Droplet-patterning of viscous adhesive assisted with microfluidic technique

Journal Pre-proof Droplet-Patterning of Viscous Adhesive Assisted with Microfluidic Technique Zheng Xu, Ping Zhu, Xiaoyu Xu, Wei Zhao, Xiaodong Wang, Junshan Liu, Liding Wang PII:

S0143-7496(19)30268-4

DOI:

https://doi.org/10.1016/j.ijadhadh.2019.102518

Reference:

JAAD 102518

To appear in:

International Journal of Adhesion and Adhesives

Received Date: 5 September 2019 Accepted Date: 24 November 2019

Please cite this article as: Xu Z, Zhu P, Xu X, Zhao W, Wang X, Liu J, Wang L, Droplet-Patterning of Viscous Adhesive Assisted with Microfluidic Technique, International Journal of Adhesion and Adhesives, https://doi.org/10.1016/j.ijadhadh.2019.102518. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Elsevier Ltd. All rights reserved.

Droplet-Patterning of Viscous Adhesive Assisted with Microfluidic Technique Zheng Xu a*, Ping Zhu a, Xiaoyu Xu a, Wei Zhao a, Xiaodong Wangb*, Junshan Liub, Liding Wang b a

Dalian University of Technology, Key Laboratory for Micro/Nano Technology and System of

Liaoning Province, Dalian, 116024, China b

Dalian University of Technology, Key Laboratory for Precision and Non-traditional Machining

Technology of Ministry of Education, Dalian 116085, China Tel.: +86-411-84707713-2193, Fax: +86-411-84707940 E-mail: [email protected], [email protected]

Abstract: In transfer process of viscous adhesive, the initial geometrical shape of droplet has crucial influence on the filling completeness. Herein a droplet-patterning method assisted with microfluidic technique was presented to control the shape of viscous adhesive, in which three kinds of adhesives were selected as samples, spanning two orders of viscosity magnitude from 83 mPa·s to 7792 mPa·s. Microfluidic technique was developed to obtain the anti-adhesive boundaries of original polygon patterns. Considering viscous dissipation, a criterion was established to confirm the droplet-volume limitation of overflowing across anti-adhesive boundary. Moreover, the capability of droplet-patterning was evaluated with the ratio of droplet-pattern to original polygon and the dynamics of patterning was analyzed. Under the conditions of low Reynolds and Weber numbers, the smaller area and more edges of polygon, lower viscosity and more non-polar groups of adhesive are helpful to improve the patterning capability. Keywords: Droplet-Patterning; Anti-adhesive; Microfluidic technique

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0. Introduction The ability to control the shape of viscous adhesive in nano-micro liter level is essential for adhesive microbonding, 3D-printing, and soft-lithography etc [1-3]. For example, in chip package process with adhesive microbonding, the shape-optimization in the range of a few millimeters could eliminate encapsulation void which was responsible for over-stress failure [4]. In micro-3D-printing field, the shape control of viscous ink was helpful to the structural homogeneity and the reduction of residual stress [5]. However, comparing with the development of dispensing technique, the research on patterning mechanism of viscous liquid is still insufficient. In practice, the manual brushing-and-smearing is still widely used to adjust the shape of viscous droplet that extremely depends on the skill of operator. In the past decades, the shape-controlling of droplet based the microfluidic technique had attracted lots of attention. Of particular interest is the tailoring of droplet shape via the wetting difference of substrate surface. Raj et al. showed the ability to control the shapes of aqueous droplet on the surface of micropillar array [6]. According to their results, as the pitch of these micropillars along horizontal direction increased, the contact angles of droplet along horizontal and vertical directions were not the same. Thus the anisotropy phenomenon of contact angle can be used to convert a droplet into various shapes. More recently, some scholars focused on the control of non-aqueous or hybrid droplets. Feng et al. studied the manipulation of water-ethanol droplet on the substrate with micropyramid array [7]. As the ethanol concentration increased from 0 to 30%, the contact angles along the horizontal and diagonal directions of micropyramid array decreased respectively by 15° and 21°. The difference of contact angle resulted in a droplet with octagonal wetted area being transformed into to a quasi-quadrilateral droplet. Courbin et al.

