Non-monotonic dependence of induced-charge electro-osmosis on ion concentration

Abstract

Induced-charge electro-osmosis (ICEO) is an important phenomenon in microfluidics as it shows great potential for various microfluidic devices. However, many basic mechanisms of ICEO remain unclear. The effect of ion concentration on ICEO is controversial. Literature has reported that ICEO velocity either monotonically increases or decreases by increasing the ion concentration. In this paper, we experimentally captured that ICEO velocity increases at first and then decays as sodium chloride (NaCl) concentration increases, which covers the whole variation range in the existing literature. The NaCl concentration of peak ICEO velocity varies as the electric field strength increases. In addition, it is known that ICEO velocity achieves a maximum value upon increasing the electric field frequency, and the frequency of peak velocity is determined by the microfluidic chip geometry and the ion concentration based on the volumeless ion assumption. However, we found that it reduces as the electric field strength increases because of the steric effect of ions in real electrolyte solutions. Moreover, an inflow-outflow asymmetry is experimentally observed in ICEO vortex positions, which shows a dependence on the electric field and NaCl concentration. The vortices become close to the cylinder surface and the two vortices along the x axis tend to merge as NaCl concentration increases. The new findings could enrich the knowledge of ICEO.