Lubrication breakdown in hydrodynamic simulations of concentrated colloids

We report simulation studies of model colloid spheres interacting via squeeze, shear and rotation lubrication interactions. The results are both sensitive to the numerical scheme and its associated timestep. We find evidence that suggests the ideal smooth hard sphere problem is singular in that clusters of particles form within which very narrow gaps exist between neighbours — gaps which are unphysically narrow in colloid terms. The inclusion of a surface interaction or Brownian forces can prevent this catastrophe although for high enough shear rates and/or weak enough coats very narrow gaps again may form. Shear induced structure in these suspensions can be very sensitive to system size, but using large boxes extended along the flow direction gives structures likely realistic of true colloid flow under simple shear.