  Using Brownian dynamics simulations, density functional theory, and
analytical perturbation theory we study the collapse of a patch of
interfacially trapped, micrometer-sized colloidal particles, driven by
long-ranged capillary attraction. This attraction {is formally analogous} to
two--dimensional (2D) screened Newtonian gravity with the capillary length
\hat{\lambda} as the screening length. Whereas the limit \hat{\lambda} \to
\infty corresponds to the global collapse of a self--gravitating fluid, for
finite \hat{\lambda} we predict theoretically and observe in simulations a
ringlike density peak at the outer rim of a disclike patch, moving as an
inbound shock wave. Possible experimental realizations are discussed.
