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NOVEL COMBINATION OF HYDROPHILIC/HYDROPHOBIC SURFACE FOR LARGE WETTABILITY DIFFERENCE AND ITS APPLICATION TO LIQUID MANIPULATION T. Kobayashi1*, K. Shimizu2, Y. Kaizuma3, S. Konishi1 1
Ritsumeikan University, Japan 2 Kyoto University, Japan 3 Shinko Seiki Co., LTD, Japan
ABSTRACT This paper reports novel combination of hydrophilic/hydrophobic materials for the evolution of liquid manipulation. Generally, droplets can naturally be formed on hydrophilic regions, and the shape is defined by surface tension. Droplet generation based on hydrophilic/hydrophobic mechanism is a promising method for high accurate liquid manipulations [1]. However, it used to be difficult to split fluid into droplets in microchannels due to its narrow cross-section. Therefore the enhancement of wettability difference is strongly required. Our group successfully enhanced difference of contact angle from 47.5 to 94.3. Novel droplet generation and liquid transportation technologies are also described. KEYWORDS: Droplet generation, Hydrophobic, Hydrophilic, Micro Fluidics, Surface Morphology INTRODUCTION On-chip manipulation technique for minute volume of liquid is essential for miniaturization of biochemical diagnostic devices or micro reactors. Hydrophilic/hydrophobic functional surface is an attractive component for accurate droplet generation technique as shown in Fig. 1(a). Figure 1(b) shows a conceptual image. Droplet generation based on hydrophilic/hydrophobic mechanism requires large wettability difference. In this paper, we employ surface roughness controlled silicon oxide as a superhydrophilic material. It is well known that the large surface area of hydrophilic material reduces the contact angle. The relationship between the surface area and the contact angle is defined by Wenzel’s equation. In this study, we successfully developed a novel combination of hydrophilic/hydrophobic surface to increase the wettability difference. EXPERIMENTAL
Figure 1: Hydrophilic and hydrophobic combination for PTAS applications: (a) Construction of hydrophilic islands, (b) Integration image of hydrophilic islands for micro fluidic applications.
Figure 2: Key factors to obtain large difference of contact angles between hydrophilic and hydrophobic materials: (a) Fabrication process of hydrophilic islands surrounded by hydrophobic region, (b) AFM image of surface roughness controlled SiOx for superhydrophilicity.
978-0-9798064-3-8/µTAS 2010/$20©2010 CBMS
1082
14th International Conference on Miniaturized Systems for Chemistry and Life Sciences 3 - 7 October 2010, Groningen, The Netherlands
Figure 2(a) shows fabrication process. Silicon oxide deposited by pulse plasma CVD (PPCVD-SiOx) was employed as a superhydrophilic material. The surface roughness was controlled by pulse patterns of RF power in the PPCVD process for enhancement of the hydrophilicity. CYTOP® was patterned by O2 plasma etching. Our group has reported surface wettability control of hydrophobic patterns using perfluoropolymer [2]. Key factors to obtain high hydrophobic patterns are the RIE mask material and the post-annealing process. Figure 2(b) shows an AFM image of PPCVD-SiOx. The surface of PPCVD-SiOx showed rough morphology in nanometer-scale. Table 1 shows contact angles and the surface roughness on SiOx formed by different methods. PPCVD drastically reduced contact angle from 42.7 to less than 5. The largest surface area of SiOx showed lower contact angle than others. Contact angle of patterned CYTOP® was successfully improved from 80.2 to 109.3 by employing Cu as RIE mask and post-annealing process[2]. The difference of contact angles was improved from 47.5 to 94.3. Next, droplet generation and droplet transportation with various conditions was evaluated. Parameters are shown in Table 2. In our experiments, two types of droplets were generated. Procedure of the droplet generation is shown in Fig. 3(a). One type was domed-shape droplet utilizing one-side hydrophilic islands as shown in Fig. 3(b), the other type was suspended-shape droplet utilizing both-side hydrophilic islands as shown in Fig. 3(c). Figure 3(d) shows procedure of the droplet transportation based on centrifugation. RESULTS AND DISCUSSION Figure 4(a)-(c) are photographs of generated droplets at flow rate of 0.1, 1 and 10 ml/min, respectively. Faster flow rate could increase area of droplets. PPCVD-SiOx islands surrounded by CYTOP® could successfully be filled with droplets as shown in Fig. 4(c). Figure 4(c)-(e) are photographs of generated droplets with various wettability difference, parameters are given in Table 2. CYTOPp of samples (d) and (e) were etched through masks of Cu and Al, respectively. The sample (d) showed larger wettability difference than the sample (e). Both samples (d) and (e) employed borosilicate glass as hydrophilic substrates. The sample (d) could increase volume of the generated droplet, compared with the sample (e). PPCVD-SiOx and CYTOP® was most suitable combination for droplet generation based on hydrophilic/hydrophobic mechanism. Next, Suspended-shape droplet can be generated using hydrophilic islands formed on both the ceiling and the bottom in a flow channel. Figure 5 (a) and (b) show photographs of conventional domed-shape droplets focused on the bottom and the top, respectively. Domed-shape droplet was not suitable for observation of floating beads inside of the droplet due to optical refraction by curvature and air layer as shown in panel (b). Figure 5 (c) and (d) show photographs of suspended-shape droplets focused on the bottom and the top of droplets, respectively. Even floating substances were clearly observed by the use of suspended-shape droplet as shown in Fig. 5 (c) and (d). It is suggested that suspended-shape droplet have strong potential for optical based analysis applications [3]. Table 1. Comparison of water contact angles of on candidate materials for a hydrophilic layer: TO, CPCVD and PPCVD stand for Thermal Oxidation, Continuous Plasma CVD and Pulse Plasma CVD, respectively. Borosilicate glass Before ‘5’ of Fig. 2(a) After ‘5’ of Fig. 2(a)
55 Not measured
Sorts of Materials CPCVDTO-SiOx SiOx 42.7
47.6
24 Not measured
Area % 0.342 0.0019 0.797 (%) Area %= (Surface Area –Projection Area)/Projection Area
Table 2. Parameters of evaluation for droplet generation
(a) (b) (c) (d)
PPCVDSiOx
