As reviews of the year in science focus inevitably and rightly on the discovery of a Higgs-like boson, it’s more tempting than ever to regard the Large Hadron Collider’s awesome tunnels as representative of the current state of play in physics. So it is good to be reminded that there is still an abundance of far more modest, mundane and much closer-to-home questions for physicists to solve besides looking for the origins of mass. If you’re planning to sit down to a roast turkey at Christmas, you might like to know that the question of exactly how the carving knife cuts the moist flesh into slices has finally received a thorough scientific answer.
No one tries to cut up a turkey by simply pressing down hard with the blade – that’s likely merely to squash the bird. As with cutting bread, you’ll instead use a sawing motion, sliding the knife through the soft meat. But why is slicing more effective than chopping?
That’s the question investigated by Lakshminarayanan Mahadevan, an applied mathematician at Harvard University in Cambridge, Massachusetts, and his colleagues. It is the sort of problem that Mahadevan has made his speciality. In previous work he has explained why flags flutter, how wet paper curls and drapery folds, why opened envelopes have jagged tears, and why pieces of dry breakfast cereal (such as Cheerios) floating in milk tend to clump together.
These could seem like whimsical issues. But very often, these phenomena manifest themselves in a host of both natural and technological situations, and understanding them might help to address some important practical problems. This sort of work harks back to the days when eminent physicists such as Lord Rayleigh and Lord Kelvin investigated the world as they found it, rather than the world beyond our senses (as is the case with nuclear and high-energy physics). They studied bubbles, cracks, flowing fluids and soaring seabirds – and in an age of huge, costly international collaborations, it is heartening to know that good science can still be done with a piece of wire and a block of jelly.
That is essentially what Mahedevan and colleagues have used to understand the physics of slicing and cutting. Their soft solid is a polysaccharide gelling agent extracted from algae, known as agar and widely used as a food ingredient in Asia. The researchers cut a block of this stuff using either metal wire or thin fishing tackle, stretched tautly like a cheese wire. They investigated the forces that develop in the gel when the wire is either just pushed down through the block or when it is also pulled across the surface in a slicing action.
The two cases give different results. When the wire is simply pushed downwards, the gel surface is squashed into a V shape several millimetres deep before suddenly the wire cuts through it and the downward force on the block plummets. But with the slicing action of a tangentially moving wire, there is hardly any initial resistance or squashing before the wire cuts through – in effect, you don’t have to press anything like as hard.
Why is this? The researchers point out that in both cases cutting the gel entails initiating a crack. But the key difference between 'chopping" and slicing is that, in the former case the wire mostly just compresses the gel, whereas in the latter case the sideways movement of the wire also stretches it. Soft solids tend to be more resistant to compression than to stretching: a jelly can be squashed considerably without ripping, whereas if you pull on it then it will soon split apart. Mahadevan and colleagues carried out detailed calculations of the forces throughout the gel to show how it is much easier to cut by slicing.
But there’s one proviso. The stretching caused by slicing arises through friction between the sideways-sliding wire and the gel surface. If there’s not enough friction, the wire can’t gain traction and some of the advantage of slicing is lost. So very smooth wires don’t cut as well as rougher ones, and the wire can’t tug so effectively either if it is pulled too fast across the gel surface. Ideally one should find a configuration in which the force required to move the blade against friction is more or less exactly equal to the force needed to initiate a crack.
It’s not just the slicing action of carving knives that the study helps us to understand. For example, a paper cutter uses a hinged or moving blade so that the cutting edge moves steadily across the paper surface rather than pressing down on it all at once. The same is true, the researchers mordantly note, of the angled blade of the French guillotine, ensuring a clean slice rather than a messy, squeezing chop. Whether or not you choose to divulge this morsel of information as the Christmas turkey is being carved is, of course, up to you.
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