  The diffusing-diffusivity mechanism proposed by Chubynsky and Slater [PRL
113, 098302, 2014] 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 wall. 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.
