Can colloidal swarms settle faster than isolated particles? (iSoDays)
Abstract
Besides its practical importance in many industrial and science applications, sedimentation has been highly influential in the development of statistical physics too, thanks to the celebrated experiments by Perrin. By experimenting on model colloids where depletion forces can be carefully tuned and quantified, we show that attractive interactions consistently “promote” particle settling. For a suspension of hard spheres, the settling velocity decreases with particle volume fraction $\phi$, whereas, in presence of strong attractive forces, in a moderately concentrated dispersion it can even exceed its single-particle value. At larger $\phi$, however, hydrodynamic hindrance eventually takes over. Hence, $v(\phi)$ actually displays a nonmonotonic trend that may threaten the stability of the settling front to thermal perturbations. Finally, by discussing the case of BLGA protein, we show that these results are relevant to the investigation of protein association effects by ultracentrifugation.
