Nucleation Processes in Liquid Helium

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This low temperature physics project will study the properties of electrons in liquid helium. Previous work has shown that when an electron is introduced into liquid helium, it forces open a small cavity in the liquid. This "electron bubble" has a radius of approximately 20 Angstroms. Application of a negative pressure to the liquid causes this bubble to grow, and at a critical pressure, the bubble becomes unstable and explodes. This provides a convenient way to detect the electron bubbles. It is planned to study in detail what happens to the bubbles when they are illuminated by light and the electron is excited to a higher energy state. This causes the size and shape of the bubble to undergo a sudden change. In addition to the ultrasonic experiments, measurements of ion mobility will be performed. Students involved in this project are trained in a range of experimental techniques that prepares them for careers in academe, industry, or government.Bubbles form in liquids as a result of a number of different processes. They can be produced when the pressure on a liquid that contains gas is reduced: For example, when a cola can is opened. They also occur when a fluid passes rapidly by a solid surface, such as a propeller blade, a sometimes-damaging process known as cavitation. In this research project, bubbles are produced in liquid helium, a cryogenic fluid with a boiling point of -268.93 C. Liquid helium is chosen in part because it is ultra-pure, so effects that might arise from impurities can be controlled or eliminated. It is also chosen because it is possible to create bubbles in liquid helium via a highly controllable, though rather esoteric, process. It has been found that when an electron is introduced into liquid helium, it forces open a small cavity in the liquid. This unusual phenomenon and the high purity of cryogenic liquid helium combine to provide a model system for the study of bubbles. The goal is to understand and control bubble initiation and growth, and, where possible, apply the knowledge to practical situations such as cavitation. Students involved in this project are trained in a range of experimental techniques and are well prepared for careers in academe, industry, or government.