Droplet Mechanical Hand Based on Anisotropic Water Adhesion of

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Droplet Mechanical Hand Based on Anisotropic Water Adhesion of Hydrophobic-Superhydrophobic Patterned surfaces Xiaolong Yang, Won Tae Choi, Jiyu Liu, and Xin Liu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b03969 • Publication Date (Web): 10 Jan 2019 Downloaded from http://pubs.acs.org on January 15, 2019

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Droplet Mechanical Hand Based on Anisotropic Water Adhesion of Hydrophobic-Superhydrophobic Patterned surfaces Xiaolong Yang,†,*,1 Won Tae Choi,‡,1 Jiyu Liu,§ Xin Liu§ †National

Key Laboratory of Science and Technology on Helicopter Transmission,

Nanjing University of Aeronautics and Astronautics, Nanjing 210016, PR China ‡Department

of Chemistry, The University of Texas at Austin, Austin, Texas 78712, United States

§Key

Laboratory for Precision and Non-traditional Machining Technology of the

Ministry of Education, Dalian University of Technology, Dalian 116023, PR China 1These

authors contributed equally to this work.

KEYWORDS: hydrophobic/superhydrophobic patterned surfaces; sliding resistance; anisotropic sliding; droplet manipulations

ACS Paragon Plus Environment

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ABSTRACT:

Superhydrophobic

copper

surfaces

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patterned

with

non-round

hydrophobic areas were fabricated by combining through-mask chemical oxidation and fluorocarbon film deposition technique. Anisotropic sliding resistance of droplets on typical non-round hydrophobic patterns such as semicircle, V-shaped and line segment hydrophobic patterns was observed. The dependence of sliding anisotropy on the pattern shape and dimensions was investigated. Results showed that experimental sliding resistance was in good agreement with the calculated data using classical drag-resistance model (Furmidge equation). By taking advantage of the anisotropic sliding resistance, these patterned surfaces can be used as droplet mechanical hand to capture, transfer, mix and in-situ release micro droplets by simply moving the surface in different directions. A droplet pinned on a non-round hydrophobic pattern can be captured by lifting a surface with another non-round hydrophobic pattern in large-sliding-resistance direction after touching it while the captured droplet can be in-situ released with nearly no mass loss by horizontally moving the surface in low-sliding-resistance direction. The lossless droplet manipulations using the hydrophobic/superhydrophobic patterned surfaces have advantages of low-cost and easy-to-operate and may have great promising applications to high throughput drug screening, molecular detection and other lab-on-chip devices.


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INTRODUCTION Micro-sized droplet manipulations such as droplet storage1-3, transfer4-7 and mixing8-10 have attracted considerable attentions due to their great potential applications in high throughput cell screening11-15, biochemical synthesis16-19, molecular sensing20-21 and other lab-on-a-chip devices22-23. For instance, spherical droplet arrays pinned on a solid surface can serve as miniature vessels to culture cells for high throughput cell transfection and drug screening11-13. Compared with microplate technology, the above methods can reduce reagents and cell consumption by 15000 times12. Seeding molecules to be detected into the droplet and enriching the droplet by controlled volatilization can greatly increase the molecular target concentration in the droplet, increase the reaction probability of the probe and target, and thus improve the detection accuracy of the molecular target. Xu et al. reported that detection limit of 2.3×10-16 M can be achieved using this droplet-based analysis method21. In the past few years, various methods have been developed to achieve manipulations of multiple droplets for diverse applications24-39. For example, Cheng et al. fabricated a functional surface with pH-responsive droplet adhesion by assembling pH-sensitive hydroxyl and hydrophobic alkyl groups on Cu(OH)2 nanowire structures. The surfaces can be used as a smart droplet mechanical hand to selectively capture and transfer alkaline droplets due to its high adhesive forces30. Jiang et al. reported that droplet adhesion control can be realized by changing the temperature of a thermo-sensitive surface which was prepared by grafting the thermo-sensitive isopropyl acrylamide onto the surface of silicon nanowires. Underwater oil droplet manipulations such as oil droplet capture and in-situ release were performed on basis of thermo-sensitive adhesion control33. The above-mentioned droplet manipulation strategies are usually dependent of harsh external stimuli such as pH, temperature,