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discussed the spreading phenomenon of isopropanol droplet over microdecorated substrate, in which several shapes were obtained by changing the wettability of substrate [8]. However, the liquid samples in previous researches have relatively low viscosity. Until now, there is still no report about the droplet-patterning mechanism for highly viscous adhesives. Herein, we investigated the patterning mechanism for adhesives, in which three kinds of adhesives were selected as samples, spanning two orders of viscosity magnitude from 83 mPa·s to 7792 mPa·s. An efficient method of droplet-patterning based on microfluidic technique was developed to obtain the anti-adhesive boundaries of original polygon pattern. Considering the effect of viscous dissipation and surface tension, a criterion was established to confirm the initial droplet-volume limitation of overflowing across the anti-adhesive boundary. Moreover the effect of droplet-patterning was evaluated with the ratio of droplet-pattern to original polygon, and the influences of viscosity, geometric structure of polygon, and capillary effect were analyzed.

1. Materials and methods 1.1. Materials Three adhesives of different viscosities and frequently-used in microengineering were selected as samples: For the assembly of MEMS (Micro-Electro-Mechanical System), it is necessary to choose high-viscosity adhesives that can provide sufficient bonding strength. An UV-curable adhesive AA3321 was selected for experiments since its features (ultimate tensile strength 18.6 MPa and shearing strength 5.2 MPa) meet the common bonding requirement. To compare to AA3321, we also selected the relatively lower viscous photoresist (BN308-450 and BN303-100) that used in dip pen lithography to obtain micropatterns. UV-curable adhesive AA3321 (viscosity and density at 25°C: 7792 mPa·s and 1.08 g/cm3) was purchased from Henkel

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Loctite (Germany). Negative photoresist BN308-450 (456 mPa·s and 0.86 g/cm3) and BN303-100 (83 mPa·s and 0.86 g/cm3) were purchased from Kempur Microelectronics (China). Chemical modifier, 1H-1H-2H-2H-perfluorooctyltriethoxysilane (PFOTCS) was purchased from JY Chemicals (China). Chemical modifier and alcohol solution were formulated into chemical modification solution by a volume ratio of 1:20. Polydimethylsiloxane (PDMS, Sylgard 184) was obtained from Dow Corning (USA). 1.2. Methods Most experimental processes were carried out in a clean room with constant temperature 25°C, humidity 55%, and the illumination of yellow light. Each operation was repeated five times. Both surface tension (pendant drop method) and contact angle were measured by a drop shape analyzer (DSA100, Kruss, Germany) as shown in Fig. 1(a). Viscosity was measured with a rheometer (DHR2, TA Instruments-Waters LLC, USA). Due to the dispensing method of contact transfer of highly viscous adhesives (Fig. 3(d)), the patterning of adhesive droplets was performed at low shear rate (<1 s-1). Thus, the viscosities of adhesives were taken at the low shear rate and were considered to be approximately constant. Droplet-patterns of adhesives were observed by stereoscopic microscope (SZX-BI30, Olympus, Japan). All droplets were initially generated by a dispenser (ML-808GX, Musashi, Japan) installed on a 3D precise positioning platform as shown in Fig. 1(b). The optical unit, consisting of a camera (MER-200-14GC, Daheng, China) and a 0.75× microscope was used to obtain the dispensing position.

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Fig. 1. (a) Measurement of advancing contact angle (b) 3D positioning platform for droplet-patterning.

2. Experimental Section 2.1. Development of microfluidic chip As shown in Fig. 2(a), the designed microfluidic chip contains eight polygon units with a star-like layout. In each polygon unit, the black region is the polygon (here the circle is regarded as one with infinite edges) being enclosed by an excircle with 8 or 4 mm diameter. Each region between polygon and excircle is anti-adhesive with chemical modification. All polygon units are designed to share a common inlet, where chemical modification solution is injected and entered the anti-adhesive region via microchannels (W×H: 0.3 mm×0.2 mm). Herein, only circle, convex octagon, convex hexagon, and square were used.

Fig. 2. Droplet-patterning setup via microfluidic technique: (a) Star-like layout of microfluidic chip (b) Selective

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surface modification (c) Pre-dispensed positioning (d) Contact transfer process (e) Droplet-patterns on the

modified silicon wafer.

