  A comment is made on the recent PFG NMR measurements by Stallmach, et al. on
water-saturated sands [Phys. Rev. Lett. 88, 105505 (2002)]. It is pointed out
that the usual law for the time-dependent diffusion coefficient D(t) used by
these authors is not valid for a fractal surface. It is shown that (1-D(t)/D0)
\~ t^[(3-Ds)/2] at short times for a surface of fractal dimension Ds, where D0
is the bulk diffusion coefficient. Preliminary PFG NMR data on water saturated
limestone and plastic beads are presented to illustrate this analysis.