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electric and magnetic field, which may be not suitable for biochemical analysis in mild conditions25. Additionally, most of those approaches are based on specific materials of substrates, which may have poor biocompatibility. To make up these shortcomings, Wu et al. proposed a new droplet manipulation method that is driven by substrate deformations. Micropillar arrays were constructed on flexible PDMS films by combining two-beams laser interference lithography and soft lithography; spacing between micropillar tip and roughness factor of the surface vary with the film curvature; pinned and roll-down state can therefore be reversibly switched by simply bending and releasing of the PDMS films25,

40.

Similar reversible in-situ droplet

adhesion control method based on substrate stretching was developed by Wang et al31. Droplet capture and release with mass loss can be easily implemented using this adhesion control strategy. Nevertheless, fabrication of the flexible surface with arrayed micropillars requires complex processing steps and expensive equipment. In addition, continuous bending or stretching the substrates for droplet manipulations can bring about the material fatigue and fractured, which would shorten the surface’s service life. Recently, researchers found that multiple droplet manipulations using patterned non-uniform wetting surfaces (surfaces with two distinct wettability) are attracting much attentions24,

26, 29, 34-39.

For instance, Tang et al. reported a

mechano-regulated patterned surface that consists of superhydrophobic (water contact angle θ > 150°)41 background mesh and a movable hydrophilic (θ < 90°) or hydrophobic (θ > 90°)41 microfiber array. The water adhesion of this patterned surface can be in-situ switched and droplets can therefore be captured and released by stretching out and drawing back the microfiber array29. Xu et al. successfully regulated the droplet motion along horizontal and gravitational directions on a tilted hydrophobic/superhydrophobic patterned surface which was prepared by applying


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homogeneous hydrophobic coating on a morphologically heterogeneous surface24. Ghosh and Huang et al proposed that droplet dispensed on a wedge-shaped superlyophilic pattern spotted on superlyophobic background can be transported spontaneously due to Laplace-pressure gradient along the droplet moving direction34, 42.

Li and Jiao et al. fabricated micro- and nano-textured surfaces patterned with

smooth regions on titanium, silicon and nickel surfaces via site-selectively femtosecond laser ablation. Those textured patterned surfaces showed extreme wettability contrast and can be used as platforms to manipulate water, oil and bubbles36-39. Droplet manipulations on patterned surfaces have been investigated intensively due to their several advantages including simple manipulating process, low cost and high manipulation flexibility, but methods for droplet transfer and mixing without mass loss still remains as a significant challenge. In previous work, we prepared hydrophobic/superhydrophobic patterned surface by combining ink-masking-derived selective oxidation and plasma film deposition43. In this work, the method was applied to fabricate superhydrophobic surfaces patterned with various-shaped hydrophobic areas, and the relationship between anisotropic sliding resistance of droplets on the hydrophobic patterns and the geometric feature of the patterns were investigated systematically. By taking advantage of the anisotropic sliding resistance, the surface can be used as a droplet mechanical hand to capture and in-situ release droplet with nearly no mass loss simply through controlling the moving direction of the surface. This research is promising for development of new intelligent droplet manipulating platform. MATERIALS AND METHODS Fabrication of Hydrophobic-Superhydrophobic Patterned Surfaces Pre-cleaned copper foils with ink masks was directly immersed in 0.1 M NaHCO3 and


