  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.