The microfluidic chip was fabricated as follows: SU-8 mold was formed via lithography. The PDMS pre-polymer was mixed with curing agent (10:1), degassed, poured on the mold, and heated to 80 °C for 2 h. Then, the cured PDMS plate was peeled off from the mold. Subsequently, the inlet and outlets on PDMS plate were machined. Finally, the PDMS plate and a 2-inch silicon wafer were reversibly bonded together after surface treatment with oxygen-based plasma at 12 W (K1050X-190, Quorum, UK) [9]. Selective silane anti-adhesive modification was performed as follows: Firstly, chemical modification solution was injected into microchannel and kept about 30 mins as shown in Fig. 2(b). After that, the solution in microchannel was dried in a vacuum oven. Then the PDMS plate was removed from the silicon wafer as shown in Fig. 2(e). 2.2. Viscous droplet-patterning process 2.2.1. Characteristic of droplet-patterning The well-modified silicon wafer was placed on a 3D positioning platform as shown in Fig. 2(c). With contact transfer approach, a viscous droplet was dispensed onto the center of polygon as shown in Fig. 2(d). Generally, in the contact dispensing of highly viscous fluid, Reynolds ( Re = ρUL / µ ) and 2 Weber ( We = ρU L / γ LG ) numbers are very low due to the strong restriction of viscous force in

the narrow needle (inner diameter of needle no less than 0.5 mm). Where ρ is the density, µ is the viscosity, γ LG is surface tension, and U is the droplet-patterning velocity. Taking BN303 as an example: The velocity U is defined as the average velocity . Once the BN303 droplet is dispensed on the center of polygon with 2 mm inradius, it takes about 5 s to spread the distance D

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from the initial droplet diameter (red dashed line) to the meniscus flowing front (white dashed line) as shown in Fig. 7(a). Thus is 0.11 mm/s. Similarly, the velocities of BN308 and AA3321 are respectively 0.08 mm/s and 0.03 mm/s. Correspondingly, Re and We for BN303, BN308, AA3321 are respectively obtained with characteristic length L (incircle diameter: 4 mm), shown in Fig. 3. As can be seen from the figure, surface tension mainly drives the dispensed droplets to spread (We<<1, 1.2~14.7e-7), and viscous force mainly hinders the spreading of droplets (Re<<1, 0.2~45.6e-4). The relationship between surface tension and viscous force can be explained by Ohnesorge number ( Oh = µ / ρ Lγ LG ) that serves to measure the ratio of surface tension to viscous force (Fig. 3). As a result, as the viscosity increases, the viscous hindering effect on the droplet-spreading gradually strengthens. The research of Lin et al. also showed that the viscosity and surface tension are important factors to the spreading process, which is consistent with the above analysis [10]. Thus, the inertia force can be neglected and the spreading behavior of droplet is mainly dominated by viscous force and surface tension [11, 12].

Fig. 3. Parameter space defined by axes of Reynolds, Weber, and Ohnesorge numbers showing the hindering force

for initial droplet spreading after contact dispensing.

Moreover, for liquid adhesives, since the effect of viscosity is more dominant compared to the elasticity [13], the relaxation time of liquid adhesives is universally proportional to its 7

viscosity [14]. For highly viscous AA3321, the final patterning time after dispensing was set to 5 mins. For BN303 and BN308, the final patterning time was 1 min and 2 mins respectively. 2.2.2. Selection of initial droplet volume for patterning Obviously, an insufficient amount of initial droplet will lead to the short-filling, whereas an excessive one will make the droplet across anti-adhesive boundary. Thus the selection of initial droplet volume is crucial. Although an optimal initial droplet can be obtained by the final balance of forces, the establishment and solution of mechanical equation are complicated due to the dynamic interface of droplet. Therefore instead of the balance of forces, the optimal initial droplet is obtained by energy conservation that was widely used in the study of droplet dynamics [10, 15-18]. Herein, since the area of circle is equal to the area of incircle with other polygons, the acceptable initial maximum volume of droplet is evaluated with the total energy variation in droplet spreading. Most literatures on droplet dynamics were about low viscosity fluid such as aqueous one, in which only surface energy and potential energy were considered [15, 16]. But for highly viscous droplet, the viscous force plays a major role and the energy loss from viscous dissipation also needs to be considered. Therefore, a criterion about the energy barrier that avoids the overflowing at anti-adhesive boundary can be expressed as [16, 17] 0 0 1 1 γ LG ( ALG − cosθ 0 ASL ) + ρV0 gh0 ≤ γ LG ( ALG − cosθ1 ASL ) + ρV1 gh1 + ∫

t

∫

0 V1

µ (U / h1 )2 dVdt

(1)

