Fluidization of granular media wetted by liquid 4He
Abstract
We explore experimentally the fluidization of vertically agitated PMMA spheres wetted by liquid 4He. By controlling the temperature around the λ 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 (c) shows a steep increase close to the saturation of the vapor pressure in the sample cell. For helium II wetting, c starts to increase at about 75% saturation, indicating that capillary bridges are enhanced by the superflow of unsaturated helium film. Above saturation, 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.