Research published in the journal Scientific Reports describes a new kind of active matter, termed a swirlonic state. As researcher Nikolai Brilliantov explains, the swirlonic state is evident in examples of group motion, such as those seen in schools of fish or murmurations of birds that often fascinate the human eye.
Brilliantov, a mathematician at Skolkovo Institute of Science and Technology in Russia and the University of Leicester in England, says that this newly described active state contradicts Newton's second law of motion, which states that as a force applied to an object increases, its acceleration increases, and that as the object's mass increases, its acceleration decreases. While this widely accepted physical law works with passive, nonliving matter, Brilliantov points out that active, living matter often moves by its own, self-directed, force.
Using computer simulations, the researchers showed that active matter in a swirlonic state does in fact not follow Newton’s second law. When a force was applied to particles in this state, they did not accelerate, rather, they continued with a constant velocity, said Brilliantov. This finding was quite a surprise.
From their simulations, Brilliantov and his colleagues explain that an active swirlonic state is composed of what they’re calling swirlons, which are “formed by groups of active particles orbiting their common center of mass. These quasi-particles demonstrate a surprising behavior,” write the authors. “In response to an external load they move with a constant velocity proportional to the applied force, just as objects in viscous media. The swirlons attract each other and coalesce forming a larger, joint swirlon. The coalescence is extremely slow, decelerating process, resulting in a rarified state of immobile quasi-particles.”
The team says the next step in their investigation will be to conduct simulations with real-world, complex active matter. Such experiments will bring the scientists closer to understanding the physical laws that govern the schooling, swarming, and flocking of certain animals – information that can help future endeavors with self-assembling materials. "It's quite important that we see the [true] nature of active matter", Brilliantov comments.