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