where the left side is initial surface energy and potential energy of droplet in turn, the right side is orderly the surface energy, potential energy, and energy loss after spreading. Since the droplet dispensed with contact transfer initially spreads like a hemisphere restricted with surface tension and viscous force as shown in Fig. 2(d), the contact angle θ0 of initial hemispherical droplet is 90°. 0 0 ALG = 2π r 2 and ASL = π r 2 respectively are the initial liquid-gas and solid-liquid interfacial

areas, V0 = 2π r 3 / 3 is the initial volume of droplet, r is the initial radius of droplet as shown in Fig. 4, g is the gravitational acceleration, h0 = 3r / 8 is the height of initial center of gravity. 2 A1LG = π (hcap + R 2 ) is the liquid-gas interfacial area after droplet spreading, which is equivalent to

the spherical crown area, R (2 mm or 0.5 mm) is the radius of circle. hcap = R / sin θ1 − R / tan θ1

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is the height of spherical cap, θ1 is the advancing contact angle on the anti-adhesive treated Si wafer,

1 ASL = π R2

is

the

solid-liquid

interfacial

area

after

droplet

spreading,

2 2 2 V1 = π hcap (3R 2 + hcap ) / 6 is the volume of spherical cap, h1 = (2 R2 + hcap )hcap / (6R 2 + 2hcap ) is

the center of gravity with spherical cap. The energy loss can be simplified to µ (U / h )2 V1t [17, 2 ) / 6R2 is the average thickness after droplet spreading, t is the final 18], h1 ≈ hcap (3R2 + hcap

patterning time.

Fig. 4. Schematic of viscous droplet spreading.

From equation (1), the initial maximum radius rmax of droplet can be obtained, where the required surface tension and advancing contact angle are shown in Table 2. The acceptable initial 3 / 3 as shown in Table 1. Considering maximum volume of droplet is calculated by Vmax = 2π rmax

both the initial maximum volume and the previous results [19], the experimental volume is confirmed as shown in Table 1. Table 1 Dispensing parameters and droplet volumes.

Convex polygons

2.0 mm inradius

0.5 mm inradius

Experimental volume (µL)

Inner diameter of needle-tip (mm)

Contact distance (mm)

Extruded time (s)

AA3321

0.42

1.20

2.40

BN308

0.30

0.95

BN303

0.20

0.50

AA3321

0.30

BN308 BN303

Adhesives

deviation

Initial maximum volume (µL)

9.25

0.08

9.57

1.30

5.20

0.07

5.38

0.20

2.83

0.06

2.94

0.55

0.30

0.15

0.02

0.17

0.18

0.40

0.18

0.07

0.01

0.08

0.10

0.30

0.12

0.03

0.01

0.04

3. Results and Discussion 3.1. Characterization of anti-adhesive performance

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Mean

Standard

Table 2 Contact angles and surface tension.

Adhesives

Surface tension (mN/m)

Contact angle on untreated Si wafer Mean

Standard deviation

Contact angle on anti-adhesive treated Si wafer Mean

Standard deviation

Advancing contact angle on anti-adhesive treated Si wafer Mean

Standard deviation

BN303

27.9

23.3°

0.4

38.1°

0.5

49.4°

0.4

BN308

28.3

37.1°

0.4

48.6°

0.4

60.6°

0.5

AA3321

31.9

47.2°

0.5

56.9°

0.4

71.9°

0.4

Anti-adhesive performance of silanization treatment was evaluated by contact angle. Table 2 shows that silanization treatment can improve the anti-adhesive performance, but doesn't reach a hydrophobic effect of water, owing to the weak surface tension and the hybrid dipole-dipole interaction between adhesives and modified region of Si wafer [20]: The original Si wafer in air exhibits a polar feature with a polar functional group hydroxyl (-OH) on the surface. After the Si wafer is silanized, the hydroxyl group is firstly converted into a weakly polar ether bond, and the Si wafer surface is finally covered by the trifluoromethyl group (-CF3) to form a hydrophobic layer. Thus, the contact angle on the modified region of Si wafer increases after the silanization treatment, and the modified region mainly shows non-polar feature [21]. Since AA3321, BN308, and BN303 present polarity feature (Table 3), these droplets tend to clump together and repel to be attracted by non-polar molecules on the modified region. However, since both polar and non-polar groups always coexist in adhesive, the anti-adhesive effect is relatively weaker than common hydrophobic effect. Moreover, it is well known that the polarity of functional groups methyl and methine (-CH3 and -CH2) contained in both BN303 and BN308 is much less than the functional group carboxyl (-COOH) of AA3321 [22]. Therefore, AA3321 is more easily repelled by the non-polar molecules on modified region, thereby improving the anti-adhesive effect.

