Critical Casimir Effect and Wetting by Helium Mixtures

We have measured the contact angle of the interface of phase-separated ${}^3\mathrm{H}\mathrm{e}$-${}^4\mathrm{H}\mathrm{e}$ mixtures against a sapphire window. We have found that this angle is finite and does not tend to zero when the temperature approaches $T_t$, the temperature of the tricritical point. On the contrary, it increases with temperature. This behavior is a remarkable exception to what is generally observed near critical points, i.e., “critical point wetting.” We propose that it is a consequence of the “critical Casimir effect” which leads to an effective attraction of the ${}^3\mathrm{H}\mathrm{e}$-${}^4\mathrm{H}\mathrm{e}$ interface by the sapphire near $T_t$.