A discussion of mechanisms proposed to explain habit changes of vapor-grown ice crystals

Abstract

In the present work, surface kinetics processes that can contribute to the growth behavior of ice crystals from the vapor phase are revised and proposed interpretations of crystal habit changes are discussed. Following the main initial papers on this subject by Hallet, Mason et al. and Kobayashi, relationships are considered between linear growth rate and step velocity. More recent results obtained by Sei and Gonda (SG2) for molecular steps naturally formed on basal and prism surfaces are shown to confirm Hallet's interpretation of previous curves obtained for the velocity of giant steps that were artificially formed on basal surfaces only. The different behavior of the condensation coefficient α( T) characterizing growth in pure water vapor, observed by Lamb and Scott for surfaces intersecting a substrate and by Sei and Gonda for free surfaces, is discussed by considering that α is the product of the adsorption and accommodation coefficients β and γ, respectively. It is noted that, as in previous works, β=1 was assumed, the variations of α discussed to interpret crystal habit changes were made to coincide with variations of γ. However, Sei and Gonda's results show that in the temperature range where crystal habit changes are observed, values of β( T)≪1 are found. As these depend on surface orientation, they should play an important role in the phenomenon. The dependence of crystal habits on two-dimensional nucleation is also discussed on the basis of measurements carried out by Nelson and Knight of the critical supersaturation σ cr on the basal and prism surfaces. A possible relationship between the curves for σ cr( T) and those of β( T) derived from Sei and Gonda's results is suggested. The mechanisms determining the large anisotropy exhibited by crystals grown in atmospheric conditions are discussed by taking into account that the growth rate curves R( T) on the basal and prism surfaces show a correlation between maximum and minimum values, which are not observed for crystals grown in pure vapor.