Researchers at the UvA Institute of Physics have discovered the secret to understanding the motion of microscopic organisms and molecular motors inside our cells by joining miniature self-propelling toys in a chain.
Microbots known as Hexbug Nano v2 use vibrations to move forward. A chain made of elastic silicon rubber connecting several of these toys creates a “elastoactive” structure. This means that, despite the self-propelling, active ingredients that it is made of constantly attempting to push the structure in a particular direction, it will return to its former shape after being deformed.
The elastoactive chains displayed a variety of movement types, including self-oscillatory, self-synchronizing, and self-snapping, depending on the size of the chain links and whether the chains were fixed at one or both ends.
“By experimenting with these elastoactive chains, we discovered that there is an interplay between activity and elasticity: when activity dominates, the chains self-oscillate and synchronize,” says Corentin Coulais, head of the Machine Materials Laboratory at the University of Amsterdam.
He continues, “Mechanical self-oscillation and synchronization are a key feature of biological machines, features that are useful for making new types of autonomous robots. These active chains really allow us to single out the nature of these nonlinear phenomena.”
Self-oscillation, self-synchronization and self-snapping
A structure that oscillates on its own is said to be self-oscillating. The microbots in the chains may begin by bending the chain to the left. The elastic links, which are resistant to this movement since the chain is fixed at one end, realign the bots so that they begin pushing and bending the chain to the right. The elastic chain will once more oppose this movement until the robots begin to move to the left once more.
When two elastic bands are joined at one end by a strong enough rod, synchronisation occurs. The two linked chains naturally begin vibrating at the same frequency as a result of wiggling, much like sea grass being moved by the same waves.
Finally, it exhibits “self-snapping” behaviour when a single elastoactive chain is pinched at both ends. If you push firmly enough from the side after bending a playing card with your fingers, it will “snap” to bend the other way. The elastic chains do this on their own, repeatedly switching between bends to the left and right.
Instructive play
“We started this research by just playing around with the microbot toys. But more generally, the idea was to explore materials out of equilibrium. In soft matter, active fluids have been studied extensively in the last 25 years, but their solid counterparts were investigated much less,” says Coulais.
The investigation of elastoactive behaviour at lower scales, such as in so-called colloidal systems made up of tiny particles suspended in a fluid, is the next item on the agenda. Despite the fact that these are still model systems, the existence of the fluid and similar length scales make them more analogous to the biological system. It would also be intriguing to employ intelligent design to integrate many self-oscillations within a single building at any scale in order to produce movement patterns that are more intricate. It is hoped that a deeper comprehension of self-oscillations will make it possible to develop new varieties of autonomous robots.
