  The settling of colloidal particles with short-ranged attractions is
investigated via highly resolved immersed boundary simulations. At modest
volume fractions, we show that inter-colloid attractions lead to clustering
that reduces the hinderance to settling imposed by fluid back flow. For
sufficient attraction strength, increasing the particle concentration grows the
particle clusters, which further increases the mean settling rate in a physical
mode termed promoted settling. The immersed boundary simulations are compared
to recent experimental measurements of the settling rate in nanoparticle
dispersions for which particles are driven to aggregate by short-ranged
depletion attractions. The simulations are able to quantitatively reproduce the
experimental results. We show that a simple, empirical model for the settling
rate of adhesive hard sphere dispersions can be derived from a combination of
the experimental and computational data as well as analytical results valid in
certain asymptotic limits of the concentration and attraction strength. This
model naturally extends the Richardson-Zaki formalism used to describe hindered
settling of hard, repulsive spheres. Experimental measurements of the
collective diffusion coefficient in concentrated solutions of globular proteins
are used to illustrate inference of effective interaction parameters for
sticky, globular macromolecules using this new empirical model. Finally,
application of the simulation methods and empirical model to other colloidal
systems are discussed.
