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The behaviour of sedimenting particles depends on the dust-to-gas ratio ofthe fluid. Linear stability analysis shows that solids settling in the Epsteindrag regime would remain homogeneously distributed in non-rotatingincompressible fluids, even when dust-to-gas ratios reach unity. However, thenon-linear evolution has not been probed before. Here, we present numericalcalculations indicating that in a particle-dense mixture solids spontaneouslymix out of the fluid and form swarms overdense in particles by at least afactor 10. The instability is caused by mass-loaded regions locally breakingthe equilibrium background stratification. The driving mechanism depends onnon-linear perturbations of the background flow and shares some similarity tothe streaming instability in accretion discs. The resulting particle-richswarms may stimulate particle growth by coagulation. In the context ofprotoplanetary discs, the instability could be relevant for aiding smallparticles to settle to the midplane in the outer disc. Inside the gas envelopesof protoplanets, enhanced settling may lead to a reduced dust opacity, whichfacilitates the contraction of the envelope. We show that the relevant physicalset up can be recreated in a laboratory setting. This will allow our numericalcalculations to be investigated experimentally in the future.

Spontaneous concentrations of solids through two-way drag forces between gas and sedimenting particles