Wetting Phenomena on (Gradient) Wrinkle Substrates - Langmuir

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Wetting Phenomena on Langmuir is published by the American Chemical (Gradient) Wrinkle Substrates Society. 1155 Sixteenth Street N.W., Washington, DC 20036

Stephanie Hiltl, and Alexander Böker Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright Subscriber access provided by Northern Illinois University claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Langmuir, Just Accepted Manuscript • DOI: 10.1021/ acs.langmuir.6b02364 • Publication Date (Web): 12 Aug 2016 Downloaded from http://pubs.acs.org on August 15, 2016 Langmuir is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright Subscriber access provided by Northern Illinois University claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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Wetting Phenomena on (Gradient) Wrinkle Substrates Stephanie Hiltl† and Alexander Böker∗,†,‡ Fraunhofer-Institut für Angewandte Polymerforschung, D-14476 Potsdam-Golm, Germany, and Lehrstuhl für Polymermaterialien und Polymertechnologien, Universität Potsdam, D-14476 Potsdam-Golm, Germany E-mail: [email protected]

Abstract We characterize the wetting behaviour of nanostructured wrinkle and gradient wrinkle substrates. Different contact angles on both sides of a water droplet after deposition on a gradient sample induce self-propelled motion of the liquid towards smaller wrinkle dimensions. Droplet motion is self-limited by the contact angles balancing out. Due to correlation between droplet motion and contact angles, we investige the wetting behaviour of wrinkle substrates with constant dimensions (wavelengths of 400-1200 nm). Contact angles of water droplets on those substrates increase with increasing dimensions of the underlying substrate. The results are independent of the two measurement directions - parallel and perpendicular to the longitudinal axis of the nanostructure. The presented findings may be considered for designing microfluidic or related devices and initiate ideas for development of further wrinkle applications. ∗ To

whom correspondence should be addressed für Angewandte Polymerforschung, D-14476 Potsdam-Golm, Germany ‡ Lehrstuhl für Polymermaterialien und Polymertechnologien, Universität Potsdam, D-14476 Potsdam-Golm, Germany † Fraunhofer-Institut

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Introduction

Gradient substrates are substrates which change their chemical and physical properties over a certain distance or area. Many chemical and biological processes are guided by gradients: Crystal nucleation and growth, movement of bacteria towards nutrients or generation of electrical energy from batteries. 1 Chemical gradients also influence wettability properties of such surfaces and are also known to induce directed motion of liquid droplets. 2 This particular wetting phenomenon represents energy transduction - from surface energy to mechanical energy - and is important for designing microfluidic 3,4 and analytical devices 5,6 as well as planar, patterned liquid pumps. 7 Additionally, droplet motion on gradient substrates helps understanding migration 8,9 and adhesion 10,11 processes of biological cells 12 and especially muscle cells. 13 In general, droplet motion results from a chemical, 14–16 thermal 17,18 or shape gradient. 19 Also radial shapes are mentioned in the literature. 20 Recently, droplet movement was also induced by vibration 21 and reported for microstructured surfaces. 22,23 Often, motion is driven by a reaction taking place at the interface of a solid support and a liquid drop (adsorption, desorption). Either a monolayer of silane, initially dissolved in the droplet, forms on the substrate upon movement 24 or the droplet takes up tenside molecules from the substrate surface. 25,26 Redox-active surfactants provide precise electrochemical control of the liquid position. 27 For long chain alkanes close to their melting point droplet movement is rendered reversible. 28 Those surface reactions gradually change the physical properties of the surface and keep the droplet in motion. Experimental data is supported by several dynamic models 29,30 identifying a wetting gradient as driving force for droplet motion. Here, we present self-propelled motion of a water droplet directed along a dimensional gradient on a nanostructured wrinkle substrate. Droplet movement results from an imbalance of interfacial energy at both droplet ends. We correlate unequal contact angles of the droplet edges with the spacing of the underlying nanostructured substrate. Additionally, we investigate the wetting behaviour of wrinkle substrates to correlate apparent contact angles with characteristic substrate dimensions. We prepare nanostructured substrates by taking advantage of surface wrinkling 31,32 occuring as a form of strain release in bilayers where each layer comes with different mechanical properties. Our system consists of a stretched polydimethylsiloxane (PDMS) fundament on which a rigid SiOx layer is generated by plasma oxidation. Upon strain release sinusoidal wrinkles form on the substrate surface. 33 Wrinkle dimensions - amplitude A and wavelength λ - depend on the thickness h of the oxide layer. Precisely tuning mechanical properties of PDMS 34 or controlling h by a shielding mechanism lead to gradient wrinkles substrates whose detailed characterization of is described elsewhere. 35 In general, the gradient wrinkles exhibit amplitudes ranging from 7-230 nm and wavelengths between 250 nm and 900 nm.


