(161c) Microfluidic Force Fields for Biochemical and Cellular Analysis

To date, AC electric fields have been exploited to manipulate liquid (electro-osmosis) bubbles, particles, biomolecules and cells (dielectrophoresis) in microfluidic devices. Research and application in this area, however, has been limited to the interfaces formed between two immiscible metal-liquid, particle-liquid, or gas-liquid surfaces. The influence of AC electric fields across aqueous liquid-liquid interfaces remains unexplored. As the majority of microfluidic applications involve aqueous liquid flows at low Reynolds number, fluid interfaces formed between co-flowing fluid streams are a natural occurrence in microfluidic devices. Fundamentally, many electrokinetic phenomena arise from discontinuities in ionic flux and charge accumulation at electrical interfaces. These regions of net charge interact with an external electric field and a body force is exerted on the interface.

Using a microfluidic channel with embedded electrodes, two fluid streams - one with a greater electrical conductivity, the other a greater dielectric constant - were made to flow side-by-side. An AC field was applied across the flow channel and fluid was observed to displace across the interface. The displacement direction is frequency dependent, and is attributed to the Maxwell-Wagner interfacial polarization at the liquid-liquid electrical interface. At low AC frequency (< 1 MHz), below the interfacial charge relaxation time, the high conductive stream is observed to displace into the high dielectric stream. Above this frequency, the direction of liquid injection reverses, and the high dielectric stream injects into the high conductivity stream. Hence, a liquid crossover frequency is observed and is well described by Maxwell-Wagner polarization mechanics.

Utilizing this mechanism, we explore the use interfacial polarization at liquid-liquid and polymer-electrode electrical interfaces for fluidic sensing and processing. Here, we demonstrate how these small microfluidic force fields, generated at polarized electrical interfaces, can be used to produce a portable and robust microfludic platform capable of precision fluid manipulation, electrolyte ion-sensing, particle and cell separation and solute concentration control.