Stationary Electrohydrodynamic Flows Around Insulators Induced by AC Electric Fields in Conductivity Gradients

Fluid flows around insulators in microfluidic systems induced by electric fields in electrolyte conductivity gradients is often referred to as electro-diffusioosmosis (EDO). Current description of the phenomenon show how the electrokinetic boundary conditions are modified as a consequence of the interplay of the electric field and the conductivity gradient, thus modifying the induced fluid flow. In this work, we show that the volume charges induced in the fluid bulk because of the conductivity gradient in the presence of the electric field build a body force on the fluid inducing a net flow, a fact that has been overlooked in the literature. We show both theoretically and experimentally that the bulk force produces a non-zero time-average fluid flow when using AC electric fields which cannot be accounted for in the current picture of EDO in conditions where surface conductance effects are negligible. Although classical diffusioosmosis (DO) is always present, we show that the reported mechanism significantly overcomes the flows induced by DO. Here, we present a theoretical framework for the scaling of this phenomenon with electrolyte concentration gradients and the amplitude of the electric field, which is then validated by experimental observations and numerical simulations. The flows described are predicted to have significant contributions to particle motion subjected to electric fields in conductivity gradients, with potential applications in bioparticle manipulation for biomedical devices.