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Wrinkles are frequently used for assembly of hard and soft colloids 36,37 or bioparticles. 38–40 They are further utilized for evaluation of mechanical properties of thin films 41,42 and applied as tunable adhesive. 43 Literature dealing with wetting phenomena - contact angle measurements and droplet movement - on (gradient 44 ) wrinkles 45,46 is rare and mostly restricted to micronsized structures. One reason for this may be that the wetting behaviour of chemically or topographically patterned surfaces, 47 rough samples and surfaces with gradients demands for considerations exceeding the Young equation (see Eq. (1)). Where γsv = γsl + γlv cos θ

(1)

with the interfacial tension between solid and vapor (γsv ), solid and liquid (γsl ) and liquid and vapor (γlv ) phase. Eq. (1) is valid for ideal (smooth, rigid, insoluble, chemically homogeneous, non-reactive) surfaces and describes a three phase wetting system and the contact angle θ of a fluid on a solid support. 48,49 On rough surfaces no relation exists between the experimentally observed contact angle θapp and the Young contact angle θ . To overcome this problem, models take surface roughness 50 or porosity and heterogeneity 51 into account. A liquid droplet then either completely wets the surface or sits on structural maxima of the surface enclosing air pockets. By applying an external stimulus 52,53 or tuning the structure, wetting behaviour changes from hydrophilic to hydrophobic. 54 Both approaches promote the design of superhydrophobic surfaces. 55 With our research we want to contribute to characterization of nanostructured substrates, especially wrinkles and gradient wrinkles, regarding to their wetting behaviour. We discuss the wetting behaviour of those structures with low aspect ratio and special features - droplet motion on gradient substrates. Our findings may be considered for the design of analytical or microfluidic devices and open new application fields for wrinkle substrates.

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Materials and Methods

For the preparation of PDMS substrate, monomer and crosslinker of Sylgard 184 Elastomer Kit purchased from Dow Corning are mixed in a weight ratio of 10:1 and poured into a levelled polystyrene petridish up to a thickness of 2.5 mm. After degassing overnight at ambient conditions, PDMS is crosslinked for 2 h at 80 ◦ C. After cooling and removing the edges, the substrate is cut into pieces of 5 mm width and 30 mm length. Wrinkles with constant wavelength and amplitude are prepared by clamping a PDMS substrate in a custom-made stretching apparatus (see literature 56 and Figure 1) and extending it to 130%


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of the original length. After exposure to air plasma (Plasma Activate Flecto 10 USB, 100 W, 0.2 mbar) for 30-450 s the sample is relaxed and placed on a glass support for further use.

