Absence of grain boundary melting in solid helium

Crystals are often expected to start melting at their free surface or at the interface between grains. Grain boundary melting corresponds to the situation where the interface between grains is invaded by a thick liquid film at the bulk melting temperature T-m. In some cases, premelting is predicted, with liquid-like layers appearing between grains at temperatures below T-m. We review this topic, and describe our experiments on solid helium 4. We find that grain boundaries are not wetted by the liquid at T-m: they emerge at the liquid-solid interface with a non-zero contact angle. This is consistent with a general argument which predicts that, although systems with short-range forces might show grain boundary melting and premelting, in systems with long-range forces (like helium), grain boundaries can only be wetted incompletely by the liquid at T-m.