Cohen Lab Research

Nonequilibrium van der Waals Forces (with Shaul Mukamel, U. C. Irvine)

Van der Waals forces have been studied in detail for bodies in thermal equilibrium. Most of the world is not in thermal equilibrium, and van der Waals forces in this regime are very different from their equilibrium cousins.  We developed a Liouville space formalism for addressing this ituation.  In contrast to equilibrium forces, nonequilibrium forces are much stronger and may show chemical specificity. There is a friction associated with the van der Waals force between bodies in relative motion. When the bodies are at different temperatures or one body is excited, this friction may be negative. Intermolecular forces with one molecule excited are far stronger than ground-state forces and may be attractive or repulsive. This fact implies that there is a mechanical force that accompanies Fluorescence Resonance Energy Transfer (FRET). In fact, any optical effect in matter modifies the forces between the constituent molecules.

This research led to a broader question: given two quantum systems with unknown properties, is it possible to perform a set of measurements on the individual systems from which one can predict the outcome of a measurement when the two systems interact with each other? The answer to this question is yes, provided one allows a sufficiently broad class of measurements on the individual systems. I developed a simple procedure for calculating the properties of a coupled quantum system in terms of properties of its constituent parts.

The figure shows the van der Waals interaction energy of two simple harmonic oscillators, as a function of their relative frequencies, w, and relative inverse temperatures, b.  The red line indicates the scenario where the temperatures are the same, in which case there are no special features on resonance (i.e. when the two frequencies are equal). However, if the oscillators are coupled to baths of different temperatures, then the force shows strong spikes on resonance.




  • Slip and Slide, Scientific American, p. 30, Jan. 2004
  • Excited electrons cut the friction, New Scientist, p. 16, 15 Nov. 2003.
©2007 Adam E. Cohen