Visualization of He crystallization process in aerogel

Abstract

Freezing of a fluid in small pores is a common phenomenon often seen in a natural environment. Water remains in a liquid phase even below 0 Celsius when contained in a fine soil and freezes into ice at lower temperatures. However, this is sometimes a very complicated phenomenon and consists of several slow processes other than crystallization, such as a viscous fluid flow in pores and diffusion of latent heat. These hide inherent features of the crystallization in a quenched disorder by introduced by pores. Using low temperature 4He in the superfluid and the solid phases causes the flow in the pores to be fast enough to reveal the nature of crystallization dynamics. As a porous material for the experiment we use silica-aerogels. Aerogel consists of silica beads in a few nm size. Aerogel has a very large open volume and its porosity is very high, ranging from 90 to 99.5％ in volume. It introduces a quenched disorder to the phase transition of 4He within it. We can alter the strength of the disorder broadly by choosing the porosity of the aerogel. Aerogel is very transparent with a low dielectric constant and suitable for visualizing the dynamics within it. Here we report a novel dynamical transition of the crystallization of 4He in pores. The crystal-superfluid interface advances via creep at high temperatures and via avalanches at low temperatures. The transition temperature is higher at a higher interface velocity and lower in higher porosity aerogels. The transition is due to competition between thermal fluctuations and disorder for the crystallization process.