Tricritical Casimir forces and order parameter profiles in wetting films of ³He -⁴He mixtures

Tricritical Casimir forces in $^3\text{He}$ -$^4\text{He}$ wetting films are studied, within mean field theory, in therms of a suitable lattice gas model for binary liquid mixtures with short--ranged surface fields. The proposed model takes into account the continuous rotational symmetry O(2) of the superfluid degrees of freedom associated with $^4\text{He}$ and it allows, inter alia, for the occurrence of a vapor phase. As a result, the model facilitates the formation of wetting films, which provides a strengthened theoretical framework to describe available experimental data for tricritical Casimir forces acting in $^3\text{He}$ -$^4\text{He}$ wetting films.