  Experimental measurements of the surface tension of colloidal interfaces have
long been in conflict with computer simulations. In this work we show that the
surface tension of colloids as measured by surface fluctuations picks up a
gravity dependent contribution which removes the discrepancy. The presence of
this term puts a strong constraint on the structure of the interface which
allows one to identify corrections to the fundamental equation of equilibrium
capillarity and deduce bottom-up the microscopic origin of a growth model with
close relation to the Kardar-Parisi-Zhang equation.
