After 80 Years, New Look at Snow Friction

The mechanics of snow friction are central to competitive skiing, safe winter driving, efficient Polar sleds and the dynamics of avalanches. For nearly 80 years, prevailing theory has postulated self-lubrication: dry-contact sliding warms snow-grains to the melting point, and further sliding produces melt-water layers that lubricate the interface. We recently published micro-scale interface observations that contradicted this explanation: contacting snow grains abraded and did not melt under a polyethylene slider, despite low friction values. Here we summarize the key observations and theoretical evidence that abrasion can govern snow kinetic friction. We obtained coordinated infrared, visible-light, and scanning-electron micrographs that confirm that the evolving shapes observed during our tribometer tests are contacting snow grains polished by abrasion, and that the wear particles can sinter together and fill the adjacent pore spaces (Fig. 1). Furthermore, dry-contact abrasive wear reasonably predicts the evolution of snow-slider contact area, and sliding-heat-source theory confirms that contact temperatures would not reach 0°C during our tribometer tests. Importantly, existing field measurements of sled and ski interface temperatures also indicate that melting did not occur. Although prevailing theory anticipates a transition from dry to lubricated contact along a slider, our results suggest that dry-contact abrasion and heat flow can prevent this transition from occurring for snow-friction scenarios of practical interest.

Fig.1 Tribometer-evolved snow-slider contacts in infrared (a - d) and coordinated post-test IR, optical and SEM images showing the same merged contact features (circled).