I do not know of any experimental data for solids suspension under the conditions that you describe, so I must work from what I know at atmospheric conditions and extrapolate the results to low viscosity and density.

Typical variations in viscosity between 0.5 cp and 10 cp seem to have only minor effects on solids suspension.  These effects can be small increases or decreases in the difficult of suspension depending on factors, such as particle size, shape, and density.  Higher viscosities may reduce the turbulence near the bottom of the vessel, which is necessary to lift the particles initially.  However, once lifted from the bottom, the higher viscosity keeps particles in suspension by reducing the terminal settling velocity.

The effect of liquid density is adequately understood for terminal settling velocity, but I do not know of any published data concerning the effect of liquid density on agitated solids suspension.  Unfortunately, agitated solids suspension is not directly related to terminal settling velocity, which describes particles falling through a static fluid.  Agitated suspension is definitely not in a static fluid, but rather a turbulent fluid, especially for the case you describe.  Agitated solids suspension is typically influenced more by the density difference between the particle and the fluid than is expected for terminal settling.

The best and only approach I know to your problem is to assume that the terminal settling velocity for the particles will eventually dominate over the effects of turbulent agitation of the fluid.  Based on agitated suspension, the density difference between the solid and the fluid often dominate atmospheric suspension difficulty, because those differences can change by a factor of five or ten between low density and high density particles.  In your case, the density difference between a  2.6 sp.gr. silica in a 1.0 specific gravity fluid or a near zero specific gravity fluid can only increase the density difference by about 50%, an effect that could be observed by increasing the density of the particles you could test in your glass reactor liner.

The effect of viscosity on terminal settling velocity has a strong effect, whether an increase or decrease, because viscosity can change by orders of magnitude.  A decrease in viscosity by a factor of ten to 0.1 cp from 1.0 cp significantly increases the terminal settling velocity of a particle.  This large increase in settling velocity is not likely to be offset by increased turbulence near the bottom of the vessel.
A large reduction in both the fluid density and viscosity will make particle suspension much more difficult.  While I cannot give you exact numbers, especially with out more information about the density and viscosity near critical conditions, I am sure that solids suspension will be difficult, if not impossible.  Other solids suspension results and correlations would require a great deal of extrapolation, and not much additional information.  The only off-setting factor, might be that the supercritical fluid will have less difficult moving between the settled particles.

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