Is the surface fluid during surface melting? A study of the surface shear resistance of solid Gallium

Melting of 3D solids is often preceded by a melting of their surface, a distinct process which begins at a temperature lower than T_M (bulk melting temperature). Until now, surface melting was investigated exclusively by non-contact techniques like diffraction, revealing how the surface becomes progressively disordered with temperature. There is however no way of telling whether the surface actually becomes fluid. We designed a method to measure an effective shear resistance of the surface, another property which should change at melting. This property was not measured before. We applied this method to surface melting of Gallium, and found that this surface shear resistance vanishes abruptly near the onset of premelting, about 9K below T_M.