Absence of grain boundary melting in solid helium

Abstract

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