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0.02 M (NH4)2S2O8 solution to site-selectively grow rod-like nanostructures. After coating

fluorocarbon

film

on

the

surface

via

plasma

deposition,

the

hydrophobic/superhydrophobic patterned surface was obtained. The detailed fabrication process can be found in our previous work43-45. Characterization Dimensions of the patterns were measured by an electron microscope (Microvision, MV-VD030SC, China). Water contact angle and sliding angle were measured with a goniometer (Ramé-Hart 290, America). Droplet manipulation processes were recorded by a CCD camera (Lumenera Lu135, Canada) with an APO lens (Leica Z6, German). Digital photos were taken using a camera with an EFS 18-135 mm lens (Canon 700d, Japan). Scanning electron microscopy (SEM) images of the sample surfaces were taken using a Zeiss Ultra60 FE-SEM. Energy-dispersive X-ray spectroscopy (EDX) was conducted to characterize chemical compositions of patterned surfaces by using EDX detector (Oxford Instruments). RESULTS AND DISCUSSION Anisotropic Sliding on Non-Round Hydrophobic Patterns In the oxidation process, area of copper foil without ink masks contacts the solution and dense nanorod clusters grows on the surface (Fig. 1a), while the area underneath the ink masks was prevented from the oxidation and growth of the clusters because of the waterproofness of the ink masks and remained as its original copper surface with smooth morphology (Fig. 1b). The ink masks were not peeled off in the mild oxidation condition, which resulted in a clear boundary between the nano-textured and smooth area (Fig. 1c and 1d). After coating a fluorocarbon film on the oxidized copper surface via plasma-enhanced chemical vapor deposition, the nano-textured area is superhydrophobic while the smooth area shows hydrophobic, and thereby the


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hydrophobic/superhydrophobic patterned surfaces were obtained. Surfaces with various-shaped hydrophobic patterns were fabricated using this method and geometrical effect was investigated.

Figure 1. SEM images of the (a) oxidized area, (b) smooth non-oxidized area, (c) patterned area and (d) boundary area.

Figure 2 displays spatial EDX mapping of a line-patterned copper surface. The mapping shows depletion of copper content (Fig. 2b) and enrichment of oxygen content (Fig. 2c) at the background surface, which confirms a selective oxidation of copper surface without ink-masking by the immersion in a 0.1M sodium bicarbonate and 0.02M ammonium persulfate solution. In addition, elemental mapping of F content shows no distinction between line pattern and background surfaces (Fig. 2d), demonstrating a uniform deposition of fluorocarbon film on the patterned surface to create hydrophobic/super-hydrophobic patterned copper surface.


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Figure 2. Elemental analysis of the patterned copper surface. (a) Microscopic image of the line patterned copper surface. SEM-EDX mappings of the surface with elements (b) Cu, (c) O, (d) F.

In previous work, it was found that the sliding resistance of droplets on round hydrophobic dots is a function of surface tension of the droplet, dot dimension and contact angle hysteresis on the surface, which can be described as the following equations43, 46-49: FRES-ALL  FRES-HP  FRES-SHP  WHP LV (cos  RP  cos  AS )  (WDRO  WHP ) LV (cos  RS  cos  AS )

(1)

where WHP and WDRO represent widths of droplet-pattern interface and droplet-substrate interface that are perpendicular to the sliding direction, respectively, γLV is the surface tension of the liquid, θRP is the receding contact angle on the hydrophobic pattern while θRS and θRA are the receding and advancing contact angle on the superhydrophobic background respectively. It can be seen from equation (1) that sliding resistance FRES-ALL is controlled by WHP.


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For surfaces with round patterns, WHP in every sliding direction is the same (Fig. 3), thus the FRES-ALL is isotropic. In this way, FRES-ALL can only be regulated by changing the pattern dimensions. However, for surfaces with non-round patterns, FRES-ALL and WHP may vary in different sliding directions, demonstrating obvious sliding anisotropy. As shown in Fig. 4, when the droplet was dispensed on a semi-circle hydrophobic pattern, the droplet-pattern interfacial width WHP-L in sliding direction L is chord length of the semicircle while in sliding direction S, component force Fst-pe of the droplet surface tension Fst drives the droplet-pattern interface moving to the middle of the pattern (position M in Fig. 4c), which makes the WHP-S in S direction much smaller than in L direction (Fig. 4d). Thus, FRES-ALL in these two directions show significant anisotropy. Similar anisotropic sliding can also be found on V-shaped and line hydrophobic patterns (Fig. 5).

Figure 3. Schematic of droplet-pattern interfacial width WHP on a round hydrophobic pattern.


