Intuitively, you think that everything that you stretch will pull back, but you wouldn’t expect a couple of pieces of plastic to win. Yet, researchers over at [AMOLF] have figured out a way to make a mechanism that will eventually shrink once you pull it enough.
Named “Counter-snapping instabilities”, the mechanism is made out of the main sub-components that act together to stretch a certain amount until a threshold is met. Then the units work together and contract until they’re shorter than their initial length. This is possible by using compliant joints that make up each of the units. We’ve seen a similar concept in robotics.
Potentially this may be used as a unidirectional actuator, allowing movement inch by inch. In addition, one application mentioned may be somewhat surprising: damping. If a structure or body is oscillating through a positive feedback loop it may continue till it becomes uncontrollable. If these units are used, after a certain threshold of oscillation the units will lock and retract, therefore stopping further escalation.
Made possible by the wonders of compliant mechanics, these shrinking instabilities show a clever solution to some potential niche applications. If you want to explore the exciting world of compliance further, don’t be scared to check out this easy to print blaster design!
Thanks to [I’m Not Real] for the tip!
That’s pretty cool! I was initially confused when I read that it “shrinks when pulled”, though. I think of pulling being a movement, as opposed to a force. It initially extends when applying a load, and then when the load increases it contracts. I guess there’s not a short, catchy way to explain it this way.
That’s inaccurate and ultimately impossible. When the mechanism gets stretched it acts like a loose spring. When it gets stretched far enough, it switches state and starts to act like a stiff spring, which might cause it to pull back some way, but if the load keeps increasing it will keep stretching as any elastic material would.
https://www.nomaco.com/blog-new-material-thickens-when-stretched/
https://www.nist.gov/news-events/news/2024/05/new-way-designing-auxetic-materials
Those suffer from a similar confusion in language. In auxetic materials, axial elongation causes transversal elongation, meaning that they get wider when they are stretched. They do not “contract” when the “load increases”.
Contracting while stretching is an oxymoron – it’s saying the material gets shorter when it gets longer, which is simply a contradiction in terms. It’s nonsense.
Thanks for sharing! The way it reduces vibrations is quite interesting: upon resonance, the structure switches to a configuration that is stiffer, thereby changing the resonance frequency of the system and moving away from the resonance point.
Question is, how do you switch back?
by releasing the tension
I don’t see any explanation of whether the mechanism will actually flip back to the original state, or how.
It doesn’t get “shorter when pulled”, it gets shorter when you pull it past a certain point and then stop pulling, so it can retract to a length shorter than it was before.
If you keep pulling on it, it will keep getting longer and longer like any other material.
Also, every elastic object “wants to shrink when pulled”.
The real deal here is that this object exhibits two states: it’s a spring that has two different spring constants that are switched when the object is extended far enough. It will keep stretching if you keep pulling – it’s just going to be harder to do so after a point.
You can think of something, generally elastic, that grows when pulled…even in diameter.
It shortens while water is added to the load for a small part of it’s length range.
It’s a trick with parallel non-linear springs.
In the video, it can be seen that the structure contracts while the load keeps on increasing. While the applied force is increasing, the structure can suddenly contract. You do not need to stop pulling.
Here you have a semantic trick between “pulling” and “load”. If you apply a force that keeps increasing gradually, it can shrink back temporarily, but if you keep pulling it further and further (forced displacement), it will not.
What did I miss? Does the force increase when it switches. And a rubber band is a terrible example. It is nothing like a spring (but can be a good damper).
Username checks out.