  We introduce an interacting particle system in which two families of
reflected diffusions interact in a singular manner near a deterministic
interface $I$. This system can be used to model the transport of positive and
negative charges in a solar cell or the population dynamics of two segregated
species under competition. A related interacting random walk model with
discrete state spaces has recently been introduced and studied in Chen and Fan
(2014). In this paper, we establish the functional law of large numbers for
this new system, thereby extending the hydrodynamic limit in Chen and Fan
(2014) to reflected diffusions in domains with mixed-type boundary conditions,
which include absorption (harvest of electric charges). We employ a new and
direct approach that avoids going through the delicate BBGKY hierarchy.
