Air bubbles in accreted ice

On the assumption that the concentration of air bubbles formed in freezing water is a function of the freezing rate, an expression for the concentration of air bubbles in accreted ice has been deduced. This expression has been confirmed by measurements of the bubble concentrations and size distributions in frozen bulk water and in ice accreted on rotating cylinders. Consequently, in both the wet and dry growth regimes it is the rate of freezing which essentially determines the bubble concentration. The size distribution of bubbles is determined by the amount of air available for bubble growth. This depends on the rate at which air diffuses out of the individual droplets during the freezing process. In the wet growth regime the freezing rate depends on the rate at which heat is dissipated from the accreting object by forced convection and also, depending on the fraction of unfrozen water, on the way in which the spongy deposits are finally frozen. In the dry growth regime the freezing rate of the accreted droplets depends on their size and speed of impact, since these determine the extent to which the droplets spread, and on the temperature of the deposit. The air bubble concentrations and size distributions were determined using ordinary optical microscopy so that only bubbles of radius greater than 1μm were counted and sized. In the dry growth regime the visible bubble concentrations range from about 106 to 108cm−3, depending primarily on the deposit temperature. The mean radii of the bubbles in the deposits range from 1 to 4μm. In the wet growth regime the bubble concentrations generally lie between 105 and 106cm−3 while the mean radii of the bubbles range from 10 to 50μm. Consequently, the two growth regimes may be readily distinguished.