John W. Cahn
John Werner Cahn (January 9, 1928 – March 14, 2016) was an American scientist and recipient of the 1998 National Medal of Science. Born in Cologne, Weimar Germany, he was a professor in the department of metallurgy at the Massachusetts Institute of Technology (MIT) from 1964 to 1978. From 1977, he held a position at the National Institute of Standards and Technology (formerly the National Bureau of Standards). Cahn had a profound influence on the course of materials research during his career. One of foremost authorities on thermodynamics, Cahn applied the basic laws of thermodynamics to describe and predict a wide range of physical phenomena.
In 1933, Adolf Hitler became Chancellor of Germany, and the elder Cahn escaped arrest only because he had been forewarned by a fellow lawyer. The family fled Germany and eventually ended up in Amsterdam. They emigrated to America in 1939, where Hans became John. Most of his family back in Europe perished in the Holocaust.
Cahn received a bachelor's degree in chemistry in 1949 from the University of Michigan. He later earned a Ph.D in physical chemistry in 1953 from the University of California at Berkeley. His doctoral thesis was titled "The Oxidation of Isotopically Labelled Hydrazine" and his thesis advisor was R. E. Powell.
In 1954, Cahn joined the chemical metallurgy research effort at the General Electric laboratory in Schenectady, New York, led by David Turnbull. Turnbull had done pioneering work on the kinetics of nucleation, and there was a focus in the group on understanding the thermodynamics and kinetics of phase transformations in solids.
In 1964, Cahn became a professor in the Department of Metallurgy (now Materials Science) at the Massachusetts Institute of Technology. He left MIT in 1978. In 1969, Cahn began a long professional relationship with his graduate student, Francis Larché, whose work focussed on the effect of mechanical stress on the thermodynamics of solids. The Larche–Cahn approach is the cornerstone of the treatment of the thermodynamics of stressed materials. Good examples of this phenomenon are the regions near a coherent precipitate or the stress field around a dislocation.
In 1972, Cahn worked with David W. Hoffman to formulate vector-based thermodynamics to describe the thermodynamics of interfaces, a formulation which is necessary to account for anisotropic materials. This is also known as the capillary vector formulation of interface energies. The mathematics of this treatment involves the concept of norms, although Cahn and Hoffman were unaware of it at the time.
From 1984, he was an affiliate professor at the University of Washington.
In 1957, Cahn worked with John E. Hilliard to develop the Cahn–Hilliard equation which describes the thermodynamic forces driving phase separation in many systems. Their joint theory of spinodal decomposition is of interest for two primary reasons. First, it is one of the few solid-state transformations for which there is any plausible quantitative theory. The reason for this is the inherent simplicity of the reaction. Since there is no thermodynamic barrier to the reaction inside of the spinodal region, the decomposition is determined solely by diffusion. Thus, it can be treated purely as a diffusional problem, and many of the characteristics of the decomposition can be described by an approximate analytical solution to the general diffusion equation. In contrast, theories of nucleation and growth have to invoke the thermodynamics of fluctuations. And the diffusional problem involved in the growth of the nucleus is far more difficult to solve, because it is unrealistic to linearize the diffusion equation. From a more practical standpoint, spinodal decomposition provides a means of producing a very finely dispersed microstructure that can significantly enhance the physical properties of the material.  Spindoal decomposition has famously been utilized in the Vycor process to produce low thermal expansion glasses for high temperature applications.
In the theory of crystal growth, Cahn concluded that the distinguishing feature is the ability of the surface to reach an equilibrium state in the presence of a thermodynamic driving force (typically in the form of the degree of undercooling). He also concluded that for every surface or interface in a crystalline medium, there exists a critical driving force, which, if exceeded, will enable the surface or interface to advance normal to itself, and, if not exceeded, will require the lateral growth mechanism.
Thus, for sufficiently large driving forces, the interface can move uniformly without the benefit of either a heterogeneous nucleation or screw dislocation mechanism. What constitutes a sufficiently large driving force depends upon the diffuseness of the interface, so that for extremely diffuse interfaces, this critical driving force will be so small that any measurable driving force will exceed it. Alternatively, for sharp interfaces, the critical driving force will be very large, and most growth will occur by the lateral step mechanism. 
Droplets and surfaces
In 1977, Cahn published a simple mathematical treatment of the thermodynamics of wetting: the interaction between a liquid in contact with a solid surface. This paper laid out a simple formulation for describing the wetting transition—the point at which a liquid changes from forming a droplet on a surface to spreading out evenly as a liquid film over the surface. This theory had wide-ranging implications for many materials processing techniques.
In 1982, Dan Shechtman observed a new crystalline structure with puzzling features. Cahn contributed to the theory of how such a structure could be thermodynamically stable and became co-author of the seminal paper which introduced quasicrystals.
