Noise-driven ionic currents in a viscosity gradient

Gradients of voltage, pressure, temperature, or salinity can transport objects in micro- and nanofluidic systems by well-known mechanisms. I will describe the discovery of an electrokinetic transport effect driven by a viscosity gradient: An imposed liquid viscosity gradient causes an ionic current to flow inside a glass nanofluidic channel. Measurements of the current and numerical simulations reveal that the counterions in the electric double layers near the nanochannel surfaces drift in the direction of decreasing viscosity. The measurements are well described by a simple model in which the couterion drift speed equals the gradient of an ion’s local diffusivity. Drift in a viscosity gradient, or viscophoresis, is a consequence of multiplicative (state-dependent) noise, where the magnitude of the thermal fluctuations experienced by a particle depends on its position.