The article “Colloidal Swarms Can Settle Faster than Isolated Particles: Enhanced Sedimentation near Phase Separation”, written by Roberto Piazza in collaboration with Enrico Lattuada and Stefano Buzzaccaro, was published on Phys. Rev. Lett., 116, 3, (2016), and selected by the American Physical Society to be signaled on the website section APS Physics: spotting exceptional research.
This work is deemed of particular interest for the study of the processes of proteins aggregation.
For more information on the article visit the web site:
Usually, the more concentrated a particle dispersion is, the 1ower it settles in natural gravity or in a centrifuge. Yet, here we show that this must not perforce be true: in fact the sedimentation velocity v(c) of a suspension of particles interacting via attractive forces depends non-monotonically on concentration c. While at high concentration v(c) does decrease with c, for sufficiently low c the suspension settles even faster than a single particle. This evidence, obtained for a system where interactions can be finely tuned, is favorably compared to recent numerical results suggesting the occurrence of such a “promoted” sedimentation regime, which utterly contrasts with standard “hindered” settling. Our results, however, also highlight an important and previously unnoticed consequence of promoted settling on the kinetics of sedimentation. In fact, the settling front, which for hindered settling takes on a time-invariant, shock-wave profile, spreads conversely with time, becoming liable to thermal instabilities that may lead to “stratification” of the profile into distinct concentration bands. Finally, we suggest that these results may be relevant for ultracentrifuge investigations of protein association effects, where in fact promoted settling is sometimes observed and usually interpreted using semi-phenomenological “clustering models”. This is not just a “generic” claim: rather, with a specific case study, we show that the structural and dynamic information provided by light scattering measurements performed at equilibrium can be exploited to quantitatively predict the settling kinetics actually observed in an ultracentrifuge without resorting to any detailed “chemical association” models.