Preliminary Measurement of the Critical Casimir Effect near the Tricritical Point in ³He - ⁴He Mixture Films

We report a preliminary measurement of the critical Casimir effect in ³He-⁴He mixture films near the tricritical point. Whereas we had found that the pure ⁴He film adsorbed on Cu substrates thins due to the critical Casimir force near the lambda transition, in our present experiments we find that the adsorbed mixture film thickens near the tricritical point. This difference in behavior most likely reflects the difference in the universality class of these two phase transitions and the difference in boundary conditions that the order parameter must satisfy at liquid-substrate and liquid-vapor interfaces.