Figure 1: Photograph of stretching apparatus used for wrinkling of PDMS substrates. For gradient wrinkles a silicon wafer is placed on the middle of a clamped and stretched sample. At one end the wafer directly rests on the sample, while at the other end a spacer generates a distance of 1 mm between wafer and sample. After plasma treatment (conditions mentioned above, 300 s) the wafer is removed prior to sample relaxation. Details are reported in literature. 35 Wrinkles and gradient wrinkles were characterized by AFM (Bruker Dimension ICON with OTESPA tips with resonance frequency of 278-357 kHz and spring constant of 12-103 N/m). All wetting experiments were carried out in sessile drop geometry and with Mili-Q-water as dispensed medium on a DSA 100 tensiometer from KRÜSS with a blunt end canula (diameter 0.517 mm). Contact angles were measured on planar untreated and plasma treated PDMS as well as linear wrinkles. Droplets of 3 µl were dispensed on the sample prior to video capturing for 30-60 s. Contact angles were extracted from videos with Laplace fits. At least 5 spots per sample were evaluated. For wrinkle substrates contact angles detected parallel and perpendicular to the wrinkle direction were distinguished. For wetting experiments on gradient wrinkles, droplets of 2 µl were inflated and then slowly brought in contact with the surface. Their behaviour was recorded in 60 s videos. Measurements were performed parallel to wrinkle direction. Samples were dried with compressed air between individual measurements. Contact angles were extracted from the videos with Laplace fits. Figure 2 provides an overview of the gradients on the substrate and the conducted experiments. As reported, 35 gradient samples exhibit an amplitude-wavelength-gradient (orange triangle) at the edge between shielded and unshielded area. Additionally, an amplitude-gradient (blue triangle) is established in the shielded area of the sample. There, wavelength is constant within the error range (300 to 330 nm), while amplitude increases from 7 nm to 40 nm. Experiment 1 takes place at amplitude-wavelength-gradients, while experiment 2 takes place at amplitude-gradients.


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Figure 2: Sketch of gradient sample with positions of water droplets (dark blue, numbered experiments for later reference) for wetting experiments. Orange triangle represents amplitudewavelength-gradient, blue triangle depicts amplitude-gradient.

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Results and Discussion

Contact angle measurements reveal unique wetting features of gradient wrinkles substrates. We use gradient substrates prepared and characterized according to literature. 35 In short, the substrates exhibit an amplitude-wavelength-gradient extending over a few milimeters. Amplitudes between 7 nm and 230 nm and wavelengths between 300 nm and 900 nm are covered on one sample. Figure 3 shows a sketch of the experiment (Experiment conducted on amplitude-wavelength-gradient as described in Figure 2 experiment 1.). The water droplet moves along the wrinkle gradient towards smaller wrinkle dimensions. The gradient extends over a length of twice the droplet diameter (2 mm). As it is not possible to determine the wrinkle dimensions underneath the droplet during the contact angle measurement, AFM height images provide only an estimation of the dimensions in the respective area. AFM images were obtained during characterization of the gradient sample prior to observation of droplet movement. We observe two special wetting features upon deposition of a water droplet on a gradient wrinkle sample. During deposition the water droplet ’jumps’ sideways, which is followed by droplet movement along the gradient. Figure 4 compares deposition of a water droplet on a planar, plasma treated PDMS (A) with deposition on a wrinkle substrate with constant wavelength of 550 nm (B) and a gradient substrate (C). In all cases the droplet is inflated above the sample surface. Deposition occurs by approaching the sample towards the droplet and subsequent withdrawal with the droplet resting on the substrate. For the planar, plasma treated PDMS substrate (A) a contact angle of 80.0 ◦ forms at left and right three phase contact point. The symmetrical droplet rests directly below the canula. On the wrinkle substrate the contact angle changes to 117.4 ◦ for both three phase contact points due to the nanostructure on the substrate surface (B). Still, the droplet stays centered below the canula. In contrast, if an experiment is conducted according to , the droplet on the gradient sample (C) ’jumps’ to the left with regard to the initial position of the inflated droplet marked by blue bars in Figure 4 (Experiment conducted as described in Figure 2 experiment 1.). The left side is translated by 0.6 mm, while the right side shows a shift of 0.2 mm. This ’jump’ occurs upon first contact


