  The ability to induce regions of high and low ionic concentration adjacent to
a permeselective membrane or nanochannel subject to an externally applied
electric field (a phenomenon termed concentration-polarization) has been used
for a broad spectrum of applications ranging from on-chip desalination,
bacteria filtration to biomolecule preconcentration. But these applications
have been limited by the ability to control the length of the diffusion layer
that is commonly indirectly prescribed by the fixed geometric and surface
properties of the nanofluidic system. Here, we demonstrate that the depletion
layer can be dynamically varied by inducing controlled electrothermal flow
driven by the interaction of temperature gradients with the applied electric
field. To this end, a series of microscale heaters, which can be individually
activated on demand are embedded at the bottom of the microchannel and the
relationship between their activation and ionic concentration is characterized.
Such spatio-temporal control of the diffusion layer can be used to enhance
on-chip electro-dialysis by producing shorter depletion layers, to dynamically
reduce the microchannel resistance relative to that of the nanochannel for
nanochannel based (bio)sensing, to generate current rectification reminiscent
of a diode like behavior and control the location of the preconcentrated plug
of analytes or the interface of brine and desalted streams.
