Sliding on ice: Real contact area, melted film thickness, and friction force

•We experimentally measure the contact area and friction force arising from high-speed sliding of quartz on ice, with the relative velocity ranging from 1 to 10 m/s.•Contact area of ice and quartz is visualized using an optical setup based on the total internal reflection.•The friction force is experimentally shown to be nearly proportional to the contact area if the film thickness changes little.•Scaling laws are constructed to predict the temporal evolutions of the contact area, the friction force and the melted film thickness. It is easy to slide on ice because of water films arising as a consequence of frictional melting. Although the friction force of ice and a slider critically depends on the area and thickness of the liquid film as well as the sliding speed, direct experimental visualization and quantification of temporal evolutions of the contact area, film thickness, and the resulting friction force have been scarce to date. Here we develop an experimental technique to visualize the contact area of ice asperities and a high-speed sliding surface in situ based on the optical principle of total internal reflection. We construct a hydrodynamic model to predict the contact area, liquid film thickness and friction force of a model system of hemispherical ice on a flat solid surface. Upon showing good agreement between theory and experiment, we briefly discuss how the fundamental understanding of the friction behavior of a single ice bump can be extended to understand the friction behavior of flat ice surfaces.