ABSTRACT
The habit or morphology of crystals plays a very important role in crystallization processes. Often crystals grow faceted with flat, well-defined faces, but in many cases some of the faces appear as rounded-off roughened faces. In these cases the growth velocity of such faces is much higher than the flat ones. In extreme cases even dendritic, or cellular, growth occurs. An intermediate case is the one for which spherulitic crystals grow. This case is often encountered for fat crystals. The crystals then appear as spherulites made up of many faceted crystals growing out of an ill-defined nucleus. In this chapter we analyze the morphology of the individual faceted crystals of β-2 crystals of fats. Although we treat the case of faceted crystals, it will turn out that the morphology of these crystals, to a large extent, is determined by the tendency of some of their faces to become rough easily. Closely connected to this tendency is the roughening temperature of flat face above which they appear as rounded-off. This effect is known as thermal roughening. The growth velocity of these faces becomes high below this critical temperature. Besides, as a result of its roughening temperature a flat face can become rough beyond a critical supersaturation, known as kinetical roughening, resulting in comparable high growth velocities. These roughening effects can
explain the elongated morphology of fat crystals, as explained in this chapter, and, in particular, the morphology of the top facets of fat crystals of the β-2 structure will be explained on the basis of the roughening theory of crystal faces.
