British Association For Crystal Growth Annual Conference 2017
Secondary Nucleation by Crystal Collisions
Ronald W, Rousseau
Georgia Institute of Technology, USA
[email protected]
Quantifying the role of secondary nucleation due to collisions of crystals was a focus of research in the late
1960s and early 1970s. Early experiments by Strickland-Constable and co-workers (1, 2) demonstrated
essential features of what was called contact nucleation or collision breeding. McCabe and Clontz (3) further
explored the phenomenon by quantifying the effects of collision energy on the number of crystals resulting
from impacts of a metal rod with a growing MgSO4∙7H2O seed crystal. The work representing the beginning
of the present author’s work on crystallization used the McCabe and Clontz (3) as a foundation and refined
the results on MgSO4∙7H2O; additionally, three more water-soluble solutes were examined: potash alum,
potassium sulfate, and citric acid (4). The apparatus used in the research is shown in Figure1, which
illustrates how the contacting device in Figure 2 was positioned in a flowing supersaturated solution.
Cooling tubes
Heating tubes
Rotameter
Solution at Constant T
Dropped weight
Contacted crystal
Rod
Thermister
Fig. 1. Flow System
Receiver
Fig. 2. Crystal Contacting Device
Experiments described in these studies (4) were all performed at supersaturations where primary nucleation
was not observed: i.e., in metastable zones. Nucleation occurred only when there were collisions of the
metal contacting rod with seed crystals, and the number of crystals produced from such collisions depended
on collision energy and prevailing supersaturation (shown in Figure 3 for MgSO4·7H2O).
Fig. 3. MgSO4·7H2O crystals formed as a function of impact energy (left) and supersaturation (right).
British Association For Crystal Growth Annual Conference 2017
Later work (5) demonstrated that both the supersaturation at the point of crystal collision and in the
subsequent environment contributed to determining the number of crystals resulting from a collision. In
comparing outcomes for the crystal species examined, it was postulated that hardness of the crystal surface
influenced the productivity of impacts with seed crystals. Potassium sulfate, which was estimated to have a
hardness approximately twice that of the other crystals, exhibited a threshold contact energy that had to be
exceeded before collisions resulted in the formation of new crystals.
The results from this research and subsequent investigations in other laboratories confirmed the role of
crystal collisions in secondary nucleation. There was at the time vigorous discussion as to the source of
nuclei resulting from such collisions, and various arguments were constructed in support of there simply
being fragments of the seed crystal dislodged by energetic impacts. Other opinions postulated that the
impacts disrupted and scattered embryos from a semi-ordered region surrounding a growing crystal.
Substantial research also noted and studied the varying growth rates observed among crystals resulting from
collisions of the type noted here.
References:
[1] R.E.A. Mason and R.F. Strickland-Constable, Transactions of the Royal Society, 1965, 62, 455.
[2] D. P. Lal, R.E.A. Mason and R.F. Strickland-Constable, J. Crystal Growth, 1969, 5, 1.
[3] N.S. Clontz and W.L. McCabe, Chemical Engineering Progress Symposium Series No. 110, 1971, 67, 6.
[4] C.Y. Tai, W.L. McCabe and R.W. Rousseau, AIChE Journal, 1975, 21, 351.
[5] R.W. Rousseau, W.L. McCabe and C.Y. Tai, AIChE Journal, 1975, 21, 1017.