Casimir Effect Found in Many Systems

Moses Chan and Rafael Garcia of Penn State University have published an article describing their recent measurements of the Casimir effect observed in thin films of 3He-4He mixtures. According to the authors, that the same mathematical formulation can be used to describe phenomena in widely different systems is one of the most beautiful aspects of physics. H. B. G. Casimir first reported these effects in 1948 in measurements of the confinement of zero-point electromagnetic fluctuations between two plates in a vacuum. The critical Casimir effect has been observed more recently in thin films of pure helium absorbed on copper surfaces, and in binary liquid films. The tricritcal point of 3He-4He mixtures provides yet another region where the critical fluctuations are limited by the size of the measuring apparatus, so one expects a Casimir force to appear.

In the phase diagram of 3He-4He mixtures, the tricritical point is the point where the line of superfluid transitions terminates at the top of the coexistence region. As this point is approached, the system is expected to exhibit an attractive Casimir force for symmetric order parameter boundary conditions, but the force will be repulsive if nonsymmetric boundary conditions prevail. The measurements reported in this paper find that the Casimir force in this system is a repulsive force.

The details can be found in the authors' paper "Critical Casimir Effect near the 3He-4He Tricritical Point," Physical Review Letters 88, 086101 (2002).