DROPLET MANIPULATION ON HIGH ADHESION

SUPERHYDROPHOBIC SURFACES

Daisuke Ishii, Masatusgu Shimomura

WPI-AIMR, Tohoku University,2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan

CREST, Japan Science and Technology Agency, 4-1-8 Hon-cho, Kawaguchi 332-0012, Japan

Hiroshi Yabu

IMRAM, Tohoku University,2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan

CREST, Japan Science and Technology Agency, 4-1-8 Hon-cho, Kawaguchi 332-0012, Japan

Keywords: Superhydrophobicity, Microfluidics, Water droplet, Adhesion, Lotus effect.

Abstract: Micro droplet handling is very important for micro and nano fluidic devices and an intelligent bio interface.

Micro droplet transfer via high adhesion superhydrophobic surfaces has been reported in recent years. We

demonstrated water droplet adhesion controllable superhydrophobic metalpolymer surfaces. Moreover we

achieved micro droplet transfer between superhydrophobic surfaces by using different droplet adhesion

properties. Water micro droplets were transferred from a low-adhesive superhydrophobic surface to a

middle-adhesive superhydrophobic surface via a high-adhesive superhydrophobic surface without any mass

loss. After transferred droplet possessed high water contact angle over 150 degrees. These moving processes

were performed repeatedly. Droplet handlings on the adhesion superhydrophobic surfaces will be expected

for fluidic bio devises with energy saving.

1 INTRODUCTION

Droplet manipulations mimicking behaviours on

plant or insect surfaces such as lotus leaf effect are

now interesting because simple surface structures

provide amazing functionalities. Superhydrophobic

surfaces which have the water contact angle lager

than or near 150° are much paid attention, since its

good water repellent property is used various

applications in coating and electronic technologies

(Zhang, 2008). Many researchers have been reported

to obtain strong water repellent surface such as a

hydrophobic fractal surface (Onda, 1996) and a

nanopin array surface (Hosono, 2005). Recently

several reports were published about water droplet

adhesive superhydrophobic surfaces in mimicry of

gecko’s feet (Cho, 2008) and rose’s petals (Feng,

2008). These adhesion properties were caused by

van der Waals’ force on large real surface area

against small apparent surface area. It was difficult

to control the adhesion forces because the adhesion

was caused by the surface structures.

Herein we demonstrated that a superhydrophobic

metal−polymer (MP) surface with different droplet

adhesion properties. The adhesive superhydrophobic

surfaces were composed of hexagonally ordered

polymer pillar arrays made from a self-organized

honeycomb-patterned polystyrene film (Yabu, 2005)

and metal micro domes deposited by nickel

electroless plating (Ishii, 2008). The dome density

was changed by catalyzation process for electroless

plating.

Droplet manipulations such as a transfer were

achieved by using the MP surface possessing

different water adhesion force. Micro droplet

handling by control of wettability is important for

further understanding of superhydrophobic surfaces

and application in microfluidic bio devices.

2 EXPERIMENTAL

2.1 Preparation Method

The superhydrophobic metal−polymer surface (MP

surface) composed of hydrophobic polymer pillar

113

Ishii D., Shimomura M. and Yabu H. (2009).

DROPLET MANIPULATION ON HIGH ADHESION SUPERHYDROPHOBIC SURFACES.

In Proceedings of the International Conference on Biomedical Electronics and Devices, pages 113-116

DOI: 10.5220/0001553901130116

 SciTePress

arrays and metal micro domes was fabricated by

electroless plating for honeycomb-patterned polymer

films and peeling process (See Figure 2).

According to our previous report (Karthaus,

2000), the honeycomb films were prepared by

casting a chloroform solution of 10:1 mixture of

polystyrene (PS; M

= 280 000 g mol

-1

) and

synthesized amphiphilic copolymer (CAP; M

270 000 g mol

-1

) on a glass substrate with

hexagonally condensed water droplet arrays. The

honeycomb film cut to 1 × 1 cm

was soaked in a

catalytic mixture solution of 6.0 ml containing 0.010

mol dm

-3

poly(allylamine hydrochloride) (PAH; M

= 14 000 g mol

-1

) and 0.010 mol dm

-3

PdCl

at 25°C.

