Daniel Chonghan Hong, a professor of physics at Lehigh University and the originator of the diffusing void model of granular flow, died of cardiac arrest, after a period of hospitalization, on 6 July 2002 in Hershey, Pennsylvania.

Dan was born in Seoul, South Korea, on 3 March 1956. He earned his BS (1979) and MS (1981) degrees in physics from Seoul National University and his PhD in theoretical condensed matter physics from Boston University in 1985. He did postdoctoral work at the Institute for Theoretical Physics (ITP; now the Kavli Institute for Theoretical Physics) at the University of California, Santa Barbara (UCSB), for the next three years and at Emory University from 1987 to 1988. He then joined the faculty at Lehigh.

As a graduate student, Dan worked with H. Eugene Stanley on fractal properties of the percolation backbone, random walks on percolation clusters, and transport in random composites. At the ITP, he turned his attention to pattern formation, which was then just beginning to attract wide interest among statistical physicists. With James Langer and Angelo Barbieri, Dan worked out the theory of microscopic solvability, showing that the Stefan model of solidification does not allow steady-state “needle crystal” solutions unless the surface tension in the model is anisotropic. Dan subsequently investigated several ramifications of microscopic solvability, especially for the Saffman–Taylor problem.

While working on both pattern formation and polymer physics at Lehigh, Dan added granular flow to his expanding list of interests in nonlinear dynamics. He approached the topic from a dizzying range of perspectives: discrete computer simulations, static stress analyses, thermodynamic analogues, Langevin-, Boltzmann-, and Enskog-equation approaches, and continuum hydrodynamic formulations. His very first effort in this field was noteworthy: With Hugo Caram, he formulated a simple granular-flow model that focused on the diffusion of voids in the assembly of particles. They published their findings in Physical Review Letters in 1991. The diffusing void model is now recognized as an effective theoretical model for treating a broad range of dynamical phenomena in granular media. In his office, Dan prominently displayed a Lucite® chamber containing internal barriers and plastic spheres; the chamber demonstrated some of the model’s first predictions.

Dan’s range of interests, and his worldwide circle of collaborators and students, continued growing vigorously until his final months. He branched out into fracture and stick–slip failure, traffic flow, and a variety of pattern-formation problems, including surfactant effects in viscous fingering, ripple marks, and washboard roads. His final work, with Stefan Luding (then with the Institute for Computer Applications in Stuttgart, Germany) and Paul Quinn (then at Lehigh), on the behavior of a vibrated bed of dissimilar grains predicted the conditions under which larger grains may sink. This “reverse Brazil nut problem” remains a focus of active investigation worldwide. Dan was pleasantly surprised—and non-plussed—when his paper, with Joseph Both of Lehigh, on controlling the size of popcorn attracted international media attention before its publication in Physica A in 2000.

In addition to his technical publications, Dan wrote popular articles on science, philosophy, and religion for the Roman Catholic monthly Dulsum Nalsum and several Korean-American publications. He edited the AKPA Newsletter for the Association of Korean Physicists in America from 1995 to 1997 and from 1999 to 2000.

Dan’s intellectual curiosity and discipline extended beyond physics. A born-again Christian, he had studied Greek to gain a better understanding of the Bible and was planning to study Hebrew as well.

Dan was an innovative and thorough researcher who related his work to everyday phenomena that other people could understand. He had an unusual ability to catch the simple physics behind a complex phenomenon. He was the source of a ceaseless stream of new ideas that few could keep up with, and he pursued those ideas with an energy and intensity that few could match. He had a keen sense of humor and infectious cheerfulness. We remember him as an invaluable colleague and a wonderful friend.

Daniel Chonghan Hong