Electrokinetic Current Driven by a Viscosity Gradient

Abstract

Gradients of voltage, pressure, temperature, and salinity can transport objects in micro-and nanofluidic systems by well known mechanisms. Here we report the discovery of a transport effect driven by viscosity gradients, which cause an ionic current to flow inside a glass nanofluidic channel. Measurements of the current are well described by a simple model wherein counterions in the electric double layers near the surfaces drift in the direction of decreasing viscosity with a drift speed equal to the gradient of the ions' local diffusivity. Drift in a viscosity gradient is a consequence of multiplicative (state-dependent) noise, which results from a particle's thermal fluctuations depending on its position. This surprisingly large effect, measured in a highly controlled nanofluidic environment, reveals fundamental behavior that is relevant to a broad range of systems.

A theoretical understanding of ionic current through a nanochannel driven by a viscosity gradient

“…26 and 37 to obtain the MS diffusion coefficient (D 12 (x)) and successively the flux of species. As Wiener and Stein [20] mentioned, their experimental data are consistent with the isothermal rule [35] in stochastic displacement models. According to the Hanggi model [35], a particle drifts toward the lower viscosity (higher diffusion) with a speed…”

“…Recently, Wiener and Stein [20] have experimentally shown that a nanochannel bridging two microchannels filled with different viscosity solutions containing dissolved ionic species will drive an ionic current, even when the bulk ionic concentration in both fluids is equal. Fig.…”

“…These methods of ion transport phenomena have found numerous applications in energy harvesting [8][9][10][11], water treatment [12][13][14][15], or logical part of nanofluidic chips [4,[16][17][18][19]. Recently, Wiener and Stein [20] have shown that by applying a viscosity gradient along a nanochannel one can drive an ionic current, similar to the other asymmetrical conditions. They deduced that this ionic current is a result of ions drifting toward the lower viscosity solvent, in which ions encounter a higher diffusivity.…”

“…Considering the above mentioned works, it is clear that different terminology was proposed (visco-taxis, viscophoresis) for describing a particle movement inside a viscosity gradient. In this paper, we aim at a mechanistic understanding of the Wiener and Stein [20] method in driving ionic species by employing a viscosity gradient. Since we are dealing with ionic species, we suggest visco-migration as the terminology for drifting ions in a viscosity gradient, analogous to electro-migration in which ions migrate under the influence of an external electric field.…”

“…Successively, a simple equation will be developed to describe the relation of ionic drift velocity and the ionic current. For ideal fluids, Wiener and Stein [20] developed a simple relation to obtain the diffusion and as a result the viscosity along the nanochannel by employing the Maxwell-Stefan equations. Their modeling results are in good agreement with their experimental measurements, but we extend the framework to a mechanistic approach which compared the ideal and non-ideal fluids with increasing the deviation of the fluid from the ideal scenario.…”