(535g) Dielectrophoresis at Conductive Liquid Interfaces

Dielectrophoresis (DEP) is a popular technique for cell and particle manipulation at the microscale, and is commonly used to describe the motion of suspending particles or cells under the application of a non-uniform electric field.

The DEP force results from electric field induced charge accumulation at the cell or particle surface due to a discontinuity in the electrical conductivity and dielectric constant at the particle-liquid interface. During exposure to an AC electric field, the interface charges much like a capacitor ? governed by conductive charging at low (typically < 500 kHz) frequency and dielectric polarization at high frequency.

Commonly, small submicron particles and the interior of most cell types have a higher conductivity and lower dielectric constant than their surrounding media. Hence, the resulting direction of the induced particle dipole often reverses when the applied AC field frequency exceeds the inverse relaxation (RC) timescale of the media-particle interface, known as the DEP crossover frequency (cof). Interfacial polarization, hence, plays an important role in dictating the magnitude and direction of the field induced DEP force exerted on a particle or cell.

Typically, DEP has been applied to particles, cells and liquid droplets in suspension. At its most basic element, however, DEP is simply a force that arises from unbalanced charging at an electrical interface. Here, I explore a different electrical interface - that of two or more adjacent liquid streams flowing within the confines of a microfluidic channel network.

Due to the laminar flow profile and slow diffusion timescale associated with microchannel fluid flow, two or more liquid streams can flow side by side without mixing. As both the conductivity and dielectric constant of aqueous solution can be readily adjusted with soluble salts and zwitterions, two solutions of different electrical properties can be allowed to flow side by side to generate an electrical liquid-liquid interface.

Much like a suspending particle surface, a microfluidic generated liquid-liquid electrical interface can also be polarized with embedded microelectrodes. Here, I characterize the unique phenomena that arise at polarized electrical liquid-liquid interfaces. Furthermore, I apply them to electrokinetic cellular manipulation applications and biological studies and demonstrate the potential for this new and flexible microfluidic platform.