Measurement of the viscous resistive torque on a sphere near wall using generalized dielectrophoresis and optical tweezer
Date Issued
2012
Date
2012
Author(s)
Huang, Chia-Ching
Abstract
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.
Subjects
viscous torque on a rotating sphere
wall effect
electrorotation
dielectroporesis
optical tweezer
Type
thesis
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