EFFECT OF BUBBLES ON GRAIN-GROWTH IN ICE

Grain growth is studied in artificial samples of pure ice. It is shown that the mean grain diameter D can be represented by the equation D-2 = D-0(2) + 4kt, both in bubble free ice and in growth regimes where b ubbles are present on boundaries. In the second case it is found k << k(i), where k(i) is the intrinsic grain growth rate obtained for bubbl e free ice. This behavior is explained by considering that the drag ef fect P-b of migration bubbles on boundaries occurs in the low-velocity regime, where P-b = v/M-b with v and M-b the migrate velocity and mob ility of bubbles, respectively. From the values of k(i), the free boun dary mobility M is calculated and the values of M-b are derived from t he equation k = k(i)(1 + M/M-b)(-1). The results obtained by different authors for k, both in artificial and glacial ice, are compared in an Arrhenius plot. It is shown that the results for glacial ice and for bubbly laboratory ice can be grouped in the same region where k << k(i ). These results are interpreted by assuming that also in glacial ice grain growth is affected by migrating bubble drag. It is noted that th is interpretation is not in contradiction with the fact that in glacia l ice bubbles gradually separate from boundaries, i.e., that the pheno menon would occur in the high-velocity regime. Correlation between the experimental and theoretical values of M-b, the latter calculated on the basis of molecular diffusion processes, is discussed.