Wetting Properties of Grain Boundaries in Solid Helium 4

TL;DR

This study investigates the wetting behavior of grain boundaries in solid helium-4, revealing partial wetting and the formation of liquid channels, which has implications for understanding supersolidity.

Contribution

It provides experimental measurements of grain boundary wetting angles and liquid channels in solid helium-4, supporting recent simulation predictions and advancing understanding of grain boundary properties.

Findings

Grain boundaries form grooves with non-zero dihedral angles.

Some grain boundaries are not fully wetted by liquid helium.

Liquid channels can form along solid walls with widths matching simple model predictions.

Abstract

We have observed boundaries between hcp $^4$He crystal grains in equilibrium with liquid $^4$He. We have found that, when emerging at the liquid-solid interface, a grain boundary makes a groove whose dihedral angle $2\theta$ is non-zero. 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 $\theta$, 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 $^4$He.