Modeling electrokinetic phenomena: A new approach to an old problem

Mr. Tirumani N. Swaminathan
Ph.D. Candidate
Advisor: Professor Howard H. Hu
Department of Mechanical Engineering
University of Pennsylvania

Abstract
Recent advances in fabrication technologies have enabled researchers to construct small-scale devices which have sparked a renewed interest in microfluidics. Electrokinetic forces are ideal for manipulating small objects and performing fluidic operations in these tiny devices by taking advantage of the small length scales. Currently, experimental exploration outpaces the theoretical understanding of these electrochemical phenomenons. In this study, an old and well known problem of dielectrophoresis in ionic media has been reexamined. Most suspensions involve the formation of ionic double layers next to the surface of particles. The double layer formed due to the induced-charge on the particle affects its motion even under AC electric fields through a phenomenon termed as induced-charge electro-osmosis. A method to numerically evaluate the effect of the double 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 a 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 nanoparticles in electric fields.

Thursday, May 4
337 Towne Bldg., 2pm