Article pubs.acs.org/Langmuir
Droplet Motion on a Shape Gradient Surface Yanfen Zheng, Jiang Cheng,* Cailong Zhou, Haiting Xing, Xiufang Wen, Pihui Pi, and Shouping Xu School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, People’s Republic of China S Supporting Information *
ABSTRACT: We demonstrate a facile method to induce water droplet motion on an wedge-shaped superhydrophobic copper surface combining with a poly(dimethylsiloxane) (PDMS) oil layer on it. The unbalanced interfacial tension from the shape gradient offers the actuating force. The superhydrophobicity critically eliminates the droplet contact line pinning and the slippery PDMS oil layer lubricates the droplet motion, which makes the droplet move easily. The maximum velocity and furthest position of droplet motion were recorded and found to be influenced by the gradient angle. The mechanism of droplet motion on the shape gradient surface is systematically discussed, and the theoretical model analysis is well matched with the experimental results.
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INTRODUCTION Droplet microfluidics technology continues to attract considerable research interest due to many potential applications such as DNA sequencing,1 single-cell analysis,2,3 and microchemical reactors4 where minimal consumption of reagents can be performed. Recent efforts have been made to induce droplet motion onto a solid surface based on unbalanced interfacial tension via chemical,5−7 thermal,8 thermocapillary,9 photochemical,10 and electrokinetic11 methods. Popular approaches also realize droplet motion by spatially altering surface shape through physical texturing,12−15 chemical patterning,16−19 or a composite interface of both.20 Commonly irregular shapes are patterned such as wedge13,14,17,21−23 and star shapes24 which contain interfacial tension gradients so that droplets can be transported in a predetermined direction. Contact angle hysteresis is the major hurdle for droplet motion which decelerates or even stops droplet motion. Although a droplet can wet and rapidly spread onto the crevices of structured surfaces due to capillary force, the back end of a droplet pins at the gradient’s start,13,14,21−23 causing surface contamination by impurities and reagent loss. Grafting polymer chains onto surfaces25,26 or slippery liquidinfused porous surfaces (SLIPS)27−31 were reported for eliminating pinning spots on the surface and obtaining a low contact angle hysteresis. Either air or infused liquid is locked in solid surfaces and blocks the contact of other liquids (droplets) with the underlying solid so that discrete droplets move smoothly. Similarly, lubricating oil film is also used in electrowetting in microfluidic devices.32,33 The droplets are commonly manipulated in a continuous phase of immiscible oil, where oil has several roles including facilitating the isolation of biomolecules, lubricating droplet motion, and eliminating crosscontamination caused by surface interactions. © 2017 American Chemical Society
Since the irregular shape surface can provide an interfacial tension gradient and hence induce droplet spreading, while oil film can exhibit a low contact angle hysteresis, we propose a simple approach to realize discrete droplet motion (without the back end of droplet pinning) horizontally and obliquely on the wedge-shaped copper substrate combining superhydrophobicity with a poly(dimethylsiloxane) (PDMS) oil layer. Superhydrophobicity of the modified copper surface ensures PDMS oil wetting the surface and the water droplet does not contact directly with the substrate; thus the back-end pinning on the surface in precious works is eliminated so that the droplet can move forward maintaining the shape of a spherical crown under the lubrication of slippery PDMS oil. Droplet deposited on the narrow end of the shape gradient surface obtains actuation force from boundary compressing, and its contact line moves toward the decreasing interfacial tension direction. The maximum velocity and furthest position from the origin (taking the intersection of two boundary sides of the wedge as the origin) of droplet motion are found to be influenced by the wedge-shaped gradient head angle α. Here, we discuss the mechanism and develop a simple formula behind the droplet motion. Besides the single shape gradient inducing droplet motion, we demonstrate handling droplet combination via multiple shape gradients. The results of droplets moving and mixing show great prospects in open microfluidic systems, lab chips, and microchemical reactors.
