Laboratory of Statistical Physics
Dr. Cohen is interested in theoretical descriptions of properties of microscopic, mesoscopic and macroscopic particle systems on characteristic time scales and develops equations to describe them. This allows him to understand their possibilities as well as their limitations, especially on small time scales. Dr. Cohen is currently interested in systems in nonequilibrium steady states. His research led to assigning a new probability measure for these systems to be in a certain state, using recent mathematical developments in the field of dynamical systems. Thus, connecting two previously separate fields led him to new predictions, for example, a theorem for fluctuations in the above-mentioned systems, which have since been confirmed experimentally.
Earlier, he derived expressions for the transport coefficients — such as the diffusion and viscous friction coefficients — of dense fluids and concentrated colloidal suspensions. This was accomplished by using a molecular model of hard spheres, for which explicit expressions for these coefficients can be obtained. These have been successfully adapted to apply to micelles, microemulsions and globular proteins.
Another interest of Dr. Cohen’s laboratory is the motion of a point particle over a triangular or square lattice in the plane, which jumps from one lattice site to a nearest neighbor site, by randomly placing obstacles on all lattice sites, which change the direction of the particle’s motion, according to a deterministic rule. This mixture of random and deterministic features has led to new types of particle motion where, after a characteristic time, the particle’s motion suddenly changes from a diffusive-like motion to a regular propagating motion restricted to a finite strip on the lattice. The precise mechanism of this behavior is not well understood.
In 1961, Dr. Cohen predicted theoretically the possibility of an incomplete phase separation in liquid mixtures of the two helium isotopes at very low temperatures. This was later verified experimentally, leading to the design of the helium dilution refrigerator, still the main very low temperature producing cooling system available. Recently, he has studied a very old but still unsolved problem, which is ubiquitous in fields from geology to biology: that of the friction between two solids moving over each other. Having solved an equation proposed by French physicist Pierre-Gilles de Gennes for a simple model, Dr. Cohen was able to obtain the observed stick-slip mode of motion.
Originally from the Netherlands, Dr. Cohen received his Ph.D. at the University of Amsterdam in 1957. He was a research associate for a year at the University of Michigan and The Johns Hopkins University and an associate professor at the Institute for Theoretical Physics at the University of Amsterdam before coming to The Rockefeller University as a professor in 1963.
In 1961, he founded an international summer school in statistical mechanics in the Netherlands and is an editor of the series Fundamental Problems in Statistical Mechanics, which contains the lectures given at this school and gives a history of the field over 50 years. In 2004, Dr. Cohen received the triannual Boltzmann Medal from the Union of Pure and Applied Physics, its highest award for contributions to statistical mechanics, and he was honored with a Royal Knighthood in the Order of the Dutch Lion.