Phase-separation of miscible liquids in a centrifuge

We show that a liquid mixture in the thermodynamically stable homogeneous phase can undergo a phase-separation transition when rotated at sufficiently high frequency ω. This phase-transition is different from the usual case where two liquids are immiscible or where the slow sedimentation process of one component (e.g. a polymer) is accelerated due to centrifugation. For a binary mixture, the main coupling is due to a term ∝ Δ ρ (ω r)², where Δρ is the difference between the two liquid densities and r the distance from the rotation axis. Below the critical temperature there is a critical rotation frequency ω_c, below which smooth density gradients occur. When ω > ω_c, we find a sharp interface between the low density liquid close to the center of the centrifuge and a high density liquid far from the center. These findings may be relevant to various separation processes and to the control of chemical reactions, in particular their kinetics. To cite this article: Y. Tsori, L. Leibler, C. R. Physique 8 (2007).