F. Caupin, S. Balibar, X. Chavanne, C. Appert, D. D'Humières
By focusing an intense sound wave in an ultrapure liquid, we have studied homogeneous cavitation, i.e. the limiting depression beyond which this liquid becomes intrinsically unstable relatively to its vapor. Two liquids are particularly pure, because they are colder than all others, helium 4 and helium 3. The properties of these two liquids are so well-known that the theory made it possible to calculate the “spinodal limits” close to which cavitation must take place: -9,5 bar for helium 4 and -3,1 bar for helium 3. By performing a study under pressure, we have shown that cavitation indeed occurred in helium 4 between -8 and -10,5 bar and in helium 3 between -2,4 and -2,9 bar.
In addition, a previous study had shown that, in helium 4, there is a transition between a classical regime (where cavitation is stochastic and depends on the temperature; indeed, the nucleation of bubbles results from the overcoming of an energy barrier by thermal fluctuations) and a quantum regime (where nucleation takes place by quantum tunnelling through the barrier). By carrying out for the first time measurements on helium 3, we have discovered that the transition towards quantum cavitation did not occur at the expected temperature; conversely the cavitation pressure becomes more negative around 40 mK. We then proposed the following interpretation: at low temperature, helium 3 is a “Fermi liquid” where only the energy of the long wavelength fluctuations decreases close to the spinodal limit; the energy of the short wavelength fluctuations remains high because of the existence of a quantum stiffness responsible for the “zero sound”. In order for bubble nucleation to occur at low temperature, it is thus necessary to bring helium 3 much closer to its spinodal limit than was predicted before, in order for the size of the critical nucleus to be large.
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