Watergel in Liquid Helium

Abstract

Semitransparent soft water clouds forming in superfluid He-II by condensation of a gaseous mixture of ⁴He with water impurities transform with time to more rigid, highly porous ‘icebergs’. The icebergs suspended in the bulk of He-II are stable at a constant temperature T≤1.6 K, and they can beak down to small ice pieces on heating the liquid above T_λ. The temperature of decomposition of the icebergs in He-I depends strongly on the vapor pressure above the surface of the liquid: at P∼0.2 atm they start to decompose at T_d∼2.5 K, but increasing the pressure to 1 atm causes T_d to rise to ∼4 K. When withdrawn from He-II the “dry” icebergs segregate to an ice powder and He on heating them above 1.8 K in He gas atmosphere. The total content of the water in the bulk of the icebergs, estimated from the ratio by volume of icebergs and powder, is ≤10²⁰ molecules/cm³. From observations of acoustic oscillations in the cell filled with He-1 (the ratio of amplitudes of vibrations of the iceberg and He-I level is about 0.2–0.3) it can be estimated that the density of the iceberg is only a few percent higher than the density of the surrounding liquid. We suggest that the highly porous water condensate (watergel) is composed of water nanoclusters, coated with 1-2 layers of solidified helium, which form a dispersed system of gel, and of liquid Helium, filling the pores between these van-der-Waals complexes, which serves as the dispersion medium of the gel.