  The onset of the Rayleigh-Taylor instability is studied a compressible
Brownian Yukawa fluid mixture on the ``molecular'' length and time scales of
the individual particles. As a model, a two-dimensional phase-separated
symmetric binary mixture of colloidal particles of type $A$ and $B$ with a
fluid-fluid interface separating an $A$-rich phase from a $B$-rich phase is
investigated by Brownian computer simulations when brought into non-equilibrium
via a constant external driving field which acts differently on the different
particles and perpendicular to the interface. Two different scenarios are
observed which occur either for high or for low interfacial free energies as
compared to the driving force. In the first scenario for high interfacial
tension, the critical wavelength $\lambda_c$ of the unstable interface modes is
in good agreement with the classical Rayleigh-Taylor formula provided
dynamically rescaled values for the interfacial tension are used. The
wavelength $\lambda_{c}$ increases with time representing a self-healing effect
of the interface due to a local density increase near the interface. The
Rayleigh-Taylor formula is confirmed even if $\lambda_c$ is of the order of a
molecular correlation length. In the second scenario for very large driving
forces as compared to the interfacial line tensions, on the other hand, the
particle penetrate easily the interface by the driving field and form
microscopic lanes with a width different from the predictions of the classical
Rayleigh-Taylor formula.
