Exploring Electron Bubbles in Liquid Helium using Cavitation

Abstract

An electron entering liquid helium experiences a repulsive potential of 1 eV. This originates in the interaction between the injected electron and electrons of the closed shells of helium atoms through Pauli exclusion principle. If the energy of the electron exceeds 1 eV, it can penetrate the liquid, form a cavity free from helium atoms and subsequently localize itself within the cavity. This is known as a single electron bubble (SEB). In another configuration, a system of electrons of energy less than 1 eV can form a floating charged layer above the surface of liquid helium: a two-dimensional system that has been studied in great detail over last few decades. If the number of electrons in this layer exceeds a critical value of 2 × 10^13electrons/m^2, an electrohydrodynamic instability sets in, giving rise to multielectron bubbles (MEBs), referring to micron to mm sized cavities containing a large number of electrons. In the experiments to be discussed in this thesis, the primary technique is based on cavitation of liquid helium using pulsed ultrasound. After a brief introduction of the experimental technique, we will present the main results obtained during my Ph.D., as follows: The first result is related to the phenomenon of cavitation in superfluid helium. We have observed that after cavitation, the bubble is pushed out of the acoustic focus because of the radiation pressure and can grow up to a size as large as a millimetre. The growth and collapse of these bubbles can be understood through Rayleigh-Plesset equation at low temperatures, and by condensation of vapour (limited by the thermal diffusivity of helium) above lambda but this description fails near the lambda transition. We suspect this is related to the large density of vortices nucleated near the bubble surface during the growth of the cavitating bubble. Second, we have observed a new species which cavitates at a negative pressure approximately 80 and 70 % lower magnitude than SEBs. We conclude that these are multielectron bubbles...