Critical Onset of Layering in Sedimenting Suspensions of Nanoparticles

We quantitatively study the critical onset of layering in suspensions of nanoparticles in a solvent, where an initially homogeneous suspension, subject to an effective gravity a in a centrifuge, spontaneously forms well-defined layers of constant particle density, so that the density changes in a staircaselike manner along the axis of gravity. This phenomenon is well known; yet, it has never been quantitatively studied under reproducible conditions: therefore, its physical mechanism remained controversial and the role of thermal diffusion in this phenomenon was never explored. We demonstrate that the number of layers forming in the sample exhibits a critical scaling as a function of a; a critical dependence on sample height and transverse temperature gradient is established as well. We reproduce our experiments by theoretical calculations, which attribute the layering to a diffusion-limited convective instability, fully elucidating the physical mechanism of layering.