Franklin Jerrel (2011-10). Finite Element Studies of Colloidal Mixtures Influenced by Electric Fields. Master's Thesis. Drummond - Texas A&M University (TAMU) Scholar

A further understanding of colloidal mixture behavior under applied electric fields would greatly benefit the design of smart material systems such as electrorheological fluidic devices and microfluidic reconfigurable antennas. This thesis presents a finite element analysis of colloidal mixture electrokinetic behavior. Computations of particle forces as a function of applied frequency and particle shape were performed. An effective medium property method was also studied. Fluidic and electric forces were obtained with various applied excitation frequencies throughout three locations in a coplanar microelectrode domain. This domain consists of two 50 nanometers thick gold electrodes separated by a 30 micrometers gap. The three locations are 1.2 micrometers, 40 micrometers, and 90 micrometers from the gap center. Total force vectors were computed by integrating Maxwell and Cauchy stress tensors to determine whether the particles are pushed toward or away from the electrode gap at frequencies of 10 Hz, 1 kHz, and 100 kHz. It was determined that particles were pushed outside the gap at median frequencies of 1kHz (indicating ac electroosmotic force domination) and began to be pushed back toward the gap at higher frequencies of 100 kHz (indicating dielectrophoretic force intensification). Particle shape effects were examined by calculating the electrical interparticle force between two particles at various incidences with respect to a uniform electric field. Particle attraction occurs when the line between the particle centers is aligned with the electric field; repulsion occurs when this center line is perpendicular. The incidence angle at which the particles switch from attraction to repulsion is defined as ?cr. The aspect ratio and particle edge separation distances used in this study were 1, 5,12.92 and 0.25 micrometers, 0.50 micrometers, 2.0 micrometers, respectively. The results indicate that higher aspect ratio particles tend to have smaller ?cr values and larger interparticle force magnitudes for given separation distances. Finally, effective dielectric constant simulations utilizing periodic crystalline arrangements of colloidal structure were performed. The results show good agreement with the Maxwell Garnett mixing rule at volume fractions above 30 percent. Less canonical structures of cubic particles were also modeled.