Habit of ice crystals

Fig. 5.5. The growth habits of ice crystals, grown in a diffusion chamber, in relation to temperature and supersaturation (Hallett Mason, 1958).

Rubinsky, B. DeVries, A.L. (1989). Effect of ice crystal habit on the viability of red blood cells. Cryobiol. 26, 580 (abstract). 

Some clue as to the structure of a crystal can usually be found from observation of the habits of natural crystals, particularly when these have grown in an environment which is reasonably symmetrical. Ice crystals which have grown by direct sublimation... 

Crystal Growth Modification. As a practical example of crystal habit modification, one can consider the growth of ice crystals in ice cream. If the crystals grow too large or attain certain shapes, the organoleptic or perceived quality of the product will be reduced significantly—the ice cream becomes sandy. In practice, the crystallization phenomenon is controlled by the addition of various natural gums (e.g., locust bean gum) that presumably adsorb on specific crystal faces and retard or prevent further deposition of water molecules. 

Sei, T. and Gonda, T. (1989) The growth mechanism and the habit change of ice crystals growing from the vapor-phase. J. Cryst. Groarth, 94 (3), 697-707. 

Chen, J. and D. Lamb, 1994 The theoretical basis for the parameterization of ice crystal habits growth by vapor deposition. J. Atmos. Sci. 51,1206-1221. 

Hydrothermal, low temperature precipitation, and vapor deposition produce different crystal habits and sizes due to differences in growth rates, diffusion rates, and surface energy. Most Fe-Mn oxide-hydroxide phases are formed at low temperatures (below 100° C), but several are known to form unde hydrothermal conditions (above 100° C and up to several kbars pressure). Larger crystals are produced from hydrothermal processes as transport is much more rapid, and solubility of the growing phase in the growth medium (solution) is high. The latter growth process can produce crystals up to 10 cm in size. Ice crystals (snowflakes) are the most commonly observed vapor deposition-produced natural phases. These usually show complex dendritic patterns caused by rapid... 

Crystals show enormous variation in external shape or habit, although all these shapes arise from ordered stacking of unit cells. This is illustrated in Figure 15.5, which speaks for itself. Flowever, for a given unit cell, the angles between faces that can exist are fixed. This thus allows determination of the crystal system and of the dimensions of the unit cell from a crystal (if it is perfect and not too small). Figure 15.5 shows that considerable variation in shape can be encountered. Even far more intricate shapes are observed in ice crystals present in snow (which are formed by desublimation of water from the air). The variation in shape is caused by variation in the growth rate of the various faces of a crystal, which rates often depend on the composition of the solution see Section 15.2. 

If the growth of an ice crystal from the vapour were a simple near-equilibrium process, then the resulting crystal habit could be determined by Wulff s theorem (Wulif, 1901) which states that, in equilibrium, the distance of any face from the centre of the crystal... 

Fig. 5.6. Ice crystal growth habit as a function of temperature and of vapour density excess over the equilibrium value for ice at the temperature in question (after Kobayashi, 1958).

Ice crystals can grow in two simple distinct ways either by the freezing of liquid water or by direct sublimation from the vapour phase. In each case the mechanisms which determine the rate and habit of growth are the transport of water molecules to the point of growth and their accommodation into the growing interface, together with the transport of latent heat away from this interface. Many different physical situations can occur, of course, but they are all controlled by these basic mechanisms. 

The surface of ice shows structural transitions, such as surface roughening and surface melting, at temperature, T, below T. The occurrence of structural transitions at the surface of ice causes alterations in the dynamic, mechanical, elastic, and electric properties of the surface. Moreover, anisotropy in structural transitions among several crystallographic plane surfaces of ice is vital for understanding habit changes of snow crystals [9]. Thus, the surface ofice near is an important subject... 

---