  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.
