bradley paul lipovsky
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Why does one slip and fall on ice? The immediate conclusion from such memorable encounters is that ice has low friction. But why is the ice slippery? Despite the familiarity of this commonplace experience, the micromechanical basis for ice friction remains an active area of investigation today. I am interested in the friction of ice in two capacities: as a physicist I want to understand the micron and nanometer-scale physics of ice; as an Earth scientist, I am interested in how friction acts in various Earth systems.

Ice friction acts to dissipate energy in a diverse collection of Earth systems. Basal friction is a fundamental component of glacier and ice sheet dynamics where it acts to dissipate the gravitational forces that drive ice flow. I've explored how modern, laboratory-derived friction laws explain stick-slip motion in the West Antarctic Ice Sheet. I've also used the same friction laws to investigate the mechanics of glacial earthquakes that originate at the ice-bed interface of fast-flowing ice.

Glacier stick-slup motion on the Whillans Ice Plain, West Antarctica

Episodic ice motion is fundamentally related to the frictional properties of ice.

Ice friction is also a topic of study that is interesting in and of itself. A long standing debate even surrounds the very origin of the slipperiness of ice. Several processes come to mind. First, a simple argument due to Bowden and Hughes [1838] argues that pressure-melting shouldn't be very relevant. A more modern line of argumentation relates ice's low friction coefficient to the formation of nanometer-scale water films that act to lubricate the sliding interface. A third process implicated in ice friction is the fact that ice has a well-estiblished temperature dependent rheology. With undergraduate researcher Vladislav Sevostianov, I've conducted laboratory experiments to measure the actual area of contact of rough ice in contact with a rigid, transparent surface. This work was done with the assistance of Professor Rubinstein using the total internal reflection imaging technique that he pioneered in his PhD work. The proximal goal of this study was to demonstrate the use of the TRI technique at temperatures near the melting point. An ongoing study (Fall 2017) seeks to relate contact area measurements to the frictional properties of ice.

Ice friction experiment

Image from a typical lab friction experiment. Although the circular ice sample (center, actual size about 4 cm) appears to be in contact, illumination shows that only part of this apparent contact area actually touches the glass plate.

Ice friction is also fundamentally related to the operation of skis and snowboards. Give the enormous enjoyment associated with these sports, even a small increase in our technical knowledge of ski friction promises great returns (joke). In a landmark paper perhaps motivated by this type of logic, F.P. Bowden was the first to point out that polytetrafluorethylene (related to Teflon) makes an excellent ski wax. But why, might we ask, does ski wax work to begin with? I would argue that the thermal properties of wax are an essential component. Consistent with Jim Rice's theory of flash weakening of microscopic asperities, the action of ski wax as an insulator eases the process of flash weakening and therefore lowers the threshold for rapid weakening. Please send me an email if you'd like to discuss this idea further!