The Physics community lost a major figure in the study of confined matter with the death, on November 28, 2010, of J. Gregory Dash.
Greg obtained his BS from City College, NY, in 1944. After a stint in the US Navy as a radar technician during World War II, he entered graduate school at Columbia University. He earned his PhD in Physics in 1951 with a thesis on super uid helium, a topic in which he maintained interest throughout his professional career. He joined the scientific staff of Los Alamos National Laboratory in 1951. There he took part in carrying out one of the first direct measurements of the heat of mixing of liquid 3He and 4He at low temperatures.
Following a year as Guggenheim Fellow in Cambridge, Greg took a position at the University of Washington in 1960. There he became interested in the properties of systems confined to two dimensions, ones which he hoped to realize by adsorbing gases, such as helium, on smooth surfaces. The principal difficulty lay in obtaining a surface that was sufficiently smooth. Early work utilizing sintered copper substrates showed that no matter how little gas was adsorbed it aggregated into a two-dimensional solid phase, presumably due to the in uence of substrate inhomogeneities. Further work on argon-plated copper yielded similar results. Finally Greg's student, Michael Bretz, utilizing a substrate of exfoliated graphite obtained a helium heat capacity signal which approached Boltzmann's constant at high temperature, the long-desired signal of a two-dimensional ideal gas. A fruitful period of discovery ensued including the observation and interpretation (with two of us, Oscar Vilches and Michael Schick, and our students) of the differences in the behavior of 3He and 4He due to their different statistics, and an intriguing order-disorder transition to a √3 × √3 phase in which the helium is in registry with the substrate. This transition was shown to be in the universality class of the three-state Potts model, and measurements of its heat capacity provided an early test of the idea of universality classes and the prediction, by Marcel den Nijs, of the exact values of the critical exponents of this model. For this work, Greg was awarded the 1985 Davisson-Germer Prize of the American Physical Society.
An outgrowth of the adsorption experiments was a new interest in the manner in which the properties of a two-dimensional system would cross over to those of a three-dimensional one as the thickness of the system increased. It is fair to say that his experiments on the growth of films sparked a revival of interest in both the theory of "wetting" and its experimental realizations. Of particular interest to Greg was the phenomenon of surface melting, the wetting of the solid-vapor interface by its own liquid phase as the triple point is approached. This again sparked great theoretical interest, this time in the way in which the surface properties of crystals, such as their facets, are lost as the temperature of the solid is increased. With one of us (John Wettlaufer), colleagues and students Greg investigated the basic physics and the geophysical consequences of surface layers of water on ice, and related phenomena, such as frost heave. He then carried these results back to helium physics.
In spite of the acquisition of new directions, Greg never lost his interest in physical adsorption with an infectious enthusiasm that drew Rudolph Peierls into the fray in the 70's, and which he maintained through the publication of his last paper. This work, with two colleagues (David Cobden and Oscar Vilches) and their students, described a nanotube balance which utilized the change in the tube's resonant frequency to measure adsorbed mass. Its sensitivity is a few atoms.
Greg was a fine and prolific writer, not only authoring scientific papers, but also a widely used book, Films on Solid Surfaces, and reports for the more general audiences of Scientific American, La Recherche, and Physics Today. He continued to teach, in collaboration with Ernest Henley, at the UW Transition School for gifted high school students until 2009.
He also wrote articles such as "Where responsibility lies" and "Nuclear-free and people-free zones" for the Bulletin of Atomic Scientists as issues of Peace were close to his heart. Greg was the Chair of the Graduate Committee on Confict Studies, on the board of the World Without War Council of Seattle, the Chair of Advisory Committees to both Congressmen Brock Adams and Mike Lowry, and an active participant in the Pugwash Conferences.
With his passing, we have lost a probing and inquisitive physicist, a wise and valued mentor, and a warm and cherished friend. He is survived by his wife of 65 years, Joan, children Michael, Tony and Liz, grandchildren Nate and Sam, and brother Robert.
