As you step out of your house on to an icy path and desperately windmill your arms in an attempt to regain your balance, you probably don't ask yourself why you are falling. After all, everybody knows that ice is slippery. So, what is it about ice that makes it different from the concrete of your path?

Ice and Sensible Shoes

If you were to step out of your door on a sunny summer's day, the soft sole of your shoe would mould itself to the shape of the concrete below. Close up, concrete consists of a number of small mountains and valleys, each one only a few millimetres across. As your weight pushes the sole of your shoe down onto the path, the rubber deforms and parts of it are bent upwards toward your foot and others are pressed down to fill the valleys below. Because the two surfaces are now precisely matched, you cannot slide your foot in any direction until you take your weight off it and the rubber returns to its natural shape.

With concrete it is fairly clear that the surface is rough, but even apparently smooth surfaces are rough if you look close enough. For instance, if your garden was paved with smooth marble, you would not expect to fall over in rubber soled shoes as on a microscopic scale the rubber would still be filling the tiny hills and valleys below.

Go back a moment now to the moment just before your shoe hits the ground. The sole of the shoe hovering centimetres above the concrete is itself not particularly smooth. If you looked closely enough with a powerful microscope, you would see that the surface consisted of millions of little pointed spikes, all of different heights and all ready to flatten and deform when they hit the ground. However, as some are higher than others, the longest ones must hit the ground first. In that instant, the weight of your body is being borne almost entirely by the uppermost tips of those tallest peaks. At a microscopic level, as your foot hits the ground, your body weight is not spread evenly over the surface of your sole, but is entirely concentrated into these few tiny regions. The pressure at these points is immense.

Ice is of course frozen water. In England for example, the temperature is rarely far below freezing, so only the smallest rise in the temperature of ice will cause it to melt. The huge pressure generated at the microscopic points of rubber bearing your whole weight is enough to raise the temperature on impact by the few degrees necessary to melt the ice. Concrete melts at a far higher temperature, and so remains unaffected. As your foot continues down onto the ice, more and more of the little peaks come into contact with the ice, which melts in more and more places. In these tiny areas, you are walking on water.

As the ice melts, the resultant water rushes out from under your shoe. If the ice is thick enough, your shoe sinks in slightly and the whole process is repeated as the peaks of the rubber encounter fresh, unmelted ice. If you were to stand perfectly still, everything would be fine, but of course you want to move about and as there is no microscopic meshing of hills and valleys there is nothing to prevent your foot from sliding out from under you and down you go...