posted on 2025-01-17, 15:03authored byAnindita Bhattacharya, Suman Chakraborty
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.