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 behavior of particles in a deterministic lateral displacement (DLD) separation device with DC and AC electric fields applied orthogonal to the fluid flow. As proof of principle, we demonstrate tunable microparticle and nanoparticle 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 nonuniform electrophoretic velocity orthogonal to the primary flow direction. This phenomenon causes particles to focus on 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 nanoscale in microscale devices.

Electric fields are commonly used for manipulating particles and liquids in microfluidic systems. In this work, we report stationary electro-osmotic flow vortices around dielectric micropillars 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 nonuniform zeta potential induced around the pillar and a rectified electric field induced by the ion concentration gradients. We refer to this fluid flow as concentration-polarization electro-osmosis (CPEO). We present experimental data in support of the theoretical predictions. The magnitude and frequency dependence of the electro-osmotic velocity are in agreement with the theoretical estimates and are significantly different from predictions based on the standard theory for induced-charge electro-osmosis, 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.

We experimentally demonstrate quadrupolar electro-osmotic flows around charged dielectric microspheres immersed in an electrolyte when subjected to an alternating current electric field. We present an electrokinetic model that predicts the flow characteristics based on the phenomena of surface conductance and polarization of the electrolyte concentration around the particles. We refer to these flows as concentration polarization electro-osmosis. We anticipate that these flows may play a major role in the electric-field-induced assembly of colloids and on the electrokinetic manipulation of dielectric micro- and nanoparticles.

Polydimethylsiloxane (PDMS) is ubiquitously used in microfluidics. However, PDMS is porous and hydrophobic, potentially leading to small molecule partitioning. Although many studies addressed this issue and suggested surface/bulk modifications to overcome it, most were not quantitative, did not address which variables besides hydrophobicity governed molecule absorption, and no modification has been shown to completely obviate it. We evaluated qualitatively (confocal microscopy) and quantitatively (fluorescence spectroscopy) the effects of solute/solvent pairings, concentration, and residence time on molecule partitioning into PDMS. Additionally, we tested previously reported surface/bulk modifications, aiming to determine whether reduced PDMS hydrophobicity was stable and hindered molecule partitioning. Partitioning was more significant at lower concentrations, with the relative concentration of rhodamine-B at 20 µM remaining around 90% vs. 10% at 1 µM. Solute/solvent pairings were demonstrated to be determinant by the dramatically higher partitioning of Nile-red in a PBS-based solvent as opposed to ethanol. A paraffin coating slightly decreased the partitioning of Nile-red, and a sol–gel modification hindered the rhodamine-B diffusion into the PDMS bulk. However, there was no direct correlation between reduced surface hydrophobicity and molecule partitioning. This work highlighted the need for pre-assessing the absorption of test molecules into the microfluidic substrates and considering alternative materials for fabrication.

We describe fractionation of sub-micron vesicles and particles suspended in high conductivity electrolytes using an electrokinetically biased Deterministic Lateral Displacement (DLD) device. An optimised, asymmetric array of micron-sized pillars and gaps, with an AC electric field applied orthogonal to the fluid flow gives an approximately ten-fold reduction in the intrinsic critical diameter (D_c) of the device. The asymmetry in the device maximises the throughput. Fractionation of populations of 100 nm and 400 nm extruded vesicles is achieved in 690 mS m⁻¹ KCl, and 100 nm, 200 nm and 500 nm polystyrene particles in 105 mS m⁻¹ KCl. The electrokinetically biased DLD may provide solutions for simple and rapid isolation of extracellular vesicles.