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Figure 4. Schematic of anisotropic sliding on a semicircle hydrophobic pattern: (a) schematic of the semicircle hydrophobic pattern; schematic of droplet sliding on the semicircle pattern in (b) L and (c, d) S directions.

Figure 5. Schematic of anisotropic sliding on (a) V-shaped and (b) line hydrophobic patterns.

To figure out how the width of droplet-pattern interface affect the droplet sliding anisotropy, superhydrophobic surfaces with various shaped hydrophobic patterns were fabricated using the method which was detailedly described in our previous work, droplet-pattern interfacial widths WHP-L and WHP-S were measured (Table 1), and FRES-ALL in the two directions were calculated using equation 1(Table 2). Comparing these FRES-ALL for the three different shaped hydrophobic patterns, it can be seen that, for semicircle hydrophobic patterns, the minimum and maximum relative error between the experimental and calculated FRES-ALL are 0.8% and 17.8% respectively, while figure for V-shaped semicircle hydrophobic patterns are 0.5% and 19.0%, respectively, both indicating that calculated FRES-ALL is in good agreement with the experimental values. As for line hydrophobic pattern, the maximum relative error for FRES-ALL in S direction is 13.0%, but that value in L direction reaches 88.4%,


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showing obvious deviation. In order to present a more visualized comparison, the FRES-ALL were represented in line chart as shown in Fig. 6. This deviation is caused by the fact that when droplet on the line hydrophobic pattern slides in L direction, the droplet outline and the line pattern intersect, component force of the air/liquid surface tension perpendicular to line at the position E (Fst-pe in Fig. 7a) drives the air/liquid/solid interface to move toward the middle of the line, making the final W* HP-L smaller than the measured one (Fig. 7b). On the contrary, outline of droplet on semicircle hydrophobic pattern almost matches the pattern, Fst-pe is negligible (Fig. 7c), which results in a reasonable deviation. Therefore, to achieve accurate anisotropic sliding control, non-round patterns matching air/liquid/solid interface should be a better choice. Table 1. Shape and droplet-pattern interfacial width of the hydrophobic patterns

Surface

Semicircle/mm

V-shaped /mm

Line/mm

No.

WHP-L

WHP-S

WHP-L

WHP-S

WHP-L

WHP-S

1

3.40

1.65

2.45

1.81

1.79

0.65

2

4.27

1.71

4.94

1.84

3.27

0.62

3

5.89

1.64

6.64

1.65

5.05

0.66


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Table 2. Calculated and experimental values of FRES-ALL in L and S directions

Semicircle/μN

Surface No. 1 2 3

V-shaped/μN

Line/μN

Calc

Expl

Calc

Expl

Calc

Expl

L

257.7

255.6 ± 3.5

192.8

212.4 ± 3.8

135.7

123.2 ± 1.2

S

127.5

111.5 ± 1.5

146.6

147.4 ± 5.1

53.2

56.1 ± 0.2

L

323.7 309.9 ± 22.6

382.0

376.2 ± 5.1

247.9

163.5 ± 6.2

S

137.4

158.5 ± 2.1

148.7

143.7 ± 1.0

56.4

64.8 ± 2.6

L

450.6 489.1 ± 21.4

504.8

424.2 ± 10.0

382.8

203.2 ± 2.6

S

134.3 163.4 ± 12.3

135.0

139.8 ± 2.9

58.2

62.4 ± 1.1

Figure 6. Experimental and calculated FRES-ALL in large and small resistance directions for (a) semi-circle, (b) V-shaped and (c) line hydrophobic patterns.

Figure 7. (a) Schematic of surface tension at the tail end of the line hydrophobic pattern; (b) schematic of the droplet-pattern interface at the sliding off moment for the (b) line and (c) semicircle hydrophobic pattern.

