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Getting to grips with why we slip

Whether ice or banana peels are to blame, explaining why we fall over has friction aficionados stumped, writes New Scientist’s Michael Brooks.


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.

One popular technique for testing the slipperiness of a surface in contact with a human foot is the ramp test.

Pioneered in Germany, it involves someone walking back and forth along a sample surface, the slope of which is slowly increased. The angle at which the tester slips off provides a measure of its slipperiness.

“It’s hilarious to watch,” says Paul Lemon, a slip investigator at Hawkins, a forensic investigation consultancy based in Cambridge, UK.

Hilarious it may be, but not everyone agrees on the ramp test’s accuracy. Different people, depending on how they walk, can come up with different measures of a surface’s slipperiness, and there is no consensus on how best to carry the test out.

The UK’s Health and Safety Executive (HSE) has modified the test protocol in an attempt to control for this subjectivity.

It also uses a different standard interface: while the German national standard specifies either bare feet on a surface coated with soapy water or heavy industrial footwear on motor oil, the UK relies on a rubber-soled shoe on a surface flooded with clean water.

The confusion doesn’t end there. The ramp test is not portable, making it useless for in-situ forensic analysis at the site of an accident. Several tests have been developed to fill the gap.

One of these is the pendulum test, in which a pivoted rod ending in a rubber-encased metal shoe is swung to the floor from a set height. When the shoe hits the surface, the friction between the two impedes the pendulum’s progress. The degree to which it slows down is a measure of the coefficient of friction.

The HSE has pronounced the pendulum test the best way to assess the slipperiness of a floor. Along with the British Portable Skid Resistance Tester and friction coefficients quoted in British Pendulum Numbers it has been exported to safety authorities across the world. If you are driving on a road in Western Australia, for example, the skid resistance of its surface has probably been tested using this pendulum method.

Not everyone is impressed. “The Americans don’t like the pendulum test because it is too easy for the operator to influence it,” says Steve Thorpe, principal scientist at the HSE’s laboratory in Buxton, Derbyshire.

“They’re right; I can get any number I want out of the pendulum.” That’s why, he says, it is essential to apply the test in accordance with strict guidelines such as those laid down by the HSE laboratory.

“No single test is perfect – they all have limitations,” Thorpe says. “But one of the strengths of the pendulum test is that we know something about its limitations.”

The pendulum has another drawback: its distinctly uncool 1950s feel. “It looks quite agricultural and simplistic,” says Lemon. The pendulum is an easy object for defence lawyers to ridicule in lawsuits claiming slip-related damages.

“Judges are quite easily swayed towards the idea that a more modern test is intrinsically better,” he says.

One of these stylish interlopers is the Tortus, a self-propelled trolley that can be set loose on a surface, dragging a small rubber foot with it to give a measure of the friction coefficient. With an array of buttons and a digital display, it certainly looks more reliable than the pendulum.

It’s also more user-friendly. “With the pendulum, you can be on your knees for hours taking readings off an analogue scale,” Lemon says. “With the Tortus you just press a button and it does the job.”

Whether it does the job well is another matter. “The Tortus test may give unrealistic results when used in wet conditions on a hard, smooth surface,” says Alessandro Tenaglia of the Italian Center for Research and Testing for the Ceramics Industry, based in Bologna.

Electrostatic forces acting between water and the rubber foot can sometimes make a wet surface appear to be less slippery than the same surface when dry. Small wonder, then, that the Tortus has gained favour with ceramic tile manufacturers keen to avoid prosecution for accidents on a wet bathroom floor.

That has led to friction in the courtroom. “Historically the tile industry was keen on the Tortus as it exonerated tiles,” Bowman says. “Meanwhile, plaintiffs used the pendulum.”

The issue divides nations, too. This is proving to be a particular problem in the European Union, where a flooring or footwear product acceptable in one country should be acceptable in all the others – but may not be, because different countries’ tests can give different results.

A committee charged with establishing a single EU-wide test method is floundering in the face of conflicting national interests.

France plans to adopt a modified ramp, while Germany wishes to stick to its original method. Spain and Portugal are swinging towards the pendulum, while in Italy the Tortus is winning out.

The US is not much better, says Harvey Cohen, a safety consultant at Error Analysis in San Diego, California. “There simply are no universally accepted standard testing instruments or methods,” he says.

Cohen’s suggestion to resolve the difficulties is to use less science, not more. “My approach has been to focus on a more cognitive method,” he says. In practice, this means more signs saying “slippery when wet”.

Laughable as that may seem, one of the most dangerous situations is a rapid change in the friction coefficient. In one study, 64 per cent of 108 slip, trip and fall accidents that occurred in one hospital took place at a transitional area: dry to wet, or one type of floor to another.

The HSE is convinced that education can also play a part. Its Shattered Lives website is full of cautionary tales, and its Slip and Trip eLearning Package (STEP) aims to reduce the number of slips in the workplace.

The HSE’s researchers managed to cut this figure by 60 per cent in a fast-food chain simply by teaching the staff better floor-cleaning practices: explaining, for example, how the chemical reaction between grease and detergent takes time, so they could reduce the number of accidents simply by not rinsing the detergent away too soon.

It’s a funny old world, where PhD scientists teach restaurant cleaners how to clean floors. But the sums of money at stake in insurance, legal costs and health premiums mean that no one is laughing.

When department stores offer you a plastic bag for your dripping umbrella, it’s not a thoughtful gesture: it’s protection against being sued for damages if a customer should slip on a wet floor.

“It’s very easy to ridicule,” says Thorpe. “That’s everyone’s first reaction. But then they talk about some friend who had a terrible accident. Now that’s ridiculous, isn’t it?”