Fluctuation spectroscopy of surface melting of ice without, and with impurities

Abstract: Water, in its three phases, is ubiquitous, and the surface properties of ice is important to clarifying the process of melting, as well as to various other fields, including geophysics. As such, the subject has been studied both theoretically and experimentally, for over a hundred years, while being an active field of research today. It has been established that surface melting, or premelting, exists below the melting point, and a `liquid-like layer' (LLL) exists on the surface of ice. Here, we use the surface thermal fluctuation spectra to study the properties of LLL, including its thickness, for pure ice, and for ice with impurities. We find that the properties of LLL are consistent with those of bulk liquid water, and for layers thicker than 10\,nm, their properties are experimentally indistinguishable from those of liquid water. Measured thicknesses are found to be much smaller than the previous experimental measurements close to the bulk melting temperature. We find that the additions of impurities at ppm levels cause LLL to be thicker, as well to be quite inhomogeneous, with properties depending on the dopant. This is revealed by scanning the surface at $\mu$m level resolution, and can contribute to the slipperiness of ice in natural settings.