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Table 3 Polar properties of adhesives and substrate. BN303/308 --

AA3321 Polyurethane acrylate (monocomponent)

Si wafer Silicon wafer in air

Treated Si wafer --

Weak-polar component

M-xylene (65~90%) and cyclized rubber (5~30%)

--

--

--

Non-polar component

--

--

--

Silicon wafer silanized by PFOTCS

Polar component

3.2. Droplet-patterning in convex polygons with 2 mm inradius 3.2.1. Description and quantitative evaluation of droplet-patterning results The droplet-patterning results in four kinds of convex polygons with 2 mm inradius are shown in Fig. 5, in which all droplet-patterns are constrained within polygons. For all three adhesives, the area ratios of droplet-patterns to original polygons are shown in Fig. 6. These values of areas were obtained by imaging characteristic extraction and calculation with Matlab. From circle, octagon, hexagon to square, the overall mean area ratios of BN303 droplets are 99.84%, 99.71%, 99.37%, and 91.51% in turn. The overall mean area ratios of BN308 droplets are 99.82%, 99.67%, 99.36%, and 85.80% in sequence. Thus, for BN303 and BN308, the area ratios of droplet-patterns to original polygons except for the square are almost 100%. Since the viscous resistance of AA3321 is much higher than others, the capability of completely filling the polygons is relatively weaker: The overall mean area ratios of AA3321 droplets are orderly 99.80%, 98.53%, 94.55%, and 83.50%. Thus, for AA3321, the area ratio of droplet-pattern to circle is nearly 100%, whereas the corners of octagon and hexagon aren't fully filled. The mean area ratios at the corners of octagon and hexagon are 74.10% and 40.01% in turn.

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Fig. 5. Droplet-patterns of circle, octagon, hexagon, and square: (a) BN303 (b) BN308 (c) AA3321.

Further, the mean area ratios at the corners of square are orderly 60.06%, 33.21% and 22.40% from BN303, BN308 to AA3321. Thus, for three adhesives, the corners of square aren't fully filled and the filling completeness is BN303>BN308>AA3321. Obviously, although the contact angle on the modified region is lower than 90°, the region by silanization treatment still effectively restrains the spreading flow. Moreover, the short-filling phenomenon at the corners becomes gradually obvious with the decreasing of edge number of polygon.

Fig. 6. (a) Overall area ratios of droplet-patterns to the original polygons and (b) area ratios at corner.

3.2.2. Dynamics analysis of droplet-patterning process The patterning process can be analyzed with the effects of geometric structure, capillary effect, and boundary-friction force: At the onset of droplet-dispensing on the center of polygon, 12

the droplet prefers to spread like a hemisphere dominated by surface tension and viscous force to fill the region of incircle. Once the droplet touches the anti-adhesive boundaries, the droplet is constrained to flow along the boundaries to corners. After that, the process can be divided into two stages. Take adhesive BN303 as an example, the patterning-flow at corner is shown in Fig. 7. The green dashed line represents the boundary of square, the white dashed line represents the flowing front driven by capillary effect, and the white solid line represents the flowing front after 1 min.

Fig. 7. (a) Capillary effect drives and (b) boundary-friction force hinders BN303 motion at corner.

In the first stage, the flowing front at corner is a meniscus shape, shown in Fig. 7(a). It means that capillary effect acts to drive the adhesive to the corner. Since the contact angles in the modified region is less than 90°, the flow speed near boundaries is faster. After two adjacent boundaries at the corner are fully wetted, the capillary effect disappears since three phase interface no longer exists. The flow front at a corner gradually changes from a meniscus shape to a parabola shape, as shown in Fig. 7(b). It means that the spreading of droplet mainly depends on its kinetic energy rather than capillary effect in the second stage. Due to the influence of boundary-friction force, the profile of flow velocity gradually changes from meniscus to parabola. Thus the boundary-friction force relates to the distance di with the same vertical line (red solid line) between two adjacent boundaries, shown in Fig. 8. As the distance di increases, the droplet near

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the midpoint of distance di flows faster. Since an inner angle of octagon, hexagon, and square is 135°>120°>90°, the distance di is d1>d2>d3. Thus the droplet-patterning capability at corner gradually improves with the increasing of di. That is, as the number of sides and angle of polygon increase, the droplet-patterning capability gradually improves.