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Figure 3: Sketch of wrinkle gradient with water droplet moving towards smaller wrinkle dimensions. Gradient extension is approximately twice the droplet diameter (2 mm). AFM height images (∆z from left to right: 80 nm, 150 nm, 300 nm) with cross sections provide an estimation of wrinkle dimensions in respective areas. White bars in AFM images mark positions of cross sections. between droplet and substrate. Additionally, the droplet exhibits an asymmetrical shape due to a large contact angle difference between both three phase contact points (left: 104.3 ◦ ; right: 90.5 ◦ ). As this effect is observed neither for planar, plasma treated PDMS nor wrinkles with uniform dimensions, it is induced by the dimensional gradient in the contact area of drop and gradient substrate. After deposition, the droplet moves along the gradient towards smaller wrinkle dimensions without external agitation. Movement is perpendicular to wrinkle direction. Figure 5 shows a time resolved (44 s) study of droplet motion with regard to the position of the inflated droplet (black lines) and position of the deposited drop (red lines). Individual images were taken from a video capturing the wetting behaviour. Initially, left and right contact angle of the droplet are 104.3◦ and 90.5◦ , respectively. During movement both contact angles decrease to 89.7◦ (left) and 87.0◦ (right). Meaning that the difference between both contact angles is reduced from 14◦ at the beginning to only 2.7◦ after 44 s. Driving forces 46 for droplet movement may be reduction of contact angle hysteresis and increased surface wetting represented by lowered contact angles and equilibrium state of the drop. Slip-stick behaviour 48 governs motion of the left droplet side as it pins to the underlying nanostructure. In order to move from one pinning point to the next the drop has to overcome an energy 6 ACS Paragon Plus Environment

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Figure 4: Deposition of water droplet on (A) planar, plasma treated PDMS substrate with contact angle of 80.0◦ , on (B) wrinkles with uniform dimensions (λ =550 nm) with contact angle of 117.4 ◦ and on (C) gradient sample with different contact angle on left (104.3◦ ) and right (90.5◦ ) side. Blue lines indicate central position of inflated droplet.


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Figure 5: Time resolved motion of water droplet on gradient sample. Black lines indicate positon of inflated drop. Red lines indicate starting position of drop after deposition on the substrate. barrier. At the same time the contact angle is gradually lowered. The right droplet side behaves differently. It moves continuously over the substrate without pinning to the nanostructure. A minor reduction of drop height during movement may be due to water evaporation. Short measurement time and room temperature conditions limit this effect to a minimum. Additionally, droplets with spherical footprint may slightly distort by extending along wrinkle grooves. This anisotropic wetting is reported in literature for (chemically) nanopatterned and corrugated surfaces. 57–59 As the amplitude-wavelength-gradient extends perpendicular to wrinkle direction, droplets experience changes in contact angles and movement perpendicular to wrinkle direction. In the other direction, along wrinkle direction, substrate dimensions remain constant. Consequently, the droplet rests without the substrate inducing directed motion or contact angle changes. The video provided as supplementary information shows no droplet movement towards or away from the camera (out of focus plane), which would relate to droplet motion along wrinkle direction. We evaluate droplet movement over time regarding velocity, covered distance and change in


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Figure 6: Time dependent velocity (A), distance (B) and contact angle (C) of droplet motion on gradient sample. Data points for right droplet side are red, while data points for left droplet side are black. contact angles. Results are depicted in Figure 6. For each of this motion parameters both droplet sides are considered individually. We include the distance covered by the jump to contact of the droplet in our evaluation. Position of the inflated droplet above the substrate is defined as starting point of the motion (see black lines in Figure 5). Velocity is determined in relation to this starting point. Drop velocity (Figure 6 A) is highest for both sides directly after deposition, although the 9 ACS Paragon Plus Environment

