Fluidization of granular media wetted by liquid $^4$He

TL;DR

This study investigates how the fluidization behavior of wetted granular media changes with temperature, especially across the superfluid transition of helium, revealing the influence of superflow on capillary forces.

Contribution

It provides the first experimental analysis of fluidization in granular media wetted by superfluid helium, highlighting the effects of superflow on capillary bridge strength.

Findings

Critical acceleration increases near vapor pressure saturation for helium I.

Superfluid helium enhances capillary bridges at partial saturation.

Plateau in critical acceleration peaks at 2.1 K, linked to helium's specific heat.

Abstract

We explore experimentally the fluidization of vertically agitated PMMA spheres wetted by liquid $^4$He. By controlling the temperature around the $\lambda$ point we change the properties of the wetting liquid from a normal fluid (helium I) to a superfluid (helium II). For wetting by helium I, the critical acceleration for fluidization ($\Gamma_c$) shows a steep increase close to the saturation of the vapor pressure in the sample cell. For helium II wetting, $\Gamma_c$ starts to increase at about 75% saturation, indicating that capillary bridges are enhanced by the superflow of unsaturated helium film. Above saturation, $\Gamma_c$ enters a plateau regime where the capillary force between particles is independent of the bridge volume. The plateau value is found to vary with temperature and shows a peak at 2.1 K, which we attribute to the influence of the specific heat of liquid helium.