Mechanical Hand for Droplet Manipulation with Nearly No Mass Loss Mechanical hand for droplet manipulations such as capture, transfer and mixng have promising applications to droplet-based high throughput cell screening, biochemical


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synthesis and molecular detection. Droplet can be captured and in-situ released without mass loss by bending or stretching flexible substrates with micropillar arrays. As discussed above, the FRES-ALL of droplets on the non-round hydrophobic patterns show directional difference. By taking advantage of the FRES-ALL anisotropy, equivalent droplet manipulations such as droplet capture and in-situ release are achiveable: droplet is captured by contacting the droplet in L direction, while moving surface in S direction can in-situ release the caputred droplet, as displayed in Fig. 8. For instance, FRES-ALL of droplets on No.2 line hydrophobic pattern are 163.5 μN in L direction, and 64.8 μN in S direction, meanwhile, the force for a 4μL droplet to detach the hydrophobic dot pattern (diameter: 1.3 mm) is 141.2 μN. It is thus clear that FRES-ALL in L direction is larger than the detaching force and therefore can capture the 4μL droplet on the dot as shown in Fig. 9a, in contrast, FRES-ALL in S direction is smaller than the detaching force which means that the droplet is allowed to be in-situ released (Fig. 9b, see also Movie S1). Droplets manipulations with an greater degree of sophistication can also be realized based on those basic droplet motion control. As shown in Fig. 9c and 9d, a 4μL droplet was captured, transferred to mix with another 4μL droplet, and the mixed 8 μL droplet was finally in-situ released (see Movie S2). It should be noted that the high adhesion at the dot patterned area exceeds the inner force of droplet, which contributes to the residual of smaller droplet on the dot. Nevertheless, the droplet manipulation process can be improved by using two samples that are both patterned with non-round hydrophobic areas. As shown in Fig. 10, a V-shaped hydrophobic pattern (WHP-L=3.3 mm; WHP-S=1.5 mm) can capture a 4 μL water droplet from another V-shaped pattern (WHP-L=2.8 mm; WHP-S=1.2 mm) with nearly no mass loss when contacting the droplet and moving in the direction with large sliding resistance. Additionally, the captured droplet is able to in-situ released


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just by reversing horizontal moving direction of the upper sample. This droplet manipulation process is stable and can be repeated for many times without fault (see Movie S3). It is also worth mentioning that it would not be necessary for the manipulated droplet to be larger than hydrophobic pattern when the above mentioned droplet manipulation method was used. Droplet-pattern interfacial width varies when droplet slides off the non-round hydrophobic patterns in different directions; as a result, the sliding resistance differs, and the droplet transfer which is dependent of the anisotropic sliding resistance can be achieved even though droplet volumes is much smaller than the patterns. In addition to pattern shape and size, contact angle hysteresis of the pattern also has some effect on the droplet manipulation process. According to the discussion in section 3.1, sliding resistance is positively correlated with contact angle hysteresis of the pattern; therefore, sliding resistance of droplet on a pattern increases with the increased contact angle hysteresis. However, if contact angle hysteresis of the pattern was too large, there would be water residual on the pattern after droplet manipulations. On the contrary, too small contact angle hysteresis may be not sufficient to generate enough sliding resistance anisotropy to capture a droplet. Smooth patterns coated with hydrophobic layer that is reported in this work has a contact angle hysteresis of 37.7° and does not generate water residual after droplet sliding because of its hydrophobicity, which makes it a suitable choice of droplet mechanical hand for transferring droplets without mass loss. The loss-less droplet manipulations may be applicable to high throughput cell and drug screening because of its capability of reducing the reagent loss caused by the liquid stain. Additionally, droplet manipulations via the non-round hydrophobic patterns do not require complicated motion control, only very simple surface


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movement, like side-to-side and vertical movement are necessary, which highlights their advantages of low-cost, easy-to-operate and facility to be programmed.

Figure 8. Schematic of capturing and in-situ releasing droplets using hydrophobic/superhydrophobic patterned surfaces as droplet mechanical hand.

Figure 9. Optical images of (a) capturing, (b) in-situ releasing a 4 μL droplet, (c) droplet mixing and (d) in-situ releasing the mixed droplet.


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Figure 10. (a) Schematic of capture and in-situ release a droplet by using two samples that are both patterned with non-round (V-shaped) hydrophobic areas; optical images of (b) capturing and (c) in-situ releasing a 4 μL droplet with nearly no mass loss.

