Charge-based particle separation in microfluidic devices using combined hydrodynamic and electrokinetic effects.

A new microfluidic approach for charge-based particle separation using combined hydrodynamic and electrokinetic effects is presented. A recirculating flow pattern is employed, generated through application of bi-directional flow in a narrow glass microchannel incorporating diverging or converging segments at both ends. The bi-directional flow in turn is a result of opposing pressure-driven flow and electro-osmotic flow in the device. Trapping and preconcentration of charged particles is observed in the recirculating flow, under conditions where the average net velocity of the particles themselves approaches zero. This phenomenon is termed flow-induced electrokinetic trapping (FIET). Importantly, the electrophoretic mobility (zeta potential) of the particles determines the flow conditions required for trapping. In this paper, we exploit FIET for the first time to perform particle separations. Using a non-uniform channel, one type of particle can be trapped according to its zeta-potential, while particles with higher or lower zeta-potentials are flushed away with the pressure-driven or electro-osmotic components, respectively, of the flow. This was demonstrated using simple mixtures of two polystyrene bead types having approximately the same size (3 microm) but different zeta potentials (differences were in the order of 25 to 40 mV). To gain more insight into the separation mechanism, particle separations in straight, 3 cm-long microchannels with uniform cross-section were also studied under conditions of bi-directional flow without trapping. A thorough theoretical analysis confirmed that trapping occurs when electrokinetic and pressure-driven particle velocities are equal and opposite throughout the diverging segment. This makes it possible to predict the pressure and electric field conditions required to separate particles having defined zeta potentials.

Stabilization of two-phase octanol/water flows inside poly(dimethylsiloxane) microchannels using polymer coatings.

In this paper we present our first results on the realization of stable water/octanol, two-phase flows inside poly(dimethylsiloxane) (PDMS) microchannels. Native PDMS microchannels were coated with high molecular weight polymers to change the surface properties of the microchannels and thus stabilize the laminar flow profile. The polymers poly(2-hydroxyethyl methacrylate), poly(vinyl pyrrolidone), poly(ethylene oxide), poly(ethylene glycol), and poly(vinyl alcohol) were assessed for their quality as stabilization coatings after deposition from flowing and stationary solutions. Additionally, the influence of coating the microchannels homogeneously with a single kind of polymer or heterogeneously with two different polymers was investigated. From the experimental observations, it can be concluded that homogeneous polymer coatings with poly(2-hydroxyethyl methacrylate) and poly(vinyl pyrrolidone) led to the effective stabilization of laminar water/octanol flows. Furthermore, heterogeneous coatings led to two-phase flows which had a better-defined and more stable interface over long distances (i.e., 40-mm-long microchannels). Finally, the partitioning of fuchsin dye in the coated microchannels was demonstrated, establishing the feasibility of the use of the polymer-coated PDMS microchannels for determination of logP values in laminar octanol/water flows.