Using the brittleness of materials as a mechanical property
Depending on the circumstances, grainy materials like sand can be used to form various shapes. How do these shapes come to be, and when and why do they disintegrate again? Physicist Corentin Coulais designed a model to unravel the secrets behind this phenomenon and published a paper about it in Physical Review Letters, which was released on 7 November.
‘Several theoretical models describing the disintegration process of grainy materials already existed, but none of these had been tested in a practical environment,’ Coulais explains. In collaboration with several French colleagues, he has developed an experiment in which this process can be witnessed and studied. The versatility of grainy materials is fascinating. Studying and revealing the process of change and disintegration of these materials will also enable scientists to develop new applications for them. Grainy materials could, for instance, be used to develop biological tissues and ‘sand robots’, and even be applied in the oil industry.
Together with his French colleagues, Coulais researched what happens when force is applied to grainy substances like sand. ‘All sorts of things happen when you’re standing on sand,’ Coulais explains. ‘You slightly sink in, while the sand is also carrying your weight. It’s a mixture of stability and instability that is typical for grainy substances. The same process can be witnessed when a pile of snow develops into an avalanche, or when you’re drinking a cup of cappuccino. If you stir too forcefully, your foamy layer will disintegrate. A similar effect can be seen in other grainy substances, but up to now we didn’t know what exactly happened during the transition from stable to unstable.’
Understanding the transition phases of grainy substances is especially relevant for developing sand robots capable of grabbing and lifting. These robots are made from soft (and thus unstable) sand that can be shaped around an object. The sand then hardens (becoming stable), allowing the robot to easily lift the object in question. This technology will be very useful in various industries and on construction sites.
The experiment that Coulais and his colleagues developed revolves around a set of transparent rubber disks. Each of these disks represented a grain of sand, and their properties were studied by pressing an ever-growing balloon against them. Coulais: ‘One of the properties of these disks, is that they slightly twist when pressure is applied. The more pressure is applied, the more they distort. Subsequently measuring this distortion allowed us to calculate the amount of pressure applied on each disk.’ To his surprise, the physicist discovered that the instability of sand increases when more pressure is applied. ‘It also turned out that the disks further away from the balloon were more unstable than those adjacent to it, even though you’d except the opposite to happen.’
(10 November 2014)