CONCLUSIONS Superhydrophobic copper surfaces with different shaped hydrophobic patterns were fabricated

by

combining

the

masking

derived

selective

oxidation

and

plasma-enhanced chemical vapor deposition. Droplet-pattern interfacial widths, together with the sliding resistance of droplet sliding off a non-round hydrophobic pattern vary with the sliding directions because the air/liquid/solid interfaces move to different extent in different direction. This sliding anisotropy can be well predicted by classic drag-resistance model (Furmidge equation) except for line straight or approximate straight (V-shaped pattern with a obtuse angle) hydrophobic pattern. That’s because when droplets on these patterns slide in the larger resistance direction, the droplet outline and the line pattern intersect, the component force of the air/liquid surface tension perpendicular to pattern drives the droplet-pattern interface move to the middle of the pattern, which makes droplet-pattern interfacial widths at sliding off


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moment and the final sliding resistance smaller than the calculated ones. Therefore, to achieve precision anisotropic sliding control, non-round hydrophobic patterns with shapes that match the droplet outline, such as semicircle, should be a better choice. Droplet manipulations with nearly no mass loss like droplet capture, transfer and mixing can be achieved via the hydrophobic/superhydrophobic patterned surfaces by taking advantages of its hydrophobicity and sliding resistance anisotropy. Only very simple surface movement, like side-to-side and vertical movement are necessary for this droplet manipulation approach, which highlights advantages of low-cost and easy-to-operate. This research may have promising applications to droplet-based high throughput cell screening and molecular detection for reducing the reagent loss in droplet transfer process. ASSOCIATED CONTENT Supporting Information. Capture and in-situ release a 4 μL droplet (Movie S1), mix two 4 μL droplets and in-situ release the mixed droplet (Movie S2), capture and in-situ release a 4 μL droplet with nearly no mass loss (Movie S3). This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author. *Tel.: 86-25-84892551. E-mail: [email protected] Notes. The authors declare no competing finical interest.

ACKNOWLEDGEMENTS This work was financially supported by National Basic Research Program of China (Grant No.2015CB057304) and the Fundamental Research Funds for the Central


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The TOC graphic


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Figure 1. SEM images of the (a) oxidized area, (b) smooth non-oxidized area, (c) patterned area and (d) boundary area.


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Figure 2. Elemental analysis of the patterned copper surface. (a) Microscopic image of the line patterned copper surface. SEM-EDX mappings of the surface with elements (b) Cu, (c) O, (d) F.


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Figure 3. Schematic of droplet-pattern interfacial width WHP on a round hydrophobic pattern.


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Figure 4. Schematic of anisotropic sliding on a semicircle hydrophobic pattern: (a) schematic of the semicircle hydrophobic pattern; schematic of droplet sliding on the semicircle pattern in (b) L and (c, d) S directions.


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Figure 5. Schematic of anisotropic sliding on (a) V-shaped and (b) line hydrophobic patterns.


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Figure 6. Experimental and calculated FRES-ALL in large and small resistance directions for (a) semi-circle, (b) V-shaped and (c) line hydrophobic patterns.


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Figure 7. (a) Schematic of surface tension at the tail end of the line hydrophobic pattern; (b) schematic of the droplet-pattern interface at the sliding off moment for the (b) line and (c) semicircle hydrophobic pattern.


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Figure 8. Schematic of capturing and in-situ releasing droplets using hydrophobic/superhydrophobic patterned surfaces as droplet mechanical hand.


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Figure 9. Optical images of (a) capturing, (b) in-situ releasing a 4 μL droplet, (c) droplet mixing and (d) insitu releasing the mixed droplet using the mechanical hand.


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Figure 10. (a) Schematic of capture and in-situ release a droplet by using two samples that are both patterned with non-round (V-shaped) hydrophobic areas; optical images of (b) capturing and (c) in-situ releasing a 4 μL droplet with nearly no mass loss. 160x104mm (300 x 300 DPI)


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For Table of Contents Only 82x44mm (300 x 300 DPI)


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Droplet Mechanical Hand Based on Anisotropic Water Adhesion of

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