Preliminary measurement of the critical Casimir effect near the tricritical point in He-3-He-4 mixture films

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