ELECTROKINETIC TRANSPORT AND MANIPULATION OF PARTICLES IN CURVED MICROCHANNELS

The investigation of electrokinetic particle transport in confined microchannels has practical significances in a variety of applications ranging from traditional gel electrophoresis to electrokinetic microfluidics-based lab-on-a-chip devices. To date, however, studies on particle electrokinetics have been limited to primarily theoretical or numerical analyses in straight microchannels of simple geometries. Very little work has been done on electrokinetic particle motions in real microchannels which usually consist of one or multiple turns. This thesis is dedicated to the fundamental and applied studies of electrokinetic transport and manipulation of particles in various curved microchannels using a combined experimental, theoretical, and numerical method. First, a fundamental study of particle electrokinetics in a microchannel U-turn, a typical unit in LOC devices, was investigated. A 2-D numerical model based on finite element method was developed to understand and predict the particle motion within the U-turn. It is demonstrated that particles are deflected to the outer wall of the turn by curvature-induced dielectrophoresis (termed cDEP) due to the locally intrinsic electric field gradients. Moreover, this lateral displacement increases with the rise of either the applied electric field or the particle size. Next, we utilize the cDEP in microchannel turns to implement a continuous electrokinetic focusing of particles in serpentine microchannels. Particles are demonstrated to gradually migrate to the centerline due to the periodically switched dielectrophoretic force they experience in a serpentine microchannel. This electrokinetic focusing favors large electric fields and large particles, and also increases when the number of serpentine periods increases. Such focusing also takes place in a spiral microchannel, where, however, particles are eventually focused to a stream flowing near the outer sidewall of the channel. Then, we explore the applications of cDEP to continuous electrokinetic separation of particles in curved microchannels. We develop two different approaches based on what we have acquired from the studies of particle electrokinetics in serpentine and spiral microchannels. The first approach employs a sheath flow to focus particles to one sidewall of a serpentine microchannel, where particles are then deflected to different flow paths by cDEP and thus sorted at the exit of serpentine section. We use this method to separate particles and cells by size at low DC electric fields. The second approach eliminates the sheath flow focusing of particles by the use of particle deflection and focusing in a double-spiral microchannel. Specifically, particles are focused by cDEP to one single stream near the outer wall of the first spiral, which is then displaced by cDEP and divided into two or more sub-streams in the second spiral, enabling the continuous sorting. We use this approach to implement the separation of particles by size and by charge, respectively. Moreover, we also demonstrate a continuous ternary separation of particle by size and charge simultaneously