Electromechanics of suspensions
Abstract
Electrokinetic forces are becoming an increasing popular choice for the manipulation of tiny particles in microfluidic devices. Several electrokinetic phenomena, including electrothermal forces, electroosmosis, AC electroosmosis, electrowetting, electrophoresis and dielectrophoresis, are being exploited for these purposes. Currently, experimental exploration outpaces the theoretical understanding of these phenomena. In this work, the theoretical aspects of three of these phenomena, namely electrophoresis, dielectrophoresis and AC electroosmosis have been examined. ^ Complete modeling of these phenomena, in most cases, cannot be done in a fully analytical way because of the inherent geometrical complexities involved in these cases. Thus, to complement the theoretical work, a numerical scheme based on a finite element method has be developed to analyze these complex phenomena which can not otherwise be analytically solved for. Simulations of these three phenomena reveal interesting results of interactions between electromechanical and hydrodynamic forces in such systems. ^ In dealing with the phenomenon of electrophoresis, the effect of inertial forces on the motion of a suspension of particles has been explored. It is seen that these forces tend to bunch up the particles, especially when the suspension is concentrated, as the particles undergo electrophoresis. This bunching up is a cascading phenomenon wherein groups of particles will tend to behave like larger particles and increasing tend to attract more particles to them. ^ For the case of dielectrophoresis, the problem of dielectrophoresic suspensions in shear flow has been analyzed analytically and numerically. While dielectrophoresis is usually associated with the motion of particles in non-uniform fields, the dipole-dipole interactions are seen to chain particles together along the direction of the electric field even when this field is uniform. This effect has been simulated to determine the interplay of the dielectrophoretic and viscous forces which ultimately determines the stable length of the chains. ^ Furthermore, in ionic solutions, the double layer formation due to induced surface charges is seen to effect particle motion in AC electric fields as opposed to the fixed charge electrophoresis case wherein the double layer causes no motion in AC fields. A method to numerically evaluate the effect of this phenomenon, AC electroosmosis, layer on the dielectrophoretic motion of particles has been developed. The technique, developed herein, involves a matched asymptotic expansion of the electric field near the particle surface, where the double layer is formed, and is written as a jump-boundary-condition for the electric potential when the thickness of the double layer is small compared to the size of the particle. The developed jump-boundary-condition is amenable to numerical evaluation and has been implemented in the finite element scheme using a discontinuous Galerkin method which naturally permits for such discontinuous boundary conditions in its formulation. While classical dielectrophoretic analysis ignores the effect of the electric double layers formed in ionic solutions, our results reveal their effect to be generally non-negligible and important for accurate modeling of the behavior of small particles in electric fields. ^
Subject Area
Engineering, Mechanical
Recommended Citation
T. N Swaminathan,
"Electromechanics of suspensions"
(January 1, 2007).
Dissertations available from ProQuest.
Paper AAI3271820.
http://repository.upenn.edu/dissertations/AAI3271820