Most motorcyclists cherish their helmet like a baby. After all, that protection can save your life. There’s another reason, too. That protection can deteriorate drastically if you accidentally drop your helmet. In the event of a fairly hard impact, the cautious motorcyclist will play it safe and purchase a new one. That will soon no longer be necessary. Scientists at Ghent University (UGent, Belgium) have discovered a mechanism by which shock-absorbing material returns to its original form after a blow.
When it comes to crash barriers, helmets or the shock absorbers in your car, it’s all about being able to absorb the energy. If the impact is too great, the material deforms or the shock absorber breaks apart. The new shock absorbers consist of two elements: Water and material with very small pores or so-called ‘cages’. This is what is known as nanoporous material. These ‘cages’ are up to a hundred thousand times smaller than the thickness of a human hair. They are water-repellent and interconnected.
When this material absorbs a shock, the energy of the shock is used to push water into the water-repellent cages. The faster the shock occurs, the more energy the material absorbs. After a shock, the water flows out of the cages again and the whole absorption cycle can start all over again.
More efficient when subjected to faster shocks
Researchers at the Universiteit van Oxford first observed this new mechanism in ZIF-8, a specific nanoporous material. The researchers at UGent studied this material to understand why it can absorb mechanical shocks so efficiently. The main question was why the material becomes more efficient when subjected to faster impacts.
This turned out to be down to the specific structure of ZIF-8. Because the material is made up of interconnected water-repellent cages, water never spontaneously penetrates these cages. That only happens as soon as enough pressure is exerted on the material. For example, as a result of a hard blow. Then the first water molecules penetrate the cages despite the water-repellent nature of the material. Hydrogen bonds ensure that the molecules within the cages organize themselves into small groups. Once such a group becomes large enough – starting at about five water molecules – it becomes much easier for additional water molecules to penetrate the cages. Until they eventually fill the entire material.
This process does take some time. If the blow against the material is too fast, then there is insufficient time for these groups to form. This means that even more energy from the mechanical shock is needed for the water to penetrate the cages. This explains the higher efficiency of the materials at faster high-speed impacts.
New materials for shock absorbers
The researchers derived a number of design rules based on simulations to develop shock absorbers that work according to the abovementioned mechanism. The most important rule is that such materials must consist of water-repellent cages, so that water does not enter on its own accord. Those cages must be connected through openings large enough to allow water molecules to move from one cage to another. The larger the cages, the more water can eventually enter, and thus the better they are able to absorb the shock.
“On the basis of these design rules, we discovered about 20 materials that are not currently used as shock absorbers, but would actually be extremely suitable for this purpose. Some of these materials are now also effectively being tested experimentally, with very positive results,” says Aran Lamaire of UGent.
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