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EXPERIMENTAL SECTION
Copper foil was cut into a wedge-shaped piece (0.8 mm width of gradient start × 23.8 mm length × 5.0 mm width of gradient end) and Received: February 8, 2017 Revised: April 10, 2017 Published: April 11, 2017 4172
DOI: 10.1021/acs.langmuir.7b00227 Langmuir 2017, 33, 4172−4177
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Langmuir
Figure 1. (a) Schematic diagram of droplet on unmodified copper substrate only covered with PDMS oil and SEM image of substrate. (inset) Side view showing droplet pinning on the substrate. (b) Schematic diagram of droplet on CuO surface only covered with PDMS oil and the SEM image of CuO surface. (inset) Side view showing droplet pinning on the surface. (c) Schematic diagram of droplet on CuO surface modified with both FAS-17 and PDMS oil in sequence. Side view demonstrating the low surface hysteresis with a sliding angle of 3°. (d) Top view of droplet moving horizontally along the gradient on such surface.
Figure 2. Sequent captures of 5 μL water droplet moving on (a) a horizontal gradient surface and (b) an inclined (10°) gradient surface with the gradient angle of both surfaces of α = 12°. (c) Experimental and predicted position evolutions of a 5 μL water droplet on horizontal surface with α = 6° and 12°. (inset) Table displays the maximum position and velocities of water droplets motion on different gradients. (d) Experimental and predicted position evolutions of a 5 μL water droplet on an oblique surface at inclination angle of 10° with α = 12°. motion on the oil layer was performed by placing a 5 μL distilled water droplet onto the prepared sample and subsequent recording with a high speed camera (Germany Baumer HXC20c) at 330 frames per second (fps).
cleaned several times by consecutive ultrasonication in acetone, dilute hydrochloric acid, and distilled water for 10 min at room temperature. The cleaned copper foil was immersed into a mixed aqueous solution of 2.67 mol/L NaOH and 0.133 mol/L (NH4)2S2O8 for 2 h to form CuO structures.34 The reacted copper foil was then washed several times with distilled water and dried in N2. Superhydrophobization of the copper surface was carried out by immersing the foil into 1 wt % 1H,1H,2H,2H-perfluorodecyltrimethoxysilane (FAS-17) for 5 min and subsequent sintering under 120 °C. Finally, an excess amount of PDMS oil (η = 9.35 mPa·s) was applied onto the modified copper surfaces. The liquids were maintained on the surfaces overnight to fully saturate the microstructure. Afterward, samples were tilted vertically for 2 h to get rid of the excess of oil lubricant before use in the experiments. We also prepared a series of shape-gradient samples (0.8 mm width of the gradient start × 5.0 mm width of the gradient end) with different head angles α (6, 8, 10, and 12°). The morphology of the superhydrophobic copper surface was characterized by scanning electron microscopy (SEM). The contact angle was determined using a contact angle analyzer (Shanghai Zhongchen Powereach JC2000C1) at ambient temperature. The liquid
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RESULTS AND DISCUSSION
Effect of Surface Modification. Droplet behaviors were observed in succession on the unmodified naked copper surface, CuO surface, and FAS-17 modified CuO surface on copper substrate, all covered with PDMS oil. The droplet adheres on the unmodified copper substrate covered with PDMS oil layer even when the substrate is rotated 90° (Figure 1a). Since the oil phase does not wet the naked copper surface, the droplet touches the surface directly presenting in Wenzel state.35 Figure 1b displays the SEM image of cabbagelike CuO structures on the copper substrate with the size of these particles of approximately 5 μm. The droplet still pins on the formed CuO surface covered only with PDMS oil layer due to 4173
DOI: 10.1021/acs.langmuir.7b00227 Langmuir 2017, 33, 4172−4177
Article
Langmuir
Figure 3. (a) Schematic diagram of droplet aggregation. (b)−(i) Aggregation and motion of droplets on horizontally positioned multiple shape gradients.
