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I. INTRODUCTION

WETTABILITY plays an important role in many industrial processes. Flotation is a representative example of the application of wettability characteristics to the separation of useful materials dispersed in a bath.[1] In the near future, wettability would also play a dominant role in the space environment and the microscale environment (e.g., micromachine, microplant, microreactor, etc.). In the space environment, one of the important problems is gas-liquid separation. A number of researchers have devoted their efforts to establish the techniques of separation of bubbles from liquid phase.[2–6] The influence of buoyancy vanishes and that of surface tension dominates in space, i.e., under microgravity conditions. Therefore, it is difficult to apply the conventional techniques of gas-liquid separation taking advantage of buoyancy. We focus on the wettability of a solid body for the separation of bubbles from the liquid phase. At present, it is possible to make surfaces with a contact angle of over 2.967 radian (170 ⬚). Such a poorly wetted surface is expected to be applicable to the purpose of gas-liquid separation. Generally, the wettability is quantitatively evaluated by a contact angle. The contact angle defined for a smooth flat plate is called the Young contact angle and is regarded as an intrinsic contact angle. It is solely determined as a function of the solid-liquid, solid-gas, and liquid-gas surface tension. However, an actual material surface is more or less rough, and the contact angle reflects the effects of surface roughness. Such a contact angle is regarded as an apparent contact angle and is termed the Wenzel contact angle[7] or the Cassie contact angle.[8] The evaluation of the surface roughness and solid surface tension is eventually difficult, and the actual contact angle is a practical index of the condition of material surface. Also, the contact angle at the leading edge of a sliding droplet is different from that at the trailing edge. This hysteresis effect is common in actual materials. These two contact angles are very important for the gasliquid separation based on the wettability effect. NOZOMU SONOYAMA, Graduate Student, and MANABU IGUCHI, Professor, Division of Materials Science and Engineering, are with the Graduate School of Engineering, Hokkaido University, Hokkaido, 0608628, Japan. Contact e-mail [email protected] Manuscript submitted June 18, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS B

In this study, we predicted the shape of a bubble on the basis of the bubble-free energy using the finite element method. The contact angles were determined by observing the shape of a bubble being contact with a poorly wetted plate. II. MATERIALS AND METHODS Figure 1(a) shows a schematic diagram of the experimental apparatus. An acrylic plate coated with paraffin wax and a metal plate with tetrafluoroethylene plating were used. These plates are called the paraffin plate and TFE plate, respectively. An acrylic box is filled with water and a bubble is generated from a hole (diamet