Context
Low-temperature liquid helium-4 is a model material for condensed matter physics for two reasons. First, atomic interactions are weak and quantum effects play an important role at the macroscopic scale. Second, liquid helium-4 can be obtained with remarkable purity. When preparing a sample of liquid helium-4 in an experimental cell, atomic or molecular impurities other than helium are frozen along the cell filling line or on the walls of the cell itself. It is then expected that the intrinsic properties of the liquid can be studied without being modified by the presence of impurities. In particular, liquid helium-4 is a system for probing in depth the metastable states of condensed matter and hoping to study homogeneous nucleation phenomena, i.e. not assisted by impurities in the liquid.
Our group creates metastable states of liquid helium by focusing an intense sound wave on it. In doing so, it is possible, during the decompression phases of the wave, to bring the liquid to pressures lower than the saturation vapor pressure; it is then in a metastable state where its pressure can be negative (see figure). The concept of negative pressure may be surprising but is easily understood by imagining a real fluid contained in a container. When the fluid particles push on the walls of the container, the fluid pressure is positive, when they pull on it thanks to van der Waals interactions, the pressure is negative!
For sufficient acoustic intensities, the liquid destabilizes and a bubble appears in the bulk of the liquid at the acoustic focus: this phenomenon is called cavitation. The aim of our experimental work is to provide measurements of thermodynamic properties of the liquid in the vicinity of the cavitation threshold in order to test predictions of nucleation theory in a quantum liquid.