In 2004, Cahn and Bendersky presented evidence that an isotropic non-crystalline metallic phase (dubbed "q-glass") could be grown from the melt. This phase is the first phase, or "primary phase," to form in the Al-Fe-Si system during rapid cooling. Experimental evidence indicates that this phase forms by a first-order transition. TEM images show that the q-glass nucleates from the melt as discrete particles, which grow spherically with a uniform growth rate in all directions. The diffraction pattern shows it to be an isotropic glassy phase. Yet there is a nucleation barrier, which implies an interfacial discontinuity (or internal surface) between the glass and the melt.
Research in retirement
In his retirement, Cahn accepted a position at the University of Washington as an affiliate professor in the Departments of Materials Science and Engineering and Physics. In his office in the new Physics/Astronomy Tower, Cahn was working on a project that includes a glass that grows from a melt like a crystal – as if by a first-order transition.
Honors and awards
2011 The Kyoto Prize, Inamori Foundation
2002 Bower Prize, Franklin Institute
2001 Emil Heyn Medal, German Metallurgical Society
2001 Honorary Life Member, American Ceramic Society
1999 Bakhuys Roozeboon Lecturer and Gold medal, Netherlands Academy of Sciences
1998 National Medal of Science
1998 Member, National Academy of Engineering
1998 Distinguished GE Lecturer in Materials Science at RPI
'69 & `98 MacDonald Lecturer, Canadian Metallurgical Society
1996 Doctor Honoris Causis, Universite d'Évry, France
1994 Rockwell Medal; Hall of Fame for Engineering, Science and Technology, and Medal, International Technology Institute.
1994 Gold Medal, Honorary Member, Japan Institute of Metals.
1993 Inland Steel Lecture, Northwestern University.
1993 Hume–Rothery Award, TMS.
1993 Cyril Stanley Smith Lecturer, University of Chicago.
1992 Honorary member, MRS-India.
1991 Michelson and Morley Prize, Case Western University.
1990 Honorary Sc. D., Northwestern University; Hilliard Lecturer.
1989 Sauveur Award, ASM International.
1987 Distinguished Physics Lecturer, Boston University.
1986 Stratton Award, National Bureau of Standards.
1985 Von Hippel Award, Materials Research Society.
1984 Gold Medal, US Department of Commerce.
1983 Distinguished Lecturer, University of Connecticut.
1982 Golick Lecturer, University of Missouri, Rolla, MO.
1981 Fellow, Japan Society for the Promotion of Science.
1981 Dickson Prize, Carnegie–Mellon University.
1980 Honorary Professor, Jiao Tong University, Shanghai, China.
1979 Van Horn Lecturer, Case-Western University.
1978 Dorn Lecturer, Northwestern University.
1977 Acta Metallurgica Gold Medal.
1974 Fellow, American Academy of Arts and Sciences.
1973 Member, National Academy of Sciences.
1968 Institute of Metals Lecturer, AIME.
1966 S. B. Meyer Award, American Ceramic Society.
1960–61 Guggenheim Fellowship spent at the University of Cambridge, Goldsmith Laboratory.
1951 Allied Chemical and Dye Fellowship at University of California, Berkeley.
- "The Selected Works of John W. Cahn" (PDF).
- "John W. Cahn".
- "John W. Cahn: Foremost metallurgist fled Nazi Germany". The Seattle Times. 15 March 2016.
- Cahn, J,W. and Hilliard, J.E., Free Energy of a Nonuniform System. I. Interfacial Free Energy, J. Chem. Phys., Vol. 28, p. 258 (1958)
- Cahn, J.W., Spinodal Decomposition, 1967 Institute of Metals Lecture, Trans. Met. Soc. ASME, Vol. 242, p. 168 (1968)
- Hilliard, J.E., Spinodal Decomposition, in Phase Transformations p. 497 (American Society of Metals, Metals Park, 1970)
- Cahn, J.W., On spinodal decomposition in cubic crystals, Acta Met., Vol. 10, p. 179 (1962)
- Hilliard, J.E. and Cahn, J.W., On the Nature of the Interface Between a Solid Metal and Its Melt, Acta Met., Vol. 6, p. 772 (1958)
- Cahn, J.W., Theory of crystal growth and interface motion in crystalline materials, Acta Met, Vol. 8, p. 554 (1960)
- Cahn, J.W., Coherent fluctuations and nucleation in isotropic solids, Acta Met., Vol. 10, p. 907 (1962)
- Cahn, J.W., Hillig, W.B., Sears, G.W., The molecular mechanism of solidification, Acta Met., Vol. 12, p. 1421 (1964)
- Cahn, J.W.; Bendersky, L.A. "Formation of Glass by a First Order Transition". Metallurgy Division Publications – NISTIR 7127. Retrieved 2009-06-06.
- "The President's National Medal of Science: Recipient Details - NSF - National Science Foundation".