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left drop side moves with more than twice the velocity of the right side. Within three seconds velocity decreases drastically for both sides, then almost stays constant in the remaining time. Consequently, the distance plot (Figure 6 B) shows the left drop side moving twice as far as the right side: 1 mm versus 0.5 mm. The higher left contact angle and the higher velocity lead to the conclusion that balancing out contact angles is the driving force for droplet motion. The experiment described above was performed on a wavelength-amplitude-gradient proceeding perpedicular to the wrinkle direction (Experiment conducted as described in Figure 2 experiment 1.). In contrast, we also deposited droplets on amplitude-gradients proceeding along the wrinkle direction (Experiment conducted as described in Figure 2 experiment 2.). In the latter case, the droplets show no motion, neither parallel nor perpendicular to wrinkle direction. Also no jump occured upon establishing first contact between droplet and sample. On the amplitudegradient the droplet is centered directly under the canula. As wrinkle wavelength is constant along the amplitude-gradient no contact angle hysteresis is observed for left and right contact angle. Consequently, no driving force acts on the droplet or induces directed motion. This control experiment with a resting drop (experiment 2) shows that a wavelength difference (as provided in experiment 1) in the contact area of drop and substrate is crucial for a contact angle difference between left and right droplet side. Only the non-euqilibrium state of contact angle hysteresis forces the droplet to move. Due to the correlation between contact angle difference and droplet motion, we also investigate the wetting behaviour of wrinkles with uniform dimension. Anisotropic wetting of nanostructured substrates 57–59 results in different contact angles parallel and perpendicular to the wrinkle grooves. Contact angles for both directions were measured, averaged and plotted with trendlines against the wavelengths of the nanostructured substrates as shown in Figure 7. We observe increasing contact angles for both measurement directions. For perpendicular measurements the slope is higher than for parallel measurements. Also, higher scattering of the values was found perpendicular to the wrinkle direction. This is probably due to elongation/spreading of the water droplet along the grooves of the nanostructure and leads to larger error bars. Nevertheless, the results show that the water contact angle on wrinkles depends on the dimensions of the underlying structure. Surprisingly, the trend of increasing contact angles for increasing wavelengths on substrates with uniform dimensions opposes our findings on gradient substrates. On amplitude-wavelength-gradients a droplet exhibits a large contact angle (left: 104.3 ◦ ) for small wavelengths and a small contact angle (right: 90.5 ◦ ) for large wavelengths. A possible explanation may be provided by the different states of water droplets after deposition. A sessile water droplet deposited on uniform wrinkles is in an equilibrium state, as both contact angles are equal and the drop is not distorted by the deposition or the canula. This is not the case for a water droplet on a gradient substrate: during deposition the droplet is already deflected to one side and finally 10 ACS Paragon Plus Environment

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Figure 7: Contact angles versus wrinkle wavelength for measurements parallel (A) and perpendicular (B) to wrinkle direction. Trendlines show increasing contact angles with increasing wrinkle wavelengths. jumps into contact (see Figure 4 (C)). The resulting situation resembles the case of advancing and receding contact angles. Due to this unstable, non-equilibrated state contact angles conflicting the above mentioned correlation are detected. The resulting contact angle imbalance in left an right three-phase-point provides sufficient driving force inducing droplet motion until both contact angles equalize. With small wavelengths correlating to small contact angles, the final contact angles are again in agreement with the ones expected from measurements on uniform wrinkle substrates.

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Conclusion

We observe and evaluate self-propelled motion of a water droplet on nanostructured wrinkle substrates exhibiting a dimensional gradient. The gradient proceeds over several milimeters and guides the droplet towards smaller wrinkle dimensions. Motion originates from different contact angles (∆θ =14◦ ) at the three phase contact points of the droplet. The droplet migrates over a distance of approcximately 1 mm with maximum speed directly after deposition. With time droplet velocity decreases as the contact angle difference is constantly reduced during motion. If both contact angles are almost even, the droplet stops moving due to the self-limitation of the process. Besides droplet motion, we determine water contact angles on uniform wrinkles with wavelengths between 400 nm and 1.2 µm. Due to anisotropic spreading of water droplets on nanostructured substrates, we perform separate measurements parallel and perpendicular to the wrinkle direction. Contact angles slightly increase with increasing dimensions of the underlying substrate. The measurement direction - parallel or perpendicular in respect to the wrinkle grooves - has no 11 ACS Paragon Plus Environment

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influence on this general trend. Our results may be useful to create microfluidic devices without walls or channels but for droplets moving along predetermined trajectories. Additionally, droplets could transport reagents or drugs or basically deliver small amounts of water to specific positions. As droplet motion is purely induced by the surface structure without further chemical modification necessary, also bioinspired applications like sensors or drug delivery systems are possible.

Supporting Information Available Video of water droplet moving on gradient wrinkle sample. This material is available free of charge via the Internet at http://pubs.acs.org/.

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Wetting Phenomena on (Gradient) Wrinkle Substrates - Langmuir

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