  We study the statistical fluctuations of the Casimir potential felt by an
atom approaching a dielectric disordered medium. Starting from a microscopic
model for the disorder, we calculate the variance of potential fluctuations in
the limit of a weak density of heterogeneities. We show that fluctuations are
essentially governed by scattering of the radiation on a single heterogeneity,
and discuss in which limits they become larger than the average value predicted
by effective medium theory. Finally, for denser disorder we show that multiple
scattering processes become relevant.
