We describe a novel particle separation technique that combines deterministic lateral displacement (DLD) with orthogonal electrokinetic forces to separate particles below the critical diameter.

This paper describes the behaviour of particles in a Deterministic Lateral Displacement (DLD) separation device with DC and AC electric fields applied orthogonal to the fluid flow. As a proof of principle, we demonstrate tunable microand nano-particle separation and fractionation depending on both particle size and zeta potential. DLD is a microfluidic technique that performs size-based binary separation of particles in a continuous flow. Here, we explore how the application of both DC and AC electric fields (separate or together) can be used to improve separation in a DLD device. We show that particles significantly smaller than the critical diameter of the device can be efficiently separated by applying orthogonal electric fields. Following the application of a DC voltage, Faradaic processes at the electrodes cause local changes in medium conductivity. This conductivity change creates an electric field gradient across the channel that results in a non-uniform electrophoretic velocity orthogonal to the primary flow direction. This phenomenon causes particles to focus into tight bands as they flow along the channel countering the effect of particle diffusion. It is shown that the final lateral displacement of particles depends on both particle size and zeta potential. Experiments with six different types of negatively charged particles and five different sizes (from 100 nm to 3 µm) and different zeta potential demonstrate how a DC electric field combined with AC electric fields (that causes negative-dielectrophoresis particle deviation) could be used for fractionation of particles on the nano-scale in micro-scale devices.

Abstract

Electric fields are commonly used for manipulating particles and liquids in microfluidic systems. In this work, we report stationary electroosmotic flow vortices around dielectric micro-pillars induced by AC electric fields in electrolytes. The flow characteristics are theoretically predicted based on the well-known phenomena of surface conductance and concentration polarization around a charged object. The stationary flows arise from two distinct contributions working together: an oscillating non-uniform zeta-potential induced around the pillar and a rectified electric field induced by the ion concentration gradients. We present experimental data in support of the theoretical predictions. The magnitude and frequency dependence of the electroosmotic velocity are in agreement with the theoretical estimates and are significantly different from predictions based on the standard theory for induced-charge electroosmosis, which has previously been postulated as the origin of the stationary flow around dielectric objects. In addition to furthering our understanding of the influence of AC fields on fluid flows, we anticipate that this work will also expand the use of AC fields for flow control in microfluidic systems.