WINTER: 鈥檛is the season of reduced friction. Depending on where you are, you might be anticipating the first icy days of the year鈥檚 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鈥檚 a picture comparable to those in the US and the UK. 鈥淪lip resistance is a global problem,鈥� says Richard Bowman, a slip consultant at Intertile Research in Melbourne.
That鈥檚 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 鈥� except where it is suddenly absent 鈥� but it turns out to be surprisingly difficult to get to grips with.
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Even supercomputers capable of calculating what goes on inside stars or modelling the most complex characteristics of the atomic world slip up on friction鈥檚 intricacies. 鈥淔riction is not a material property, it鈥檚 a system response,鈥� explains of Lule氓 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. 鈥淚f you give me two surfaces and ask me to predict the friction when I rub them together, I just can鈥檛: it鈥檚 too complex,鈥� 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 鈥渃oefficient of friction鈥� between two surfaces, a number that quantifies the frictional force that arises when you press them together with a certain force. But there鈥檚 the rub: take any two friction aficionados and you will probably find that they do their measurements in completely different ways 鈥� quite possibly reaching two different answers. 鈥淭here is no overall, agreed way to do this,鈥� says , 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. 鈥淚t鈥檚 hilarious to watch,鈥� says Paul Lemon, a slip investigator at , a forensic investigation consultancy based in Cambridge, UK (see video at ).
Bare feet or rubber shoes?
Hilarious it may be, but not everyone agrees on the ramp test鈥檚 accuracy. Different people, depending on how they walk, can come up with different measures of a surface鈥檚 slipperiness, and there is no consensus on how best to carry the test out. The UK鈥檚 (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鈥檛 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 (see video at ). When the shoe hits the surface, the friction between the two impedes the pendulum鈥檚 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 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 using this pendulum method.
Not everyone is impressed. 鈥淭he Americans don鈥檛 like the pendulum test because it is too easy for the operator to influence it,鈥� says , principal scientist at the HSE鈥檚 laboratory in Buxton, Derbyshire. 鈥淭hey鈥檙e right; I can get any number I want out of the pendulum.鈥� That鈥檚 why, he says, it is essential to apply the test in accordance with strict guidelines such as those laid down by the HSE laboratory. 鈥淣o single test is perfect 鈥� they all have limitations,鈥� Thorpe says. 鈥淏ut one of the strengths of the pendulum test is that we know something about its limitations.鈥�
The pendulum has another drawback: its distinctly uncool 1950s feel. 鈥淚t looks quite agricultural and simplistic,鈥� says Lemon. The pendulum is an easy object for defence lawyers to ridicule in lawsuits claiming slip-related damages. 鈥淛udges are quite easily swayed towards the idea that a more modern test is intrinsically better,鈥� 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 (see a video at ). It鈥檚 also more user-friendly. 鈥淲ith the pendulum, you can be on your knees for hours taking readings off an analogue scale,鈥� Lemon says. 鈥淲ith the Tortus you just press a button and it does the job.鈥�
Whether it does the job well is another matter. 鈥淭he Tortus test may give unrealistic results when used in wet conditions on a hard, smooth surface,鈥� says Alessandro Tenaglia of the , 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. 鈥淗istorically the tile industry was keen on the Tortus as it exonerated tiles,鈥� Bowman says. 鈥淢eanwhile, plaintiffs used the pendulum.鈥�
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 鈥� but may not be, because different countries鈥� 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 in San Diego, California. 鈥淭here simply are no universally accepted standard testing instruments or methods,鈥� he says.
Cohen鈥檚 suggestion to resolve the difficulties is to use less science, not more. 鈥淢y approach has been to focus on a more cognitive method,鈥� he says. In practice, this means more signs saying 鈥渟lippery when wet鈥�. 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 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鈥檚 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鈥檚 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鈥檚 not a thoughtful gesture: it鈥檚 protection against being sued for damages if a customer should slip on a wet floor.
鈥淲hen stores offer you a bag for your dripping umbrella, it鈥檚 not just a thoughtful gesture鈥�
鈥淚t鈥檚 very easy to ridicule,鈥� says Thorpe. 鈥淭hat鈥檚 everyone鈥檚 first reaction. But then they talk about some friend who had a terrible accident. Now that鈥檚 ridiculous, isn鈥檛 it?鈥�
To wax or not to wax?
Not a conundrum involving unwanted body hair, but a controversy in the world of downhill skiing that illustrates the frictions friction can cause. The problem revolves around a claim many skiers make: that skis run faster when coated with a slippery wax. This belief was dealt a blow in 2006, when , a Swedish cross-country skiing trainer and a researcher at Mid Sweden University in 脰stersund, declared that skis go faster without wax ().
That assertion did not go unchallenged. Roland Larsson of Lule氓 University of Technology, Sweden, points out that Kuzmin knew when his skis were waxed and when they weren鈥檛, leaving him open to bias. He was also skiing on different ice each time he went down the slope. 鈥淲henever you rub the surface, it鈥檚 different for next time,鈥� he says. 鈥淚f you measure a 1 per cent difference in friction, is that because of the wax, or because the ice changed?鈥�
A similar problem has plagued Larsson鈥檚 own work. Earlier this year, he was driving trucks around a frozen lake to test which kinds of tyres cope best with icy conditions. The first unsatisfactory conclusion was that each type of tyre suited a different type of ice. Then it turned out that the results seemed to depend on which tyre was tested first.
This is because it is not the ice itself that is slippery, but a layer of liquid that separates the ice from the air above at temperatures around freezing. In Scandinavia鈥檚 frigid winters, it is often so cold that this layer does not form, so the icy surfaces are not slippery.
In Larsson鈥檚 tests, the pressure from the passage of tyres across the ice was constantly changing the consistency of the surface layer 鈥� and the passage of time didn鈥檛 help either. 鈥淵ou start testing in the morning, then the sun comes up and melts a thin layer of ice. After an hour, you have a different surface.鈥� Larsson says. 鈥淭hat鈥檚 a big problem when you want to compare two tyres.鈥�
- Michael Brooks is the author of 13 Things that Don鈥檛 Make Sense (Doubleday/Profile, 2009)
