Abstract
ABSTRACTThe mechanics of snow friction are central to competitive skiing, safe winter driving and efficient polar sleds. For nearly 80 years, prevailing theory has postulated that self-lubrication accounts for low kinetic friction on snow: dry-contact sliding warms snow grains to the melting point, and further sliding produces meltwater layers that lubricate the interface. We sought to verify that self-lubrication occurs at the grain scale and to quantify the evolution of real contact area to aid modeling. We used high-resolution (15 µm) infrared thermography to observe the warming of stationary snow under a rotating polyethylene slider. Surprisingly, we did not observe melting at contacting snow grains despite low friction values. In some cases, slider shear failed inter-granular bonds and produced widespread snow movement with no persistent contacts to melt (μ < 0.03). When the snow grains did not move and persistent contacts evolved, the slider abraded rather than melted the grains at low resistance (μ < 0.05). Optical microscopy revealed that the abraded particles deposited in air pockets between grains and thereby carried heat away from the interface, a process not included in current models. Overall, our results challenge whether self-lubrication is indeed the dominant mechanism underlying low snow kinetic friction.
Highlights
The currently accepted explanation for the slipperiness of ice and snow has a long history. Reynolds (1899), having developed the theory of hydrodynamic lubrication, described a ‘eureka’ moment wherein he postulated that a thin water film formed by pressure melting could account for the slipperiness of ice
Our study objectives were to verify that self-lubrication occurs at snow-slider contacts and to quantify the evolution of real contact area and the production and loss of meltwater to help constrain snow-friction models
Considering the widespread belief that self-lubrication accounts for low kinetic friction on snow, we were understandably surprised to find contradictory results
Summary
The currently accepted explanation for the slipperiness of ice and snow has a long history. Reynolds (1899), having developed the theory of hydrodynamic lubrication, described a ‘eureka’ moment wherein he postulated that a thin water film formed by pressure melting could account for the slipperiness of ice. Reynolds (1899), having developed the theory of hydrodynamic lubrication, described a ‘eureka’ moment wherein he postulated that a thin water film formed by pressure melting could account for the slipperiness of ice. Bowden and Hughes (1939) published the first systematic study, and while they agreed that lubrication was likely, they proposed a different mechanism. Bowden and Hughes (1939) published the first systematic study, and while they agreed that lubrication was likely, they proposed a different mechanism They pressed small sliders against rotating disks of solid ice and compacted snow and suggested that self-lubrication from frictional heating accounted for low sliding (kinetic) friction on both substrates.
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