Layering in sedimenting nanoparticle suspensions: The order-inducing role of randomness

Sedimentation of nanoparticles (NPs) is common in science and technology; yet, the dynamics of sedimentation is sometimes unexpected. We study the sedimentation of NPs under an elevated gravity in a centrifuge, in presence of a small temperature gradient, transverse to the axis of sedimentation. We observe spontaneous ordering of the sedimenting fluids into well defined layers of constant number density; the corresponding density profiles thus adopt a staircase-like appearance. While the physical mechanism of these phenomena has been very recently elucidated [8], the quantitative description of these effects still remained incomplete. We fill this gap by direct quantitative measurements of the layering effect. In this work, we employ much smaller Fe₂O₃ NPs, in addition to the previously-used Ag and Cu@Ag NPs; the Fe₂O₃ particles are suspended in water, while only organic solvents were used in our previous work. Remarkably, in both cases, the layering effect remains largely unchanged. We demonstrate that particle polydispersity, an archetypal source of disorder, counterintuitively, promotes formation of ordered density staircase structures. Furthermore, we demonstrate complex layer dynamics and 'glitches' of particle concentration at the interfaces between the layers; these phenomena have been predicted by theory, yet their experimental validation was missing.