Drop Cargo Transfer via Unidirectional Lubricant Spreading on

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Drop Cargo Transfer via Unidirectional Lubricant Spreading on Peristome-Mimetic Surface Cunlong Yu,† Longhao Zhang,† Yunfei Ru,† Ning Li,† Chuxin Li,‡ Can Gao,‡ Zhichao Dong,*,‡ and Lei Jiang†,‡

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Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education, School of Chemistry, Beijing Advanced Innovation Center for Biomedical Engineering, Beihang University, Beijing 100191, P. R. China ‡ CAS Key Laboratory of Bio-Inspired Materials and Interfacial Sciences, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China S Supporting Information *

ABSTRACT: To promote drop mobility, lubricating the gap between liquid drop and solid surface is a facile method which has been widely exploited by nature. Examples include lotus and rice leaves using entrapped air to “lubricate” water and Nepenthes pitcher plant using a slippery water layer to trap insects. Inspired by these, here, we report a strategy for transporting drop cargoes via the unidirectional spreading of immiscible lubricants on the peristome-mimetic surface. Oleophilic/hydrophobic peristome-mimetic surfaces were fabricated through replicating three-dimensional printed samples. The peristome-mimetic surface, via unidirectional immiscible hexadecane spreading, can transport a wide diversity of drop cargoes over a long distance with no loss with controllable drop volumes and velocities, hence mixing multiphase liquids and even reacting liquids. We anticipate this unidirectional drop cargo transport technique will find use in microfluidics, microreactors, water harvesting systems, etc. KEYWORDS: drop cargo transfer, 3D printing, lubricant, peristome-mimetic surface

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challenge for superhydrophobic surfaces is their fragile structures.32,33 Different from tracked superhydrophobic surfaces using sharpe microedges to repel water wetting, the Nepenthes pitcher plant uses microstructured peristome surface to lock slippery liquid layer to forbid the surface from further wetting by other liquids.34−37 When an insect aquaplanes on the rim of a pitcher plant, it will slide down from the rim to the bottom.37 Inspired by the peristome surface, recent progresses have led to lubricant-impregnated surfaces capable of almost zero contact line pinning and high drop mobility without causing drops to ball up and minimize the contact area.38−40 However, without considering the distinct structures of the peristome surface, the liquid drop’s motion on another liquid-immersed surface is still triggered by gravity, and it is still hard to control the motion trajectory over a long distance. Inspired by the trapping strategy of the pitcher plant, here, we present a way to achieve drop transfer on the lubricantcoated peristome-mimetic surface. The nature of the drop

iquid drop movements are ubiquitous in nature, daily life ,and industrial technology, and examples include the gathering of condensate water drops on spider silk1 and cactus spine2 and the liquid delivery processes for food,3 cosmetic and beverage industries,4,5 and lab-on-a−chip devices.6,7 When a sessile drop is released on the top of a solid surface, it usually spreads into a cap shape and firmly sticks on the surface even at a large title angle due to the capillary adhesion.8,9 Unless the apparent contact angle is larger than the advancing contact angle, no water drop could move on the surface.10,11 Dragging forces induced by wind,12 magnetic force,13−16 light,17,18 and gravity19−22 are therefore greatly needed for such surfaces. To reduce contact line pinning, surface chemistry needs to be coupled with microand/or nanostructures to create superhydrophobic surfaces23−27 or heat surfaces above a threshold temperature to achieve the Leidenfrost state.28,29 On these surfaces, a water drop would ball up into an almost spherical shape, thus minimizing the contact area. Previous research has demonstrated that tracked superhydrophobic surfaces with patterns could repel water wetting and guide the water drop’s motion.30,31 However, for practical applications, the big © XXXX American Chemical Society

Received: August 8, 2018 Accepted: October 16, 2018

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DOI: 10.1021/acsnano.8b06023 ACS Nano XXXX, XXX, XXX−XXX

