利用光鉗與廣義介電泳量測近牆效應下圓球的黏性阻力矩

Abstract

本論文的目標是建立一項實驗的方法來量測及驗證球形粒子在近牆效應下的黏性阻力阻。將採用兩種非接觸方法，光鉗與廣義介電泳(含介電泳力與電旋轉)應用在由微機電製程技術製作出來的四相位電旋轉槽來進行實驗。吾人使用半徑約為10

The goal of this thesis is to propose a method for the experimental validation of the theory of wall effect on the viscous torque of a spherical particle. Two non-contact techniques, optical tweezer and generalized dielectrophoresis (includes dielectrophoresis and electrorotation here), are employed for the experiment in a four-phase electrorotation chamber fabricated using MEMS techniques. The experiment was performed using Sephadex particles with radius (r) around 10 μm in KCl solution. Such a particle behaves negative dielectrophoresis and settles at a height on the vertical centreline of the chamber (equilibrium position of force balance) when a constant rotating electric field is turned on. Meanwhile, the particle rotates steadily with a constant speed (Ω) around it own axis. The particle wanders away frequency when its settling height (h) is sufficiently large, and an optical tweezer is thus employed to confine the particle to stay on the centreline of the chamber during the experiment. The tweezer also exerts a downward optical force to the particle and thus lowers its settling height. The settling height is altered mainly by changing the applied electric voltage and frequency in the present experiment, and is determined through the differences of the scales of the focus screw of the microscope when it is focused at the particle and at the bottom wall of the chamber, respectively. The horizontal particle position is determined easily through the view of a microscope. With the particle position known, the dielectrophoretic torque on the particle can be evaluated using the simulated electric field of the chamber, and thus the viscous torque (which equals the dielectrophoretic torque) is determined. The wall effect on the viscous torque, T, is characterized by comparing it with the theoretical viscous torque in an infinite medium, . The ratio, T/8πμr^3Ω , is evaluated here by measuring Ω, which is derived from the motion of the particle rotation recorded through a CCD camera mounted on a microscope along the centreline of the chamber. It is found that the present measurement agree with the theoretical result in the literature within 0.5%-38% discrepancy.