Figure 4. (a) Side view schematic diagram of forces acting on advancing droplets. (b) Top view schematic diagram of the interfacial tension profile along the shape gradient. (c) Contact angle (CA) and cosine of contact angle as a function of the radius on circular plates. The typical droplet profiles are shown on the upper side.
the same wetting state. However, after the CuO surface is modified with FAS-17 and further covered with PDMS oil, the droplet moves rashly with a low sliding angle (3°) indicating the key role of FAS-17 modification to eliminate the contact line pinning33 (Figure 1c). Surfaces fabricated with FAS-17 became strongly hydrophobic, ensuring the oil phase wets the hydrophobic surface and water droplet does not contact CuO particles directly. With the combination of the superhydrophobicity of FAS-17 and lubrication of the oil layer, the droplet displays high mobility on the copper substrate. The shape gradient surface offers gradient interfacial tension to induce droplet motion in desired directions. Due to elimination of contact line pinning and lubrication by oil, the deposited droplet moves horizontally along the axis of the gradient wedge surface (Figure 1d). Droplet Motion. Parts a and b of Figure 2 present the displacement−time profiles of a droplet moving along a horizontal surface and a 10° tilted substrate with α = 12°, respectively. The droplet is deposited on the narrow end of a wedge track, with the width of 0.8 mm significantly narrower than the droplet diameter (∼2.7 mm). Therefore, the droplet is constricted in the transverse direction by the shape boundary and moves lengthwise along the wedge track from a tighter footprint to a larger one. The droplet accelerates initially and
gradually slows down resulting from the reducing actuating force and the resistances along the gradient surface. The corresponding position evolutions along the horizontal and oblique gradients from the experimental data and model prediction are displayed in Figure 2, parts c and d, respectively. The point of intersection of the substrate boundary is set as the origin. The maximum velocity on horizontal surfaces for α = 6−12° increases from 4.0 to 14.0 mm/s, whereas the furthest position from its corresponding origin decreases from 15.8 to 12.1 mm, respectively. Experiments performed on the oblique gradient (tilted 10°) show that the gravity affects droplet motion where the velocity for α = 12° decreases from 14.0 to 4.1 mm/s and the position from 12.1 to 5.7 mm, compared with the horizontal gradient. Apparently, a larger gradient angle produces a significantly larger gradient interfacial tension in the x-direction; as a result, the droplet presents a higher velocity and stops at a nearer position whereas the narrower gradient angle is the opposite. Droplet Aggregation Using Multiple Shape Gradients. Based on the single wedge-shaped track, a more complicated shape was designed to drive aggregation of individual droplets and move forward in a specified direction (Movie S1). The substrate in the video was horizontally positioned. The shape gradient exists on each section of track as demonstrated in 4174
DOI: 10.1021/acs.langmuir.7b00227 Langmuir 2017, 33, 4172−4177
Article
Langmuir Figure 3a. When the first probe droplet is deposited onto the narrow end of the first section of track, it moves fast toward the wider transverse and can even turn a corner to the second section of track, the width of which is slightly greater than the radius of a single droplet (∼1.35 mm) as shown in Figure 3b,c. The second probe droplet then appears, repeats the steps, and aggregates with the first droplet at the second track. The radius of the combined droplet increases to nearly 1.71 mm, which enables the droplet to sustain the unbalanced interfacial tension and keep going as demonstrated in Figure 3d,e. The sequent combinations for three or more droplets can be followed from the other branches as displayed in Figure 3f−i. Therefore, simultaneous or successive combination of droplets and droplet motion in a specified direction can be operated via designing the dimension of tracks which may have potential application in open microfluidics systems. Theoretial Prediction of Droplet Motion. The droplet motion on the wedge-shaped gradient surface can be explained by a simple model such as eq 1 (Figure 4a). The temporal evolution of the total momentum P of the droplet is based on Newton’s equation:
Fix = 4γ sin
1
⎛ α⎞ r cos θr sin⎜φ − ⎟ dφ ⎝ 2⎠
⎡ R2 α 1 − tan 2 2 − x 2 tan 2 1 −1⎢ 2 φ1 = cos ⎢ R2 α 2 1 + tan 2 2 ⎢⎣ 2
(
)
(
)
α 2
⎤ ⎥ ⎥ ⎥⎦
(4)
(5)
where φ1 represents the initial polar angle at the front of the droplet. Drag Force. Considering oil flowing with a low Reynolds number (
