structures were carefully peeled off and used for further studies. The average width and height of the grid wall was 6 and 4 μm, respectively. Benzophenone-Initiated, UV-Mediated Graft Polymerization. A thin layer of PNIPAAm was grown on PDMS surfaces using benzophenone-initiated, UV-mediated graft polymerization.34 Schematic of the overall procedure is depicted in Supporting Figure S8. The patterned PDMS substrates were thoroughly cleaned with deionized water (Millipore) and immersed in the solution I (20 wt % benzophenone + acetone) for 5 min; as a result, the photoinitiator (benzophenone) gets adsorbed onto the surface and creates reactive sites. Then, the substrate was cleaned with deionized water for 5 min and dipped in solution II (10%(w/v) NIPAAm + 0.5 mM NaIO4 + 0.5 wt % benzyl alcohol) for 30 s. Then, the substrate was rotated at 2000 rpm for 30 s (SPIN 150, APT GmbH) to get a very thin layer of monomer (NIPAAm) over the structured surface. Then, the substrate was irradiated with the UV-light emitted from a high-pressure mercury lamp (Model 66905, Power 195 W, Newport) at a lamp-substrate distance of about 20 cms for 10 min. As a result, the NIPAAm molecules get covalently bonded at the previously created reactive sites. After the photopolymerization, the substrate was sequentially rinsed with deionized water and acetone to remove the unpolymerized NIPAAm. Instrumentation. The schematic of the experimental set up shown in Figure 3a consists of a high speed camera (DCC1545M, Thorlabs), syringe pump (HO-MMSP-02, Holmarc), cold water bath (RW0525G, Lab Companion), hot plate (RCT Basic, IKA), temperature measurement system (T probe and 34970A data acquisition system, Agilent), and LED light source (HO-HBL1-10W, Holmarc). All the components were placed on an optical table. To apply a temperature gradient along the length of the surface, the sample was kept bridged between two copper pads, one of which is projected from a heating stage and other connected to a cold water bath. By properly controlling the temperature of the heating stage as well as the cold water, the desired temperature gradient could be applied across the sample. The temperature of the surfaces was measured using a T type temperature probe connected to the temperature measurement device. Once the desired temperature gradient attained a steady state, a water drop was placed over the surface using the syringe pump. The droplet motion was captured using a fast camera at 440 FPS. The droplet tracking was performed using Mtrack plug-in in ImageJ. The center of the droplet baseline was chosen as the tracking point. The WCA was measured using DropSnake plug-in in ImageJ. A custom build setup was implemented for tilting the surface for the measurement of CAH. After each trial, the sample was kept in vacuum chamber at 80 °C for 30 min to remove any moisture that was left on the PNIPAAM grafted structured surface. The SEM images were taken using a HITACHI SU 6600 instrument. A thin layer of gold was sputtered over the surface for 30 s using HITACHI E1010. FTIR measurements were taken using Frontier MIR, PerkinElmer.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Theneyur Narayanaswamy Banuprasad: 0000-0003-3935-372X Thamarasseril Vijayan Vinay: 0000-0001-7685-4430 Cherumannil Karumuthil Subash: 0000-0002-6080-6028 Subramanyan Namboodiri Varanakkottu: 0000-0001-7024-3882 Author Contributions

S.N.V. developed the concept of droplet transport over textured smart surfaces. T.N.B., T.V.V., and S.N.V. performed the experiments and analyzed the data. S.D.G. contributed with the textured surfaces, also in the interpretation of the experimental results. C.K.S. and S.V. contributed with polymer fabrication and SEM analysis. S.N.V. wrote the manuscript with the contributions from S.D.G., T.N.B., and T.V.V. All authors have given approval to the final version of the manuscript. T.N.B. and T.V.V. contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.N.V. greatly acknowledges funding from Department of Science and Technology, India through INSPIRE Faculty award (Award 2016/DST/INSPIRE/04/2015/000544). T.N.B. and T.V.V. acknowledge financial support from MHRD, India. Authors acknowledge Rahul TK (School of Nano Science and Technology, NIT Calicut) for helping with the UV lamp.

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REFERENCES

(1) Baigl, D. Photo-Actuation of Liquids for Light-Driven Microfluidics: State of the Art and Perspectives. Lab Chip 2012, 12, 3637− 3653. (2) Jebrail, M. J.; Bartsch, M. S.; Patel, K. D. Digital Microfluidics: A Versatile Tool for Applications in Chemistry, Biology and Medicine. Lab Chip 2012, 12, 2452−2463. (3) Hu, S.-W.; Xu, B.-Y.; Ye, W.; Xia, X.-H.; Chen, H.-Y.; Xu, J.-J. Versatile Microfluidic Droplets Array for Bioanalysis. ACS Appl. Mater. Interfaces 2015, 7, 935−940.

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ACS Applied Materials & Interfaces (48) Eifert, A.; Paulssen, D.; Varanakkottu, S. N.; Baier, T.; Hardt, S. Simple Fabrication of Robust Water-Repellent Surfaces with Low Contact-Angle Hysteresis Based on Impregnation. Adv. Mater. Interfaces 2014, 1, 1300138.

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