Bubble dynamics and nucleation of ice in supercooled water

Abstract

The effects of mechanically induced impulses such as vibration and flow have been used to induce the solid–liquid phase transformation in liquids held below their equilibrium freezing point. Although processes used for grain refining in metallurgy rely on this artificially enhanced nucleation process, there is no clearly identified underlying physical mechanism. Experiments have been conducted with single bubbles immersed in a small volume of supercooled water and trapped in a 22-kHz resonant cell to study the action of both large amplitude radial and shape oscillations. The experimental observations reveal that large-amplitude radial oscillations of micron-size air bubbles induce the nucleation of ice when a minimum supercooling of 5 °C is reached. Shape oscillations and radially nonresonant bubbles do not induce nucleation of ice within the same ‘‘incubation’’ time. The appearance of the ice crystal at the location of the trapped bubble strongly suggests a cause and effect correlation between the bubble radial oscillations and the freezing onset, and both sonoluminescing and nonluminescing bubbles appear to induce nucleation. This mechanism also has the additional benefit of allowing the direct measurement of free dendritic growth velocity within a supercooled liquid bulk in the absence of any artificially introduced seed. [Work sponsored by NASA.]