Development of an evidence-based model of ski–snow friction

Gliding on snow using skis has a history going back thousands of years, ever then it is facilitating the transport of goods and enabling faster mobility. The basis for this is the remarkably low friction in the ski-snow interface. Nonetheless, friction changes drastically with temperature and speed. Many theories on skisnow friction ultimately revolve around the availability of liquid water, which is acting as lubricant between ski and snow. The friction coefficient is thus often illustrated in dependence of the thickness of a water layer: no or only a thin water layer results in high friction; with increasing water layer, friction comes to a minimum; with further increasing water layer thickness, friction rises again. Yet, this skisnow friction curve has never been verified experimentally, which is strongly correlated with the lack of devices capable to detect the presumed water layer. Over the years, several studies could disprove the friction curve for low gliding speeds, though. In this project, we aim to provide an experiment- based skisnow friction curve in dependence of temperature and ski velocity using novel or improved detection techniques. This should enable us to prove or disprove various mechanisms thought to play a role in ski-snow friction. We will conduct ski friction experiments at speeds up to 90 km/h and ambient temperatures down to -20C on a 24 m long linear friction tester (tribometer). Surface parameters of ski and snow will be determined with various instruments. To detect the formation of liquid water we will use different approaches: temperature sensor inside the ski base, infrared imaging of the snow surface, humidity indicator chemicals and capacitive sensors. In dedicated experiments, we will use dry ice to cool down the snow surface below -30C. This way we are minimizing the so-called quasi-liquid water layer (a layer of water a few molecules thick on the grain surface, present at temperatures down to -30C), which is thought to have an impact on ski gliding.