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Figure 1. Water cargo transport on lubricant-coated surfaces. (a) Water drop pins at the depositing point and shows little influence by the isotropic spreading of n-hexadecane (C16H34) film on the smooth hydrophobic surface. (b and c) Water drop delivered as a cargo along the +x-axis on the spreading n-hexadecane “conveyer” that bidirectionally spreads on the microgrooved surface (b) or unidirectionally spreads on the peristome-mimetic surface (c). The red arrows indicate the spreading direction of n-hexadecane, and the blue arrows represent the transport direction of water drops. (d) Variations of spreading lengths of n-hexadecane films or strips along the x-axis of the smooth or structured surface as a function of deposition time with the same liquid deposition speed. (e) The transport length of water drop cargo versus time measured on n-hexadecane coated smooth, microgrooved, and peristome-mimetic surface. As n-hexadecane spreads on the peristome-mimetic surface only along the +x-axis unidirectionally, fastest transport speeds are achieved both for n-hexadecane and water drop cargo. The bidirectional hexadecane spreading on the microgrooved surface reduces the cargo transport ability by halving transport speed and limiting the cargo transport length when compared with that on the peristome-mimetic surface.

gliding on the lubricant-coated peristome-mimetic surface is demonstrated. Gliding drop exhibits a floating-and-tumble-like behavior related to the injection speed of lubricants. About a dozen carrier−cargo combinations such as hexadecane−water, hexadecane−ethanol, and hexadecane−ethylene glycol were deposited onto the peristome-mimetic surface to investigate the transport efficiency. The understanding of drop motion behavior could help us further understand the complex

behaviors that emerge in populations of liquid transport behaviors.

RESULTS AND DISCUSSION Effective Water Drop Delivery. Smooth, microgrooved, and peristome-mimetic polydimethylsiloxane (PDMS) surfaces shown in the first row of Figures 1a−c were fabricated by replicating templates constructed through a three-dimensional (3D) high-resolution printer. Figures 1a−c show the surface B

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Figure 2. 3D presentations of sample structures using 3D X-ray microscopy. (a) A hexadecane layer and a 2 μL water droplet are successively deposited onto the peristome-mimetic surface. Then, the sample is mounted on the sample stage. (b) During micro-CT processing, a tomography data set of the sample is collected to reconstruct the 3D sample volume. (c) The 3D reconstructed volume presents the wetting state of a water droplet on the peristome-mimetic surface coated by a hexadecane layer. (d) The cross-sectional 2D reconstructed slice further shows a detailed morphology of the solid−oil−water−air interface.

substrate by a syringe pump continuously, and a water drop with a volume of 2 μL is then dropped onto the hexadecane coated substrate. Apparently, the spreading behaviors of nhexadecane and the transport behaviors of water droplets are vastly different on the three types of surfaces. As Figures 1c−e reveal, water drop transports fastest on the lubricant-coated peristome-mimetic surface. Considering the spreading tendencies of hexadecane in every direction except +x-axis are limited (Figure S1 and Movie S1), the oil “conveyer” can effectively deliver the water drop along the peristome-mimetic surface with the highest efficiency (Movie S2). In contrast, as Figure 1b reveals, the transport speed of oil and water drop on the microgrooved surface is less than the half of the transport speed of n-hexadecane or water cargo on the peristomemimetic surface. Bidirectional spreading of the lubricant phase therefore weakens the transport ability. As an extreme case, if n-hexadecane isotropically spreads in all directions on the surface, a slowest spreading speed would be achieved for the lubricant phase. Just as the time sequences shown in Figure 1a, water droplet deposited on the lubricant-coated surface hardly transports on a smooth surface under the act of isotropical

morphology of the smooth, microgrooved, and peristomemimetic surface in low and high magnifications. Particularly, peristome-mimetic surface shows the pattern with periodic and duck-billed microcavities in arrays.41−43 As the second row of Figures 1a−c reveals, the experimental apparatus is composed of two syringe-based actuators to dispense n-hexadecane and water drops, respectively, onto the horizontally mounted substrates. Microdisplacement stages are used to control the position between the substrate and nozzle positions, where the oil nozzle contacts the substrate and the water nozzle hangs in the air above the substrate with a certain distance of 2.0 mm. During the dispensing process, half-heights of the liquid drops are less than the capillary length, lc = (γ/ρg)1/2 with a value of 2.7 mm in the case of water, where γ is the surface tension, ρ is the liquid density, and g is the gravity acceleration. Hence, the gravity effect was neglected.44−47 To obtain detailed information on the liquid transport process, two high-speed macro-lens cameras captured the flow dynamics in vertical and horizontal views. Step deposition processes are utilized and shown in the third row in Figures 1a−c: n-hexadecane is first deposited onto the C

