Watergel—a new form of water condensed in liquid He4

Abstract

The semitransparent layer (cloud) of impurity condensate formed on the interface between superfluid He II and the vapor during condensation of gaseous He4 containing impurity water vapor in a cell filled with superfluid helium at a temperature T⩽1.5 K is transformed over time into an oval iceberg with an average diameter of ∼9 mm, suspended on the walls of the glass cell under the surface of the liquid. Within the liquid helium, icebergs can exist at temperatures above Tλ as well. At a vapor pressure P=150 torr over the surface of normal liquid He I the temperature Td at which intense decomposition of the icebergs occurs is 2.5 K. When the pressure is increased to 760 torr the temperature Td increases to 4 K. In an atmosphere of gaseous He4 the “dry” icebergs extracted from He II at T∼1.4 K decompose on heating above 1.8 K. The decomposition of the icebergs is accompanied by the formation of a fine powder (apparently amorphous ice) on the bottom of the cell; the volume of this powder is nearly two orders of magnitude less than the volume of the initial icebergs, i.e., the total water content in an iceberg is ⩽1020 molecules/cm3. This estimate agrees with an estimate of the difference of the density of an iceberg and the density of the surrounding liquid ρL (the ratio Δρ/ρL<0.1) from the results of observations of oscillations of the sample when thermoacoustic vibrations arise in a cell filled with liquid He I. In considering the structure of the icebergs it can be assumed that in helium vapor over the surface of He II the H2O impurity molecules agglomerate into clusters, so that the core of the impurity–helium water condensate in He II (a gel dispersion system) is formed by water nanoclusters surrounded by one or two layers of solidified helium, and the superfluid He II filling the pores between particles serves as the dispersion medium of the watergel.