The wrinkle pattern of a dried passion fruit appears to exhibit a kind of symmetry not previously known. This inspired researchers to design a gripping robot that could be useful in the future for cleaning up space debris.
Anyone who has ever had a passion fruit on the fruit bowl knows that the skin gradually changes from tight and smooth to more wrinkled than the skin of a 120-year-old. That’s exactly what happened to a box of passion fruit that was left at the office of a Chinese research group. This led to an extensive study into the origin of this ripple pattern.
The researchers developed a mathematical model that describes how the tight skin of the passion fruit changes into a wrinkled landscape when it dries out. This model not only explains the shape change, but also makes it possible to develop grippers based on this ripple pattern. The researchers demonstrated this by copying the structure on the surface of a silicone shell.
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Bend or kink
Wrinkle patterns can be found in all kinds of places in more and less natural materials. Such wrinkles can arise roughly in two ways.
The first cause is growth. For example, our brains are still quite smooth during the first period of pregnancy. But because the cerebral cortex then grows faster than the rest of the brain, it is compressed and ripples and twists are created.
The other main cause of wrinkles is dehydration. This causes, for example, the wrinkled skin of dried fruit and the wrinkling of your skin as you get older.
A material is put under pressure during both growth and dehydration. When that pressure reaches a critical point, the material can suddenly bend or kink. This process also plays a role, for example track spatter: the phenomenon in which dangerous kinks occur in railway rails when the rails expand due to the heat.
Understanding and influencing the formation of ripple patterns may have technological applications. It comes in handy, for example, when designing flexible electronics or the ‘skin’ of soft robots.
Unknown pattern
The Chinese research group discovered that the skin of a drying passion fruit first folds into a buckyball shape. That is a shape with hexagons and pentagons, like the surface of a football.
As the fruit dries out further, deeper grooves and wrinkles appear. Those turn out to rank in a previously unknown chiral cartridge. That means the ripple pattern is not identical to its mirror image. More precisely, it’s a pattern that you can’t rotate or shift to look the same as its mirror image. An example of chiral symmetry is your left and right hand.
Gripping robot
Inspired by the dried passion fruit, the researchers made a spherical silicone shell with a similar chiral pattern. They placed this shell on a robot gripper. The structure turned out to be very suitable for picking up small objects. The researchers demonstrated this with a number of objects, such as a diamond, blueberry, screw or heart-shaped candy.
This gripping structure could be used to pick up dangerous objects, such as small radioactive or explosive materials. Another application may be cleaning up space debris, such as debris from satellites or rockets floating in orbit. About half a million of those fragments are, according to NASA, about the size of a marble (1 centimeter); and millions are even smaller.
The technique may also have potential for microelectronics, writes materials scientist Francesco Dal Corso, of Italy’s University of Trento, in a News & Viewsarticle about the Chinese publication. For this, the now purely mechanical properties would have to be combined with electricity. The technology will have to be further improved for such applications.