Laboratory of Statistical Physics
Dr. Cohen develops equations to describe the properties of microscopic, mesoscopic, and macroscopicparticle systems on their characteristic timescales. He is particularly interested in systems in nonequilibriumsteady states. His research led to the application of a new measure of the probability such systems arein a particular state, using recent developments in the field of dynamical systems. Thus, connecting two previously separate fields led him to new predictions, including theorems for the behavior of fluctuations in these systems, which have since been confirmed experimentally.
Earlier, he derived expressions for the transport coefficients — such as for diffusion and viscous friction — of dense fluids and concentrated colloidal suspensions. He accomplished this using a molecular model of hard spheres for which explicit expression for these coefficients can be obtained. These have been successfully adapted to apply to micelles, microemulsions, and globular proteins.
Dr. Cohen’s lab is also interested in the motion of a point particle over a triangular, a square lattice, and recently, a honeycomb lattice in the plane. When the particle jumps from one lattice site to a nearest neighbor site and meets a scatterer placed on that lattice site, the direction of the particle’s motion changes according to a deterministic scattering rule. For a given initial state of the particle and the scatterers, the particle’s motion suddenly changes from a diffusive-like motion to a self-organized propagating motion, which is restricted to a finite strip on the lattice. On the honeycomb lattice, the particle creates intricate reflecting structures, leading to a complicated motion, but ending in a closed orbit.
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. Later verified experimentally, his prediction led to the design of the helium dilution refrigerator, still the main very low temperature cooling system available.
He also studied a very old but still-unsolved problem ubiquitous in fields from geology to biology: that of friction-induced stick-slip motion of 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, as found in earthquakes, as well as the inner ear.
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 John Hopkins University and 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 was 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.