Observation of Particle Drift in a Controlled Viscosity Gradient

We report the observation of drift by Brownian particles in a viscosity gradient inside a nanofluidic device. We set up a controlled viscosity gradientby pumping miscible fluids of known viscosity past either end of a 200 nm-deep, 100μm-long, and 150μm-wide glass channel. We tracked the motion of nanoscopic fluorescent spheres inside the channel. An analysis of the particles' stochastic trajectories based on the work of Frishman and Ronceray obtained the position-dependent self diffusivity of the particles, the mean drift, and the inferred force field. Particles were observed to drift in the direction of increasing diffusivity, consistent with the theory of Brownian motion with multiplicative noise in the an isothermal stochastic framework. We separately used electrodes to measure ionic currents flowing through the same nanofluidic channel in the direction of lower viscosity. The currents increased in proportion with the viscosity gradient.