Wetting Properties of Grain Boundaries in Solid ⁴He

We have observed boundaries between hcp ⁴He crystal grains in equilibrium with liquid ⁴He. We have found that, when emerging at the liquid-solid interface, a grain boundary makes a groove whose dihedral angle 2θ is nonzero. This measurement shows that grain boundaries are not completely wet by the liquid phase, in agreement with recent Monte Carlo simulations. Depending on the value of θ, the contact line of a grain boundary with a solid wall may be wet by the liquid. In this case, the line is a thin channel with a curved triangular cross section, whose measured width agrees with predictions from a simple model. We discuss these measurements in the context of grain boundary premelting and for a future understanding of the possible supersolidity of solid ⁴He.