Active and self-propelled particles have been attracting increasing interest from researchers for several years. You have surely seen videos showing magnificent schools of fish, schools of insects or even impressive marbles of starlings. In fact, these systems can behave abnormally, making them difficult to understand or model.
They usually form a gigantic shape that swirls while maintaining phenomenal integrity. But how are these formations possible? Think of these as oversized examples of a state of matter that in some way circumvents the laws of physics, especially Newton’s.
A new type of active ingredient
In an article published in Scientific Reports magazine in October 2020, Nikolai Brilliantov, a mathematician at the Skolkovo Institute of Science and Technology in Russia and the University of Leicester in England, argues that this type of group behavior is a new type of drug forms called vortex state.
Newton’s second law states that the acceleration of an object depends on the force acting on it and its mass. The acceleration increases with the exerted force and decreases with increasing mass of the object. It turns out that law does not apply to vertebrae. They are only passive, non-living objects.
A murmur of starlings in the air. Photo credit: Shutterstock / Albert Beukhof
Pretty amazing behavior
The computer simulations carried out by Brilliantov and his co-workers actually showed astonishing behavior in the vortex. Instead of moving faster when a force was applied to them, the groups of quasiparticles did not accelerate. “We were completely amazed to see how these quasi-particles swirl around in the active material. They behaved like individual super particles with surprising properties, in particular because they did not accelerate despite the application of force and clumped together during the collision to form vortices with greater mass, ”said Brilliantov.
Big eye Trevally Jack (Caranx sexfasciatus). Photo credit: Shutterstock / Dray van Beeck
A big difference between passive and active matter
In short, the active material behaves very differently from the passive material. In particular, the researchers discovered that different states of passive matter could coexist. Brilliantov took the case of a glass of water as an example. The liquid can gradually evaporate into a gaseous state while the water molecules are retained. This is not the case with the active material, which can only take one form: solid, liquid or gaseous.
The particles were also grouped together as quasiparticles that interacted with each other to form a circular pattern around a central cavity. Brilliantov and his colleagues then named these quasiparticles of eddies and the new state of matter they formed from the “eddy state”. The team hopes to further study the eddies, including real-world studies and experiments.