Phase separation of a binary mixture in a complex plasma in the absence of gravity


Under the influence of gravity, heterogeneous mixtures of small particles tend to demix by sedimentation of the heavier species. In the recent issue of Physical Review Letters the teams of André Melzer (Greifswald) and Alexander Piel (Kiel) report on demixing of microparticles with only a few percent difference in size under weightlessness. The technical trick to study the details of this demixing process at the level of individual particle motion was to dope one species with a fluorescent dye, which makes the particles optically distinguishable. A mixture of these particles was injected into an argon plasma, which gives each particle a negative electric charge that is proportional to its radius. It turned out that weak electric fields inside the plasma, which exert slightly different forces on the two particle species, are responsible for the demixing. The researchers have measured the diffusion flux and determined the uphill diffusion coefficients. The experiments were performed on parabolic flights with the new "ZERO-G" aircraft and are financially supported by the German Aerospace Center DLR. 

Carsten Killer et al, Phys. Rev. Lett. 116, 115002 (2016)