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

Abstract

Melting of 3D solids is often preceded by a premelting of their surface, a distinct process which begins at a temperature lower than TM (bulk melting temperature). Premelting is usually investigated by non-contact techniques like diffraction or elipsometry, revealing how the surface layer becomes progressively disordered with temperature. Although this disordered surface layer is called a “quasiliquid”, in most cases (except for ice) there is no way of telling whether it is a fluid or a disordered solid. One significant difference between these two states is in their resistance to shear, which should decrease if the surface becomes fluid. To address this question, we developed a mechanical method to measure the effective shear resistance of a metal surface. This property was not measured before. We applied this method to the surface of polycrystalline Gallium, and found that the surface shear resistance vanishes abruptly near the onset of premelting, about 9K below TM. This result confirms that the quasiliquid on the surface of Ga is a fluid.