Raindrops on windshields may yield better kidney stone slayer

Powerful soundwaves have been used for some years to break up kidney stones, one of the most painful conditions known to medicine. But the method is not perfect: if the stone is not reduced to a fine powder, remaining fragments can still cause pain and potential infections, and surgery is often necessary to remove them with all the attendant risks.

Engineers from Duke University in North Carolina now believe they have found a more efficient way to destroy the stones, using a phenomenon that was first seen when testing supersonic aircraft.

When high-speed aircraft were first tested it was found that raindrops striking the canopy can sometimes cause circular cracks. The cause of this was mysterious for some time, until two researchers from Cambridge University, Frank Philip Bowden and John Field, discovered that the culprit was surface waves: these propagate in only two dimensions rather than three, and therefore pack a more powerful punch. However, the mathematics to describe this phenomenon remained elusive, and it proved difficult to design experiments to test theories of their behaviour.