Modulating the Selective Enrichment and Depletion of Ions Using Electrorheological Fluids in Variable-Area Microchannels

Electrorheological fluids are suspensions that are characterized by a strong functional dependence of their constitutive behavior on the local electric field. While such fluids are known to be promising in different applications of microfluidics including electrokinetic flows, their capabilities of controlling ion transport and preferential solute segregation in confined fluidic systems remain to be explored. In this work, we bring out the unique role of electrorheological fluids in orchestrating the selective enrichment and depletion of charged species in variable area microfluidic channels. Our reported phenomenon is fundamentally distinctive from other types of nonlinear electrokinetic effects previously reported, in a sense that here the dependence of the flow rheology on the electric field turns out to be the central mechanism toward orchestrating the observed nonlinear ion transport. Our results indicate exclusive features of the resulting ion concentration polarization, such as more pronounced ion concentration polarization, controlled largely by the influence of the variations in the channel cross section on the driving electrokinetic forces and the resistive viscous interactions. The underlying physical mechanism is captured aptly by a simple one-dimensional area-averaged model, and validated by full-scale three-dimensional simulations. Our illustrative case study for a converging-diverging microchannel with cross-sectionally uniform solute concentrations reveals that electrorheological effect with greater contrast between the deep and shallow region depths, greater solute concentration, and larger applied axial electric field, all acting in tandem, magnifies the solute enrichment and depletion in the respective segregation zones, bearing significant implications in analytical chemistry, bioanalysis, and beyond.