Date on Master's Thesis/Doctoral Dissertation

7-2012

Document Type

Master's Thesis

Degree Name

M. Eng.

Department

Mechanical Engineering

Committee Chair

Williams, Stuart J., 1981-

Author's Keywords

Colloid; Colloid characterization; Patterning; Laser; Rapid electrokinetic patterning

Subject

Electrokinetics--Technological innovations

Abstract

Rapid Electrokinetic Patterning (REP) is a relatively new method for collecting and manipulating micrometer-scale particles at an electrode surface. REP is an AC electrokinetic technique that uses induced fluid motion to capture and manipulate particles. A laser (975 nm) is focused on the surface of the parallel-plate electrode, generating a thermal gradient in the medium. This thermal gradient is acted upon by the AC electric field in such manner as to produce a vortex. Particles are trapped on the electrode surface at the center of the vortex. It is hypothesized that AC electroosmotic flows act to hold the particles to the electrode surface more strongly than other holding forces, such as dielectrophoresis (DEP). The accumulation of single-size particles occurs in one layer, and is two-dimensional in REP, and crystalline in nature under the correct conditions. Electrostatic forces separate the particles while fluid drag forces tend to corral the particles together. The particle aggregations tend to exhibit several exclusive, characteristic behaviors: the particles will group together very closely and uniformly, the particles will form arbitrary aggregations of tightly packed particles, or the particles will exhibit a nearly-random spacing. Parameters are varied such that the average spacing is changed between the particles to transition the particles from nearly-random spacing to a uniform, tightly packed crystalline grid. The relationship between the drag forces and the electrostatic forces parallel to the electrode surface are explored for different AC frequencies, AC voltages, laser powers, laser scan rates, and laser scan lengths. The effect these parameters have on the spacing of the particles is characterized, and aggregation crystallinity is discussed. A more detailed force analysis is discussed for non-varying parameters with a dot-shaped aggregation. The ability to use this force analysis for particle spectroscopy is briefly discussed. Particle motion was analyzed with Particle Tracking Velocimetry (PTV) using Matlab scripts.

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