Investigation of the behavior of dissolved gases during freezing

Abstract

This paper deals with the freezing process of aqueous solutions of gases and the nucleation of gas bubbles at the moving ice—water interface. A cryomicroscope was used to investigate the conditions of nucleation and growth of bubbles after reaching a stationary concentration profile ahead of the phase boundary. The enrichment of gases due to the distribution coefficient was detected by means of a test bubble method, i.e., the increase in the radius of a small bubble being approached by the ice front. A distribution coefficient of 0.048 (at 0 °C) was found for oxygen. Nucleation occurs when stationary growth conditions in the solution are reached. The measured oversaturation is close to 20, i.e., about the inverse of the distribution coefficient. In highly saturated gas solutions, dendritic breakdown of the planar ice-water interface due to gas enrichment could be observed. At these positions also a considerable degree of constitutional supercooling was found. Bubbles were nucleated in interdendritic spaces. Nucleation and growth of gas bubbles was seen to be a periodic process under certain circumstances which can be explained by the continuous buildup and reduction of the concentration field in the remaining solution. The growth kinetics of the bubbles and their maximum size are governed by the velocity of the ice-water interface. During growth the gas bubbles are pushed and partially encapsulated, until they reach a radius in the order of magnitude of the diffusion boundary layer of the concentration profile, and become totally engulfed by the solid phase.