Thin strips of a transparent gel have been found to crack impossibly quickly.
鈥淲hat we found was a total surprise. This just isn鈥檛 supposed to happen,鈥� says at the Hebrew University of Jerusalem in Israel who led the team that observed the record-breaking cracks. The finding could improve our understanding of some earthquakes and how brittle materials break.
The researchers used fast high-resolution cameras to record how cracks spread through thin samples of porous gels that are commonly used for analysing molecules in medical labs. For each sample, they printed a grid of tiny squares on the gel to more easily visualise where the cracks were, and they made a shallow notch in the place where they wanted the gel to crack when stretched.
Advertisement
Then, they stretched the gel and recorded how the tip of the crack propagated. Across many recordings for many gels, these cracks moved unexpectedly quickly. The fastest were about 30 per cent faster than the speed of sound. Even when the researchers skipped the notch-making step, some still travelled 15 per cent faster than the speed of sound.
Fineberg says that both theoretical and experimental studies of materials that develop cracks had so far shown that cracks can never propagate faster than sound. This is because the speed of sound reflects how quickly mechanical energy can move from one part of the material to another, which must happen for it to crack. In fact, equations that are typically used to predict cracking behaviour return physically impossible values of energy when cracking speeds faster than sound are plugged in.
at Northeastern University in Massachusetts says that part of why these equations may not capture what happens in the new experiment may be how these gels resist being stretched near the tip of the crack. For common stretchy objects like springs, the relationship between the force that stretches them and how much they elongate is simple and direct, but that is not the case for every part of these gels, he says.
鈥淭here is no theory that really describes how a crack propagates at a very high speed in this kind of elastic medium. And it鈥檚 really puzzling how you could bring energy to the crack fast enough for this behaviour,鈥� says Karma. Resolving this puzzle could also further researchers鈥� understanding of dangerous large-scale events that involve fractures like earthquakes, where some cracks are known to propagate very quickly.
The researchers are now working on expanding their experiments to include more materials and making detailed comparisons with numerical simulations of materials like their gels. Fineberg says they are also collaborating with theorists who nearly predicted their experimental findings in little-known work from over two decades ago.
鈥淲e stumbled into this, but I believe that we will be able to get a deep understanding of it by attacking it theoretically and experimentally at once,鈥� he says.
Science
