Viscophoresis: Motion in a Viscosity Gradient*

We report the discovery of a new electrokinetic transport effect driven by viscosity gradients. We call this effect viscophoresis.
Specifically, we imposed a viscosity gradient in the solvent filling a glass nanochannel and observed the electrical current resulting from the drift of counterions in the electric double layers near the charged glass surfaces. We consistently measure a current, I, in the range of 10-100 pA, flowing in the direction of decreasing viscosity. The fundamental nature of our discovery is suggested by the simple and elegant equation of motion that the particles obey: 〈dx/dt〉 = dD/dx.

Diffusion in a viscosity gradient challenges our intuition and our mathematics alike. In addition to revealing a new mechanism for driving transport, our experiments address a basic disagreement about the effects of a solvent viscosity gradient on a diffusing particle, showing drift in the direction of lower viscosity, and addresses a longstanding question in stochastic calculus known as the Ito-Stratonovich dilemma.