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Figure 3. Characteristics and mechanism of water core transport on n-hexadecane coated peristome-mimetic surface. (a) Diagram of the experimental setup. (b) Schematic image of the wetting state of water core on the n-hexadecane coated peristome-mimetic substrate. (c) Schematic setup for measuring lateral and vertical resistance forces of water core sliding on the n-hexadecane strip. The force in x-axis and zaxis can be calculated from the tube deformation by detecting reflected light variations. (d) Lateral and vertical resistance force for a water drop (∼2 μL) that slides on the n-hexadecane coated peristome-mimetic surface. The measurement is performed at a constant linear stage velocity of 1.0 mm·s−1.

delivered. As a hypothesis, if oil can spontaneously transport fast at the precursor side, reducing the oil film height at the front of the water core, or the oil phase deposits in a faster speed at the rear side, enhancing the oil film height at the rear of the water core, a larger driving force is achieved. The control of transport speed at the precursor side was first demonstrated. Precursor oil initially spreads along the microcavity and then fills the cavity with a capillary acceleration αcapillary, finally overflowing at the edge at the top of the microcavity for the next microcavity.41,48 The capillary acceleration that acts on the liquid precursor is in relation to the contact angle, θo, and the distance of the precursor from the apex, x, which can be demonstrated as

spreading of hexadecane (Figure 1e). Peristome-mimetic surface is therefore the effective substrate for the drop cargo delivery, even after being swollen with hexadecane (Figure S2). Drop Repellency of Lubricant-Coated PeristomeMimetic Surface. Micro-CT microscope is used to investigate the microscopic wetting state in a 3D view (Figure 2). Even though water has a density higher than that of the lubricant phase, it is totally repelled by the hexadecane phase to form a drop shape. Similar results can be observed on the PVA hydrogel surface for n-hexadecane drop (Figures S3 and S4). Water drop floats on the lubricant-coated substrate with a layer of hexadecane that separates the water drop from the solid surface. Considering the energy balance,38 ΔER = r(γw cos θw − γocos θo) + γo − γw, hexadecane has a higher tendency to wet the peristome-mimetic PDMS surface, where r is the surface roughness, γw is the water surface tension, γo is the oil surface tension, θw is the water contact angle on the surface, and θo is the oil contact angle on the surface, respectively (Tables S1 and S2). Drop Cargo Delivery via Unidirectional Spreading of n-Hexadecane. The driving force, Fdriven, for the motion of the water drop is calculated from the oil height difference, Δh, at the front and rear sides of the water core (as shown in Figures 3a and b). Δh is induced by the balance of the spontaneous transport ability of the oil phase at the precursor side and the injection speed of oil phase at the rear side. In theory, the hydrodynamic pressure difference, ΔP, induced by Δh can be explained as, ΔP = ρgΔh, where ρ is the density of n-hexadecane and g is the gravitational acceleration. A larger Δh leads to a larger tendency for the water drop to be

αcapillary ≈

rocos θo αρx

(1)

Driven by the capillary rise at the precursor side, liquid with a viscosity, η, could spread along the microcavity with a length L, in an increased flow velocity, v, which is given by υ≈

αγLcosθ0 ηx

(2)

where flow velocity, v, increases as the precursor approaches the tip of the cavity. Adjusting the parameter of the peristome surface with an appropriate value of αL/x, it could lead to a large transport velocity at the precursor side. Measurement of the resistant force was then performed. To measure the resistance force during the water drop transporting process on the peristome-mimetic PDMS surface via D

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Figure 4. Liquid cargoes generalized to varied immiscible liquids. (a) A water drop with the surface tension 72.8 mN/m transports via unidirectional spreading of n-hexadecane with an axiolitic shape. The contact length, L1, is ∼2.19 mm, and the drop height, H1, is ∼1.04 mm at 0.5 s. (b) An ethylene glycol drop with the surface tension 48.0 mN/m transports via n-hexadecane unidirectionally spreading with a rodlike shape. The contact length, L2, is ∼2.88 mm, and the drop height, H2, is ∼0.90 mm at 0.5 s. (c) An ethanol drop with the surface tension 22.4 mN/m transports with unidirectional spreading of n-hexadecane with a spindle shape. The contact length, L3, is ∼3.70 mm, and the drop height, H3, is ∼0.77 mm at 0.5 s. (d) The lengths of the water drop, ethylene glycol drop, and ethanol drop cargoes versus time transported via n-hexadecane unidirectionally spreading on the peristome-mimetic surface. (e) The transport velocity of water droplet, ethylene glycol droplet, and ethanol droplet on n-hexadecane coated peristome-mimetic surface during 20 s. Ethanol droplet with lower surface tension transports faster than ethylene glycol droplet and water droplet with higher surface tension.

