Why Is Ice Slippery?

T freezing of water and melting of ice are among the most dramatic examples of phase transitions in nature. Melting ice accounts for everyday phenomena as diverse as the electrification of thunderclouds, in which the liquid layer on ice chunks facilitates the transfer of mass and charge during collisions between them; frost heave powerful enough to lift boulders from the ground (see PHYSICS TODAY, April 2003, page 23); and, of course, slippery surfaces. Everyday experience suggests why ice surfaces should be slippery: Water spilled on a kitchen floor or rainwater on asphalt or concrete can create the same kinds of hazards for walkers and drivers that ice can. Presumably, the liquid makes the surface slippery because liquids are mobile, whereas solid surfaces are relatively rigid. Asking why ice is slippery is thus roughly equivalent to asking how a liquid or liquid-like layer can occur on the ice surface in the first place.