  The mechanism of diffusing diffusivity predicts that, in environments where
the diffusivity changes gradually, the displacement distribution becomes
non-Gaussian, even though the mean-squared displacement (MSD) grows linearly
with time. Here, we report single-particle tracking measurements of the
diffusion of colloidal spheres near a planar substrate. Because the local
effective diffusivity is known, we have been able to carry out the first direct
test of this mechanism for diffusion in inhomogeneous media.
