Boundary conditions and critical Casimir forces in helium

Abstract If a fluid near its critical point is confined between two interfaces, the long-ranged critical fluctuations in the order parameter will mediate a force. This force, known as the critical Casimir force, is a direct analog of the Casimir force in electromagnetism. Dielectric constant measurements of helium films adsorbed on Cu electrodes provide evidence for the existence of the critical Casimir force near the superfluid transition in 4 He and near the tricritical point in 3 He – 4 He mixtures. In pure 4 He , we find the force is attractive but near the tricritical point the force appears repulsive, a change due to the extraordinary boundary conditions at the tricritical point.