SEPARATION of heavy particles from a fluid medium by centrifugal forces or by barodiffusion is well known1. For a homogeneous fluid mixture comprising a suspension of very fine particles (such as smoke in air), on the other hand, for which the centrifugal and barodiffusive effects are negligible, a continuum model of the suspension would suggest that it is not possible to separate the particles from the fluid by flow-induced kinematic action alone. Here we present experimental evidence to the contrary: we have observed spontaneous segregation of smoke particles from air in an initially uniform mixture, when rotational flow is induced in the medium. A definitive explanation for this effect is not yet forthcoming.
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