Getting to grips with why we slip

WINTER: 'tis the season of reduced friction. Depending on where you are, you might be anticipating the first icy days of the year's coldest season, or already be well attuned to its attendant dangers. Ice plus incaution, we all know, equals slips, slides, broken bones and mangled cars and bicycles.

Not your problem, you might think, if you are basking on Bondi beach or sunning yourself in your Florida bolt-hole. You would be wrong. Even in Australia, where ice tends to be confined to the beer cooler, slips on low-friction surfaces such as tiled bathroom floors or oil-slicked filling station forecourts result in a dozen deaths, tens of thousands of injuries and an estimated AU$ 1 billion in lost productivity each year. That's a picture comparable to those in the US and the UK. "Slip resistance is a global problem," says Richard Bowman, a slip consultant at Intertile Research in Melbourne.

That's why, in safety laboratories around the world, fearless researchers are having our accidents for us, slipping and sliding their way, they hope, towards a better understanding of the perils of reduced friction. They do not have it easy. Friction might be everywhere - except where it is suddenly absent - but it turns out to be surprisingly difficult to get to grips with.

Even supercomputers capable of calculating what goes on inside stars or modelling the most complex characteristics of the atomic world slip up on friction's intricacies. "Friction is not a material property, it's a system response," explains Roland Larsson of Luleå University of Technology in Sweden. The amount of friction between two surfaces depends not only on their atomic structures, but also on their context. The presence of a liquid between them can affect it, for example, as can whether they are moving, and if so at what relative velocity.

This means there is no simple formula for how surfaces glide across each other. "If you give me two surfaces and ask me to predict the friction when I rub them together, I just can't: it's too complex," says Larsson.

If the theorists are floored, the experimenters are at sixes and sevens. True, you can ask them to carry out measurements to define the "coefficient of friction" between two surfaces, a number that quantifies the frictional force that arises when you press them together with a certain force. But there's the rub: take any two friction aficionados and you will probably find that they do their measurements in completely different ways - quite possibly reaching two different answers. "There is no overall, agreed way to do this," says Mark Redfern, a bioengineer and slip expert based at the University of Pittsburgh in Pennsylvania.