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 4He and near the tricritical point in 3He-4He mixtures. In pure 4He, 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. © 2003 Elsevier Science B.V. All rights reserved.
Garcia, R., & Chan, M. H. W. (2003). Boundary conditions and critical Casimir forces in helium. In Physica B: Condensed Matter (Vol. 329–333, pp. 238–241). https://doi.org/10.1016/S0921-4526(02)01975-0