Dynamical trapping of colloids at the stagnation points of electro-osmotic vortices of the second kind

Abstract

By applying a stepwise overlimiting voltage to a nanoslot system in equilibrium, it is possible to follow the time evolution of the electroconvective instability vortex array via the depletion dynamics or, alternatively, by following dielectrophoretically trapped particles at the stagnation points of each of the hydrodynamic vortex pairs. Particles are advected to the stagnation point by the hydrodynamics, where they are trapped by a shortrange dielectrophoretic force. It is experimentally confirmed that the wavelength selection process occurs at the diffusive time scale and that the wavelength selection mechanism, started by the Rubinstein and Zaltzman electroconvective instability is eventually determined by the system lateral geometry and dictated by Dukhin's electro-osmosis of the second kind. The steady-state case was numerically studied by solving the fully coupled electroconvective problem, confirming that the vortex stagnation point is indeed of a converging type. The particle's planar (two-dimensional) equations of motion are solved after adding a dielectrophoretic force that accounts for quasi-three-dimensional effects. It is shown that this force can account for trapping at the nanoslot interface.

Effects of three-dimensional geometric field focusing on concentration polarization in a heterogeneous permselective system

“…This prediction was confirmed experimentally using both a wide nanoslot [11] and a nanoporous membrane [12]. The interplay between these two, i.e., Rubinstein-Zaltzman [9] and Dukhin [7], electroconvective mechanisms has recently been studied numerically for modulated surfaces [13] and observed experimentally for flat heterogeneous surfaces [14].…”

“…It is clear that in heterogeneous systems, such as membranes with partly conducting surface area [15] or fabricated micronanochannel systems [14,[16][17][18][19], the Dukhin mechanism dominates, commonly resulting in strong corner vortices that stir the flow and in turn control the length of the diffusion layer. However, aside from these electroconvection effects, field-focusing effects alone stemming from the physical twodimensional (2D) and three-dimensional (3D) geometries can affect the electrodiffusive solution to yield CP with much larger concentration gradients.…”

Observation and experimental investigation of confinement effects on ion transport and electrokinetic flows at the microscale

“…Investigations of microfluidic charge-selective systems typically consist of two or more micro-scale channels which are interconnected through a charge-selective layer101112. This charge-selective layer can be a classical membrane, such as Nafion910, or a medium providing charge selectivity through double layer overlap, such as nanochannels1314.…”

“…Many numerical studies have been executed on the prediction of these fluid flows, with different proposed explanations for the occurrence of the OLC regime5202122, depending on geometry and other system parameters. Experimentally, vortex structures and depletion zones associated with vortex-like rolls have been observed in multiple microfluidic systems781213192324, using particles and dyes. These results have shown the presence of multiple vortices in the vicinity of the CSI, but not the formation of these vortices and their growth as function of time in detail for microfluidic systems.…”

Pseudo 1-D Micro/Nanofluidic Device for Exact Electrokinetic Responses

“…This limiting current behavior was a nuisance in terms of an electrical power efficiency, since the current was saturated at the limiting value. Further application of voltage would lead to an overlimiting current behavior which was affected by various constraints such as a surface conduction 17,34 , an electroosmotic flow 16,24,34,39 and an electroosmotic instability 15,53 . These particular behaviors would imprint the signature of the system.…”