Growth distribution characteristics of micro-scale trapped air bubbles in ice and their impact on the freezing process

Icing is a nonlinear, variable density liquid-solid phase change process that is widespread and mostly negatively affected in nature and industry. Trace amounts of air dissolved in water converge and nucleate during icing due to the extrusion of ice crystals and form tiny bubbles that remain at the freezing front due to adhesion, ultimately forming micro-scale trapped air bubbles. Trapped air bubbles affect the later dynamic icing process by changing the internal structure. To accurately predict and control the icing process, then to develop and optimize various antiicing technologies, the study of the growth and distribution characteristics of trapped air bubbles is necessary. The nucleation mechanism, growth process, distribution characteristics, and stability of trapped air bubbles are reviewed from the micro and macro scales. Results show that the bubble shape is directly related to the freezing rate, and when the freezing rate is greater than 25μm/s, egg-shaped bubbles with the aspect ratio of the major and minor axes less than 5 form in ice. When the freezing rate is between 5 and 25 μm/s, needle-shaped bubbles with the aspect ratio greater than 5 form in ice. When the ice freezing rate is less than 5 μm/s, there are no bubbles in ice. This review is helpful for clarifying the complex characteristics of trapped air bubbles and enriching the theory of icing process, and it can also provide a reference for the optimal design of existing antideicing technology.