hexadecane spreading, a setup was manufactured (Figure 3c).49 As a representative example, we first placed a water drop on a peristome-mimetic surface that was preinfused with hexadecane. Moving the peristome-mimetic surface by a linear motion stage, the water droplet that adheres onto a capillary tube starts to slide. The resistance force for sliding water droplet equals the force that deforms the capillary. A laser beam incident on the capillary is reflected to a detector. The resistance forces for water droplet, Flateral and Fvertical, are recorded in Figure 3d. The results are shown in the resistance force measurement for water droplet, which exhibits wave-like curve with a maximum Flateral of ∼6.5 μN and a maximum Fvertical of ∼2.0 μN. The resistance force for water droplet via spreading hexadecane is very small, meaning that water droplet can easily transport on spreading hexadecane. Drop Cargoes with Varied Surface Tensions. Based on this model, for any two immiscible solvents on such a substrate, droplets should be efficiently delivered in a defined direction. To test this law, we tested a dozen combinations of immiscible liquids (Table S3). As Figure 4 and Movie S3 reveal, three liquids, water (72.8 mN/m), ethylene glycol (48.0 mN/m), and ethanol (22.4 mN/m), were selected to investigate the effect of surface tension difference in controlling

drop motion on the n-hexadecane coated surface. As shown in Figure 4a, the water droplet is in an axiolitic shape and is transported via the spreading of n-hexadecane, which resulted from the repellency of n-hexadecane to water. Reducing the surface tension difference, the droplet tends to be transported much quicker. Just as Figure 4b reveals, the ethylene glycol drop transports faster than the water drop on the n-hexadecane coated peristome-mimetic surface with a rod-like shape at the same oil injection speed. A similar result is also observed for ethanol droplet transport on the lubricant-coated surface (Figure 4c), which has the lowest surface tension difference but with the fastest transport speed (Figures 4d and e). A narrower surface tension difference for two immiscible liquid droplets can facilitate the transport of liquid core with a faster speed via the unidirectional spreading of n-hexadecane. Controllable Delivery of Drop Cargo. To optimize the water core delivery on the substrate, besides the surface tension difference, the volume of the drop is also discussed (Figure 5a and Figure S5). Due to it is more difficult to transport droplets with the increase of the droplet volume, a lower transport speed is achieved for the drop with a volume of 4 μL. We need to mention that the highest transport speed is acquired by the drop with a volume of 2 μL, while the lowest E

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Figure 5. Controlled transport of drop cargo. (a) Transport length versus time with a drop volume of 1, 2, 3, and 4 μL. The injection speed is 2.0 μL·s−1. (b) Variations of droplet transport length as a function of time with deposition velocities 0.5, 1.0, and 2.0 μL·s−1. The drop volume was fixed as 2 μL. (c−e) Water droplets with a uniform drop volume ranging from 1 to 3 μL were arrayed and transported along the oil-coated peristome-mimetic microgrooves.

molecules; consequently, some of ethanol molecules tend to cram in among the water molecules, hence achieving a reduced volume and an increased liquid temperature. Both effects can facilitate chemical reactions. The reduced volume means the increase of the concentration of reactants and the increased temperature, which means the increase of reaction speed. Therefore, as shown in Figures 6c and d, if we mix one ethanol drop with CuCl2 solute with one water drop with KOH solute by this oil conveyer, a microreactor would be devised. As a proof-of-concept experiment, we printed a substrate with six peristome-mimetic structured strips and mounted the nozzle onto the selected two strips to deliver the solution cargo. A hole is drilled at the intersection of these strips. A pipe contacts the hole at the bottom side of the plate and the dispensing nozzle to achieve recycling of the oil. As Figure 6b reveals, when two drops contact each other, the reaction occurs with the reacting drop floating on the top of the oil phase. A microreactor is therefore achieved.