The catalytic solution was gradually heated to 30°C,

45°C, and 60°C, respectively, and kept for 10 min

under horizontal shaking at 10 rpm. Treated

honeycomb films were immersed in a nickel plating

bath (Ishii, 2006) at 25°C containing 0.10 mol dm

-3

Ni(H

)

⋅6H

O, 0.19 mol dm

-3

, 0.030 mol

-3

COONa and 0.0098 mol dm

-3

(NH

)

without any rinse and drying. Then the plating bath

including the treated honeycomb film was heated to

70°C and kept for 2h with no stirring. After rinsing

and drying, a nickel layer was covered on the

honeycomb film. After electroless plating, metallic

faces of the nickel-covered honeycomb films were

adhered on an acryl substrate by an epoxy resin.

After heating at 70°C for 2h, a lower half layer of

the nickel-covered honeycomb film was peeled off

from the acryl substrate.

2.2 Physical Measurements

Surface structures of the MP surfaces were observed

by a scanning electron microscope (SEM; Hitachi S-

5200, Japan). A water contact angle (WCA) to 3 mg

water droplet on a surface was measured by contact

angle meter (Kyowa Interface Science DW-300,

Japan). A sliding angle (SA) was measured to tilt the

surfaces with a micro-droplet of 5 mg. Density of

the metal dome which is defined by division of the

0.8

0.2

HCl

CAP

PAH

Figure 1: Chemical structures used in this report.

70°C

(a) Honeycomb film

30°C, 45°C, 60°C

1) Catalyzation

2) Electroless plating

3) Rinse and drying

4) Peel off top layer

(b) Nickel-covered honeycomb film

70°C

(a) Honeycomb film

30°C, 45°C, 60°C

1) Catalyzation

2) Electroless plating

3) Rinse and drying

4) Peel off top layer

(b) Nickel-covered honeycomb film

Figure 2: Schematic illustrations of a preparation method

of MP surfaces. SEM images of top (left) and tilt (right)

views of (a) a honeycomb film, (b) a nickel-covered

honeycomb film fabricated by immersion in the catalytic

mixture solution at 45°C, and (c) a MP surface fabricated

by peeling off a top layer of the nickel-covered

honeycomb film shown in Figure 2b. (Scale bar: 10 μm).

number of metal domes by the number of

honeycomb holes was calculated by means of low-

magnified SEM images.

2.3 Droplet Manipulations

Droplet manipulations such as droplet transfer were

demonstrated by using the MP surfaces with

deferent adhesion properties. Figure 3 shows a

schematic illustration of water droplet transfer. The

5-mg water droplet was prepared on the MP surface

fabricated by using the catalytic mixture solution at

30°C. Then the MP surface fabricated by using the

catalytic mixture solution at 60°C was closed to the

BIODEVICES 2009 - International Conference on Biomedical Electronics and Devices

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water droplet from above and touched a little bit.

The upper MP surface was pulled up slowly from

the lower MP surface. Then the upper MP surface

catching the water droplet was closed and touched to

the other MP surface fabricated by using the

catalytic mixture solution at 45°C. Finally the upper

MP surface was pulled up again.

Touch

High-adhesion

Low-adhesion

Middle-adhesion

Pull up

Pull upTouch

Transfer

Touch

High-adhesion

Low-adhesion

Middle-adhesion

Pull up

Pull upTouch

Transfer

Figure 3: Schematic model of a micro-droplet transfer by

using the MP surfaces with different adhesion properties.

