Nonlinear Induced-charge Electrokinetics

Abstract

This dissertation presents the study of non-linear induced-charge electrokinetic phenomena and the corresponding applications. The study shows that a nonlinear induced surface charge distribution is immediately caused when a conducting object is immersed in an external applied electric field. The induced charge on the conducting surfaces results in a non-linear zeta potential distribution when the surfaces contact with an aqueous buffer solution. The non-constant zeta potential gives a varying slipping velocity along the conducting surface, which produces micro vortexes in the liquid. A numerical scheme is suggested to estimate the induced zeta potential on the conducting surfaces. The induced-charge electrokinetic flows (ICEKF) in a microchannel with embedded conducting hurdles are studied. Two-dimensional pressure-linked Navier-Stokes equation is used to model the flow field in the channel. The numerical results show flow circulations generated from the induced non-uniform zeta potential distribution along the conducting hurdle surfaces, which provide effective means to enhance the flow mixing between different solutions. The mixing enhancement effect is experimentally validated using PDMS based microchannels with embedded platinum hurdles. The dependence of the degree of mixing enhancement on the hurdle geometries and hurdle numbers is also predicted. It’s also found that, by adjusting the electric field applied through the microchannel with an asymmetric conducting triangle hurdle pair, an electrokinetic flow regulating effect can be obtained and this effect depends on the dimensions of the conducting converging-diverging section. The induced-charge electrophoretic (ICEP) motions of conducting particles in microchannel are also numerically studied. A complete theoretical model of ICEP motion is set up and a moving grid technique is utilized to fulfill the numerical simulation of the particle-liquid coupled multi-physics system under various conditions. The corresponding effects of micro vortex generation, particle-wall interaction and particle-particle interaction are discussed. The unique wall-lifting effect of the ICEP motion of conducting particles in a microchannel and the attracting and repelling effects between two conducting particles in an unbounded large field are investigated. The induced-charge electrokinetics described in this study can be used in various microfluidics and lab-on-a- chip (LOC) applications.