Observation of aerodynamic instability in the flow of a particle stream in a dilute gas

How planetary precursors (planetesimals) from out of the primordial dust grains 
present in gaseous protoplanetary discs remains an open question. While grain 
adhesion forces bind solids at small scales (micrometers-decimeters) and gravity 
on large scales (kilometers), bridging these extremes across growth barriers in 
the meter size range requires an additional solid-concentration mechanism, 
usually assumed to derive from collective gas-particle interaction. I present 
the results from ground-based experiments designed to test the most well-accepted 
theory of particle concentration due to 'streaming instability' (SI), or closely 
related dust-drag induced fluid instabilities, in protoplanetary discs. The 
experimental data demonstrate spontaneous concentration of spherical 
dust-particle analogues via fluid instability, with critical wavelength in 
agreement with theoretical predictions. The conditions involve a very dilute, 
low Reynold's number flow (Re<<1 on the particle scale), in which the particle 
diameter is comparable to the mean free path of the gas and the volume-averaged 
dust-to-gas density ratio is close to one, which meets the proper flow 
conditions for fluid drag on mm-m objects in the astrophysical context as well 
as critical values for appreciable growth of SI. Due to the scale similarity 
of the two-phase flow to that of a protoplanetary disc, the experimental results 
represent the first direct empirical support of the promising 
theory of particle concentration due to SI.