3 RESULTS AND DISCUSSION

3.1 Superhydrophobic Metal−polymer

Surface

SEM images of a honeycomb film, a nickel covered

honeycomb film fabricated by immersion in the

catalytic solution at 45°C, and a MP surface

fabricated by immersion in the catalytic solution at

45°C are inserted in Figure 2. An average diameter

of a honeycomb hole was about 7 μm. A nickel-

covered honeycomb film possessing some pores,

which were distributed in the honeycomb holes,

were obtained after electroless plating including

immersion in the catalytic mixture solution. A tilted

SEM image shown in a right column of Figure 2b

clears that the pores were openings of micro mono

vessels. When the temperature of the catalytic

mixture solution was changed low (30°C) and high

(60°C), the number of the vessels was decreasing

and increasing, respectively. In general, wettability

of all surfaces including the PS honeycomb film is

influenced by a solution temperature, because the

surface tension of all solutions is represented by

function of the solution temperature. This result

indicates that the number of the vessels was

dependent on wettability of the catalytic mixture

solution to the honeycomb film. In the case of

immersion in the catalytic solution at low

temperature, wettability of the honeycomb film was

low, so that the number of the vessels in the nickel-

covered honeycomb film was a few. On the other

hand, in the case of immersion in the one at high

temperature, the number of the vessels was much

because of good wettability to the honeycomb film.

The number of the vessels of the nickel-covered

honeycomb film was easily changed by the catalytic

mixture solution temperature.

The MP surfaces after peeling off the top half of

the nickel-covered honeycomb film were composed

of superhydrophobic PS pillar arrays and

hydrophilic nickel micro-domes as shown in Figure

2c. The nickel micro-dome was reverse side of the

micro mono vessel in the nickel-covered honeycomb

film. This result anticipates that density of the nickel

dome to the honeycomb hole is controlled indirectly

by temperature of the catalytic mixture solution.

Figure 4 shows SEM images of the MP surfaces

having different nickel dome density. The nickel

dome density of the MP surface prepared by

immersion in the catalytic mixture solution at 30°C,

45°C, and 60°C was about 3%, 15%, and 25%,

respectively. The surface properties such as surface

wettability and droplet adhesion properties were

controlled easily, because hydrophilic-hydrophobic

balance was varied by difference of the nickel dome

density (See Table 1).

(a) 30°C(b)45°C(c)60°C(a) 30°C(b)45°C(c)60°C(a) 30°C(b)45°C(c)60°C

Figure 4: SEM images of the superhydrophobic

metal−polymer surfaces fabricated by using the catalytic

mixture solution at (a) 30°C, (b) 45°C, and (c) 60°C. The

black dots indicate the nickel domes. (Scale bar: 100 μm).

Table 1: Surface properties of the MP surfaces.

Sample Density WCA SA Adhesion

30°C

155° <5°

Low

45°C

15%

150° 30°

Middle

60°C

25%

145°

N/A High

3.2 Micro-droplet Transfer

The water droplet adhesion properties were

measured by water contact angles (WCAs) and

sliding angles (SAs). The MP surface with nickel

dome density of 3% possessed a WCA of 155° and a

SA of less than 5°, and was abbreviated as a low-

adhesion MP surface. The MP surface with dome

DROPLET MANIPULATION ON HIGH ADHESION SUPERHYDROPHOBIC SURFACES

115

density of 15% possessed a WCA of 150° and a SA

of 30° (a middle-adhesion MP surface). The MP

surface with dome density of 25% possessed a WCA

of 145° and a not measured SA because the droplet

adhered the surface when turned upside down (a

high-adhesion MP surface). As the dome density

was increasing, the WCA was decreasing and the SA

was increasing, which means that the hydrophilic

nickel domes gave the adhesion behaviors. This

result made clear that the adhesion property was

controlled by the quantity of the nickel dome easily

changed by the catalytic mixture solution

temperature for electroless plating.

155° 150°

Low-adhesion surface

HIgh-adhesion surface

Middle-adhesion surface

155° 150°

Low-adhesion surface

HIgh-adhesion surface

Middle-adhesion surface

Figure 5: Droplet transfer of 5.0-mg water droplet from

the low-adhesion MP surface to the middle-adhesion MP

surface via the high-adhesion MP surface.

