Balloon bursts give clue to fast cracks. (Physics).

Ever since a fellow physicist pointed out to Robert D. Deegan that the edges of popped balloons have ripples like tiny shark teeth, he has wondered why. Now, Deegan and his colleagues at the University of Texas at Austin may have found a connection between that casual observation and a murky area of physics--the propagation of fast-moving cracks.

To study balloon rips, the researchers stretched rubber rectangles on a frame, pricked them with a pin, and videotaped them splitting at nearly the speed of sound. Scientists have predicted the trajectories of slow-moving cracks, such as tiny ones in windshields, but not the paths of faster fractures like the ones in a bursting balloon. In the Jan. 7 Physical Review Letters, Deegan's team reports that the tip of a crack in rubber under low tension goes straight. Yet when the tension is high, the tip wiggles out a path akin to shark teeth.

The researchers concede that the results may illuminate little more than the nature of ripping in balloons and other rubbery materials. However, they say their tests so far suggest that other materials, when under different degrees of strain, may exhibit similar patterns in cracking.