Ghent University: Researchers develop a new type of reusable and efficient shock absorbers

Researchers at Ghent University succeeded in building a new type of shock absorbers which can be reused over a thousand times and become more efficient the faster the shock they absorb.

When guard rails or helmets absorb large shocks, they typically deform substantially and can no longer be used afterwards. Researchers at Ghent University now unveiled a mechanism through which a new type of shock absorbers can be constructed. Through this new mechanism, shock absorbers become both reusable and more efficient.

Two elements
The new shock absorbers consists of two elements: on the one hand water, and on the other a material with very small pores or cages (a so-called nanoporous material). Those cages, which are up to a hundred thousand times smaller than the thickness of a human hair, are hydrophobic and connected with one another. When such a material absorbs a shock, the energy of the shock is used to force water into the hydrophobic cages. The faster the shock, the more energy the material absorbs. After the shock has been absorbed, the water again exits the cages, after which the whole absorption cycle can be repeated.

Why are these materials so efficient?
Researchers at the University of Oxford first observed this new shock absorption mechanism in ZIF-8, a specific nanoporous material. To understand why this material can absorb mechanical shocks so efficiently, and especially why the material becomes more efficient the faster the shock, researchers at Ghent University performed various challenging quantum mechanical calculations.

The key to this phenomenon was revealed to be the very specific ZIF-8 structure. Because this material consists of connected hydrophobic cages, water never intrudes spontaneously inside these cages. Only when the material is pressurized sufficiently, for instance through a mechanical shock, the first water molecules will intrude in the material despite its hydrophobic character. Hydrogen bonds then organize these molecules in small clusters. As soon as such a cluster inside a given cage becomes sufficiently large – from about five water molecules onwards – the intrusion of additional water molecules in the cage becomes much easier, until, eventually, water fills the complete material.

This whole water intrusion process does take some time. Hence, if the mechanical shock impacts the material very rapidly, there is insufficient time to form such clusters, and even more energy of the mechanical shock is needed to force water inside the cages. This explains the higher efficiency of these shock absorbers at faster impacts.

New design rules for shock absorbers
Based on these simulations, the researchers derived a set of design rules to develop shock absorbers exhibiting the above mechanism. The most important design rule is that these materials have to consist of hydrophobic cages, such that water does not intrude spontaneously. These cages need to be connected through apertures that are sufficiently large such that water molecules can move from one cage to an adjacent one. Finally, the larger the cages, the more water the material can absorb, and the better it can absorb the shock.

“Via these design rules, we discovered about twenty materials that are not yet used as shock absorbers at the moment, but which are actually very suitable for this job. Some of those materials were also tested experimentally, with very positive results,” according to Aran Lamaire

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