Water micro droplet transfer was attempted by

using the MP surfaces with different adhesion

properties as shown in Figure 5. A water droplet of 5

mg on the low-adhesion MP surface was carried

with the high-adhesion MP surface by means of

pulling off after slight contact from above. However,

in the case of the middle-adhesion MP surface, a

water droplet did not remove from the low-adhesion

MP surface. On the other hand, the high-adhesion

MP surface was not caught a water droplet on the

middle-adhesion MP surface from above. These

results suggest that the adhesion force of the high-

adhesion MP surface was stronger than that of the

low-adhesion MP surface plus gravity on the water

droplet, and weaker than that of the middle-adhesion

MP surface plus gravity on the water droplet. By

using this difference, the water droplet was

transferred from the low-adhesion MP surface to the

middle-adhesion MP surface via the high-adhesion

MP surface. After transfer, the water droplet on the

middle-adhesion MP surface was sliding when the

surface was tilted at about 30°. A droplet transfer

reported in the past is from a superhydrophobic

surface to a hydrophilic surface via an adhesion

superhydrophobic surface (Cho, 2008). Therefore,

after transfer, the water droplet is spreading, and is

unable to be handled. The novel transfer method in

this report remains a droplet shape after transfer, so

that the droplet was handily manipulated again.

These behaviors were useful to microfluidic devices,

bio interfaces, and micro-reactors.

4 CONCLUSIONS

We could fabricate water repellency and adhesion

properties of superhydrophobic metal-polymer

surfaces by electroless plating for self-organized

honeycomb films including immersion in a catalytic

Pd salt and a cationic polymer mixture solution. It

was found that a water contact angle and a water

droplet adhesion property were changed by metal

dome density which was easily controlled by the

temperature of the catalytic mixture solution.

Droplet transfer between superhydrophobic surfaces

was demonstrated by means of using the metal-

polymer surfaces with different adhesion properties.

REFERENCES

Cho, W. K., & Choi, I. S. (2008). Fabrication of hairy

polymeric films inspired by geckos: wetting and high

adhesion properties. Advanced Functional Materials,

18, 1089−1096.

Feng, L., Zhang, Y., Xi, J., Zhu, Y., Wang, N., Xia, F., &

Jiang, L. (2008). Petal effect: a superhydrophobic state

with high adhesive force. Langmuir, 24, 4114−4119.

Hosono, E.,Fujihara, S., Honma, I., & Zhou, H. (2005).

Superhydrophobic perpendicular nanopin film by the

bottom-up process. Journal of the American Chemical

Society, 127, 13458−13459.

Ishii, D., Yabu, H., Shimomura, M. (2008). Selective

metal deposition in hydrophobic porous cavities of

self-organized honeycomb-patterned polymer films by

all-wet electroless plating. Colloid and Surface; A

313–314, 590–594.

Ishii, D., Udatsu, M., Iyoda, T., Nagashima, T., Yamada

M., & Nakagawa, M. (2006). Electroless deposition

mechanisms on fibrous hydrogen-bonded molecular

aggregate to fabricate Ni-P hollow microfibers.

Chemistry of Materials. 18, 21522158.

Karthaus, O., Maruyama, N., Cieren, X., Shimomura, M.,

Hasegawa, H., & Hashimoto, T. (2000). Water-

assisted formation of micrometer-size honeycomb

patterns of polymers. Langmuir, 16, 6071−6076.

Onda, T., Shibuichi, S., Satoh, N., & Tsujii, K. (1996).

Super-water-repellent fractal surface. Langmuir, 12,

2125−2127.

Yabu, H., Takebayashi, M., Tanaka, M., & Shimomura,

M. (2005). Superhydrophobic and Lipophobic

Properties of Self-Organized Honeycomb and

Pincushion Structures. Langmuir, 21, 3235−3237.

Zhang, X., Shi, F., Niu, J., Jiang, Y., & Wang, Z. (2008).

Superhydrophobic surfaces: from structural control to

functional application. Journal of Materials Chemistry,

18, 621−633.

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