phenomena A Chemical Engineer’s Assessment
ymposia focused on specific phenomena are usually
S associated withWith the “pure” scientist and seldom the decreasing time span be-
with the engineer. tween conception and reduction to practice, however, the engineer’s need for such symposia has become great. This year’s Nucleation Symposium, like its predecessor on the Physics and Chemistry of Interfaces, was organized to appeal to both the scientist and the engineer. Despite the diversity in backgrounds and interests represented by the lecturers, the symposium seemed to stimulate a positive response from nearly all the attendees. As chairman, I had apprehensively planned forjust such a response. The unifying theme was nucleation in its classical sense-the spontaneous formation of fragments of a new phase within a preexisting phase. The ways, however, in which this theme was developed and manipulated to deal with and analyze processes such as condensation, atmospheric cloud formation, melt solidification, boiling, crystallization in polymers, devitrification of glasses, and recrystallization of metals, are a tribute to the versatility of the human intellect and a sobering challenge to the traditionally trained chemist and chemical engineer. Three surprising facts emerged from the symposium. First, the basic theory of nucleation and its kinetic analysis has undergone precious little modification and revision in the past 50 years. Despite the simplicity of the assumptions and the dearth of data or sophisticated experimental techniques of those early days, the relationships Gibbs and Volmer derived are surprisingly accurate and useful and subject to little more than second-order corrections deriving from elegant present-day experimental methods. That these simple relationships are useful for predicting the kinetics of widely diverse nucleation processes should encourage the practicing engineer to make more effective use of them in his workaday operations. Second, it is clear that the problem of predicting absolute nucleation kinetic parameters must await resolution until our knowledge of the fine structure of
matter and of molecular and atomic motion is in better shape. As was pointed out many times during the symposium, however, it is fortunate that a knowledge of these parameters is not essential to the successful application of nucleation theory to the prediction and control of important practical phase transformation problems. Third, a little-recognized and generally poorly understood phenomenon in phase transformation processesso-called spinodal decomposition-received considerable attention at the symposium, both from the speakers and the audience. The phenomenon involves the spontaneous, nonactivated unmixing of homogeneous multicomponent phases into zones of slightly differing composition without well defined phase boundaries. While the mechanism of the transformation and the conditions under which it is likely to occur instead of nucleation remain poorly defined for all but a few metallic and ceramic systems, it would appear that such a process may play an important part in governing the microcrystalline texture and phase compositions in many inorganic materials. Industrial chemists and chemical engineers should, in my opinion, keep a close eye on progress now being made in this field, for it would appear that spinodal decomposition may play an ‘mportant role in multicomponent distillation near the critical condition, in recrystallization processes in polymers, and in impurity removal during fractional crystallization of organic and inorganic compounds. I t also seems that much remains to be done with the semantics and nomenclature of nucleation. The publication of the proceedings of the symposium in this journal may help to improve this situation. We are confident that you will find the products of this symposium interesting, unusual, and provocative. We can only hope that, in addition, you will find them useful in areas of practical importance.
ALAN S. MICHAELS President, Amicon Corporation V O L . 57
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The reaction b e t w e e n a nucleus and its environment is represented by the radial energy and concentration fields surrounding the nucleus
