Drift without flux: Brownian walker with a space-dependent diffusion coeffcient

Space-dependent diffusion of micrometer-sized particles has been directly o bserved using digital video microscopy. The particles were trapped between two nearly parallel walls making their confinement position dependent. Cons equently, not only did we measure diffusion coefficient which depended on t he particles position, but also report and explain new effect: drift of the particles individual positions in the direction of the diffusion coefficie nt gradient, in the absence of any external force or concentration gradient .