  Dust-gas mixtures are the simplest example of a two fluid mixture. We show
that when simulating such mixtures with particles or with particles coupled to
grids a problem arises due to the need to resolve a very small length scale
when the coupling is strong. Since this is occurs in the limit when the fluids
are well coupled, we show how the dust-gas equations can be reformulated to
describe a single fluid mixture. The equations are similar to the usual fluid
equations supplemented by a diffusion equation for the dust-to-gas ratio or
alternatively the dust fraction. This solves a number of numerical problems as
well as making the physics clear.