transport speed is observed for the drop core with a volume of 1 μL. As a drop with a smaller volume that matches the volume of the microcavity, the energy barrier that meets during the transport process could reduce the delivery speed. Therefore, to achieve effective droplet transport, the droplet volume should not be much larger or smaller than the typical volume of the microgroove. In addition, the injection speed of hexadecane is another factor in controlling the transport of water drop core. As Figure 5b and Figure S6 reveal, the transport velocity of the droplet increases with hexadecane injection speed. However, if the injection speed exceeds a certain value, the unidirectional spreading of hexadecane on the peristome-mimetic PDMS surface would be converted to bidirectional spreading. In this work, we selected an injection speed of 2.0 μL/s as the perfect choice. After the fundamental demonstration of lubricant−cargo system, we then explored the feasibility to continuously deliver liquid drop sequences on the peristome-mimetic substrate. Patterned water cores, as Figures 5c−e reveal, with an average volume of 1, 2, and 3 μL, can be transported on the n-hexadecane coated substrate in an array. Drop Cargo Reactants. Besides the direct route, the water core can be delivered along the track of an Archimedes spiral with a curved trajectory (Figures 6a and b). We note that because the n-hexadecane layer at the rear side holds still, the liquid−solid contact line at the rear side is unaffected by prior trajectories and can withstand the hydrostatic pressure without further movement. Within a minute, the dyed drop was transported a distance of ∼5 cm and circled the pattern. Other liquids such as ethanol and ethylene glycol can also be delivered along the track of the Archimedes spiral. Our designed system is therefore a good candidate in drop cargo delivery. An interesting volume drop phenomenon occurs after mixing the 50 mL of water with the 50 mL of ethanol. Instead of being 100 mL of liquid, there was only ∼97.0 mL (Figure S7). The reason can be understood as following: ethanol molecules are much smaller in volume than the water

CONCLUSIONS In summary, we reported a facile strategy to realize controllable transport of drop cargoes via the unidirectional spreading of immiscible liquids on the peristome-mimetic surface. Water droplets with controllable volumes and transport velocities preferentially advance in a floating-and-tumble-like manner on the peristome-mimetic surface. Even for various cargo−carrier combinations, effective transportations were also achieved. This work has contributed to progress in realizing a fast, controllable, unidirectional, no-loss transport of drop cargo for effective combination of liquid cores or liquid reacting. We anticipate this method would find practical applications in the fields of biomedical devices, microfluidics, microreactors, water harvesting systems, and so on. EXPERIMENTAL SECTION Material Fabrication. Smooth, microgrooved, and peristomemimetic surfaces were prepared using the PDMS (Dow Corning, F

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Figure 6. Drop cargo transport for microfluidics and microreactors. (a) The optical image depicting the morphology of Nautilus pompiplius according to the Archimedean spiral. (b) A 2 μL droplet was transported through the peristome-mimetic Archimedean-spiral microgroove driven by unidirectional spreading of hexadecane without retraction. (c) Microreactor was performed between orange ethanol droplets and pink water droplets, which contain CuCl2 solute and KOH solute, respectively. Water droplets were dyed by phenolphthalein. (d) Droplets were driven by n-hexadecane unidirectionally spreading on the peristome-mimetic surface. Then, two droplets mix to a droplet and react to form brown Cu(OH)2. Sylgard 184), by replicating the morphology of corresponding 3D printed molds. During the process, the PDMS mixture (prepolymer base agent Sylgard 184A and thermal curing agent Sylgard 184B, 10:1) was extensively stirred to ensure a homogeneous mixture and degassed in a vacuum oven for 1 h to remove bubbles from the PDMS mixture. Then, the PDMS mixture was coated onto the 3D printed molds, degassed again, and then cured at 60 °C in an oven for 6 h. After the PDMS was cooled, the replicates were peeled and stored for further use. Characterization. The optical images of the prepared samples were captured by a 105 mm macro-lens using a digital camera (D5300, Nikon, Japan). Scanning electron microscope (SEM) images were obtained using a Hitachi SU-8010 field-emission scanning electron microscope at 5 kV. High-resolution 3D X-ray microscopy images and computed tomography were taken using a MicroXCT200, X-radia. Individual X-ray exposure slices reconstructed the 3D copy of the sample. The precise deformation of the capillary tube was detected by a Laser Rangefinder (LK-G5000, KEYENCE). Experiments on the Liquid Drop Transportation via nHexadecane Spreading. High-speed cameras (i-SPEED 3, OLYMPUS, Japan) were used to record the hexadecane spreading dynamics and liquid drop transporting dynamics from vertical and horizontal views. Analysis software was used to analyze the liquid behaviors. The liquid droplets and n-hexadecane were injected by

syringe pumps (TS-1A, LongerPump, China). For the liquid drop transportation experiments, liquid drops and n-hexadecane were colored with different coloring agents for optimal visualization.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.8b06023. Supporting figures and tables that demonstrate the unidirectional spreading of continuously injected nhexadecane on the peristome-mimetic surface, timelapse images of unidirectional drops transport with varied drop volumes and oil-injecting speeds, a volume reducing phenomenon in ethanol and water mix test, physical and chemical properties of the tested liquids, and comparison of cargo transport velocities for a dozen cargo−carrier combinations (PDF) Movie S1: Comparison of hexadecane spreading performance on different surfaces (AVI) Movie S2: Comparison of water droplet transportation on different surfaces (AVI) G

