Effects of Interparticle Attractions on Colloidal Sedimentation

We use a mesoscopic simulation technique to study the effect of short-ranged interparticle attractions on the steady-state sedimentation of colloidal suspensions. Attractions increase the average sedimentation velocity v_s compared to the pure hard-sphere case, and for strong enough attractions, a nonmonotonic dependence on the packing fraction ϕ with a maximum velocity at intermediate ϕ is observed. Attractions also strongly enhance hydrodynamic velocity fluctuations, which show a pronounced maximum size as a function of ϕ. These phenomena arise from a complex interplay between nonequilibrium hydrodynamic effects and the thermodynamics of transient cluster formation.