Fig. 8. Schematic of convex polygons.

In addition, viscous force plays a major role in the patterning of highly viscous droplet. The patterning can be further analyzed by the energy loss µ (U / h )2 V1t caused by viscous force as shown in equation (1). In the formula of energy loss, the ratio of µ with BN303, BN308, and AA3321 is about 1.0: 5.5: 93.9 respectively, the ratio of U is about 3.7: 2.7: 1.0, the ratio of h is about 1.0: 1.3: 1.7, and the ratio of mean V1 is about 1.0: 1.8: 3.3. Thus the energy loss is BN303
BN303 (e-9 N·m)

BN308 (e-9 N·m)

AA3321 (e-9 N·m)

Initial surface energy

220.0

333.5

551.9

Surface energy after spreading

197.3

302.9

486.7

The analysis about energy loss can be also verified from the variation of surface energy before and after the droplets spreading in the circular pattern (Table 4). Since the surface tension is BN303BN308>AA3321. However, the variation of

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surface energy in the experiment is BN303
The droplet-patterning results in the convex polygons that have 0.5 mm inradius are shown in Fig. 9. And the droplet-patterning results are similar to that of the polygons with 2 mm inradius (here the polygons were defined as large-sized polygons). Similarly, the area ratios of the droplet-patterns to the original polygons are shown in Fig. 10.

Fig. 9. Droplet-patterns of circle, octagon, hexagon and square: (a) BN303 (b) BN308 (c) AA3321.

Fig. 10. (a) Overall area ratios of droplet-patterns to the original polygons and (b) area ratios at corner.

The above results show that the filling completeness of droplet-patterns in the small-sized 15

polygons is still BN303>BN308>AA3321. However, the area ratios of droplet-patterning are obviously higher than that of the large-sized polygons. The phenomenon is mainly affected by the capillary effect and the geometric structure of polygons. Comparing to the large-sized polygons, the interior space of small-sized polygons is smaller. As a result, in the first stage of droplet-patterning at corner, capillary effect relatively strengthens in small-sized polygons. The filling ability of adhesive-droplets driven by capillary effect at corner is enhanced. Especially, since the residual area that is equal to polygon area minus incircle area in the small-sized polygons is about 16 times as small as that of the large-sized polygons, the corners of small-sized polygons are easier to be fully filled. Specially for the small-sized square, the final unfilled area is least 36 times much smaller than that of large-sized squares.

3. Conclusion Here, a novel method of droplet-patterning of adhesive was presented. Assisted with the microfluidic technique, the anti-adhesive boundaries of original polygon patterns were obtained and the anti-adhesive performance was evaluated, in which the variation tendency of contact angle is caused by the polarity difference of components in adhesives. Considering the effect of viscous dissipation, a criterion was established to confirm the droplet-volume limitation of overflowing across the anti-adhesive boundary. The capability of droplet-patterning was evaluated with the ratio of droplet-pattern to original polygon. Moreover, the droplet-patterning was analyzed by the effects of geometric structure, capillary effect, and boundary-friction force too. Briefly, smaller area and more edges of polygon, lower viscosity and more non-polar groups of adhesive are helpful to improve the droplet-patterning capability in the convex polygons under the conditions of low Reynolds and Weber numbers. We believe that the

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method could be useful for the applications of adhesive transfer etc. Further work will concern the patterning-efficiency improvement.

Acknowledgments Authors wishing to acknowledge assistance received from National Natural Science Foundation of China (No. 51975102), Major Project of Basic Scientific Research of Chinese Ministry (No. JCYK 2016 205 A003), and Fundamental Research Funds for the Central Universities (No. DUT19LAB22).

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Zheng XU and Xiaodong WANG

Please ensure that the following information is supplied for the corresponding author: E-mail address

[email protected], [email protected]

Full postal address Dalian University of Technology, Key Laboratory for Micro/Nano Technology and System of Liaoning Province, Dalian, 116024, China

Telephone and fax number

+86 411 84707713-2193, +86 411 84707940

Checklist: Are the keywords supplied? Yes Are all figures supplied with captions? Yes Are all tables supplied, and with captions? Yes Are copyright permission letters enclosed for any artwork/tables previously published, including those published on the world-wide-web? Yes If any of the illustrations are to be printed in colour please indicate this below: note that in the printed paper, colour costs you about 300 Euros per Fig, while on-line, colour is free Printed version of figures in black-and-white.