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Movie S3: Transport of drop cargoes with varied surface tensions on the peristome-mimetic PDMS surface (AVI)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Cunlong Yu: 0000-0003-2442-6703 Zhichao Dong: 0000-0002-6558-0531 Author Contributions

Z.D., and L.J. conceived and designed the experiments. C.Y., L.Z., Y.R., N.L., C.L., C.G., and Z.D. performed the experiments. C.Y. and Z.D. wrote the manuscript. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS We acknowledge project funding provided by the National Natural Science Foundation (Grant 21703270, 21431009, and 91127025), the National Key R&D Program of China (Grants 2017YFA0206900 and 2018YFA0208501), and the 111 Project (Grant B14009). REFERENCES (1) Zheng, Y.; Bai, H.; Huang, Z.; Tian, X.; Nie, F.; Zhao, Y.; Zhai, J.; Jiang, L. Directional Water Collection on Wetted Spider Silk. Nature 2010, 463, 640−643. (2) Ju, J.; Bai, H.; Zheng, Y.; Zhao, T.; Fang, R.; Jiang, L. A MultiStructural and Multi-Functional Integrated Fog Collection System in Cactus. Nat. Commun. 2012, 3, 1247. (3) Fryer, P.; Pyle, D.; Rielly, C. Chemical Engineering for the Food Industry; Springer: Blackie, London, UK, 2012. (4) Hashimoto, N.; Ishii, H.; Tanahashi, K.; Ohno, K.; Ohno, K. Fluid Mixing Element. U.S. Patent 4884894, 1989. (5) Rudick, A. Tri-Mix Sugar Based Dispensing System. U.S. Patent 4753370, 1988. (6) Stone, H.; Stroock, A.; Ajdari, A. Annu. Rev. Fluid Mech. 2004, 36, 381−411. (7) Brouzes, E.; Medkova, M.; Savenelli, N.; Marran, D.; Twardowski, M.; Hutchison, J.; Rothberg, J.; Link, D.; Perrimon, N.; Samuels, M. Droplet Microfluidic Technology for Single-Cell High-Throughput Screening. Proc. Natl. Acad. Sci. U. S. A. 2009, 106, 14195−14200. (8) Liu, M.; Wang, S.; Jiang, L. Nature-Inspired Superwettability Systems. Nat. Rev. Mater. 2017, 2, 17036. (9) Dong, Z.; Ma, J.; Jiang, L. Manipulating and Dispensing Micro/ Nanoliter Droplets by Superhydrophobic Needle Nozzles. ACS Nano 2013, 7, 10371−10379. (10) Si, Y.; Yu, C.; Dong, Z.; Jiang, L. Wetting and Spreading: Fundamental Theories to Cutting-Edge Applications. Curr. Opin. Colloid Interface Sci. 2018, 36, 10−19. (11) Hancock, M.; Sekeroglu, K.; Demirel, M. Bioinspired Directional Surfaces for Adhesion, Wetting and Transport. Adv. Funct. Mater. 2012, 22, 2223−2234. (12) Hu, H.; Yu, S.; Song, D. No-Loss Transportation of Water Droplets by Patterning a Desired Hydrophobic Path on a Superhydrophobic Surface. Langmuir 2016, 32, 7339−7345. (13) Sander, J.; Erb, R.; Denier, C.; Studart, A. Magnetic Transport, Mixing and Release of Cargo with Tailored Nanoliter Droplets. Adv. Mater. 2012, 24, 2582−2587. (14) Tian, D.; Zhang, N.; Zheng, X.; Hou, G.; Tian, Y.; Du, Y.; Jiang, L.; Dou, S. Fast Responsive and Controllable Liquid Transport on a Magnetic Fluid/Nanoarray Composite Interface. ACS Nano 2016, 10, 6220−6226. H

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DOI: 10.1021/acsnano.8b06023 ACS Nano XXXX, XXX, XXX−XXX