Controlling and Manipulating Microscopic Particles in Solution By Using Various Electric Field Geometries

Abstract

Progress in micro- and nanotechnologies depends on our capability of manipulating and interacting with microscopic particles and nanosize material. A promising approach towards this goal is the use of electric fields, which are the dominant forces at the molecular length scale. One technique for trapping nanoparticles in solution uses an externally controlled electric field, generated by two electrode pairs, to counteract the Brownian motion of a single, selected particle. Unlike other trapping tools, such as optical tweezers or magnetic tweezers, this approach scales favorably with particle size, down to the level of a single molecule. However, it depends on real-time position information from fluorescence imaging and requires a fast, well-calibrated feedback system. The approach taken in this thesis does not rely on this kind of external control; rather, we use geometric patterns (corral traps) “etched” into an otherwise conductive layer to create energy wells that are capable of trapping micro- and nanoscale particles as long as an electric voltage is applied to the device. These energy wells create a stable trapping potential that can keep a particle confined without the need for a feedback system. The main trapping forces are either electrostatic or dielectrophoretic in nature, depending on whether DC or AC voltages are applied to the corral trap electrode. We investigate the influence of the geometric shape of the electric field on the electrostatic and dielephoretic trapping behavior of charged and uncharged polarizable particles, and compare the experimental results to finite-element simulations of the electrostatic and dielectrophoretic forces generated by different experimental setups and various symmetric and asymmetric metal patterns. Also, the influence of electric field-induced flow patterns in solution, such as electro-osmosis, are investigated theoretically. Aside from pure trapping, i.e. confinement of particles to the low-energy regions, we find that particles can also be manipulated to move in a particular direction, follow pathways or trails, diverge in different directions, converge in one direction, make 90 degree turns, and even perform circular loops by utilizing various shapes and patterns on a metal surface.