Swirlons - A self-driven particles

Physical laws such as 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 — apply to passive, nonliving matter, ranging from atoms to planets. But much of the matter in the world is active matter and moves under its own, self-directed, force. Active matter is a substance comprised of active particle that demonstrate the motility, that is the ability to exhibit motion and to perform mechanical work at the expense of metabolic energy. 

The 'swirlon' - a novel state of active matter - displayed a stunning behavior whereby instead of moving with acceleration, the quasi-particle groups moved with a constant velocity, proportional to the applied force and in the same direction of the force. This conduct seemingly violates the Second Newton's Law.

Professor Nikolai Brilliantov who led the investigation at the University of Leicester said:- "We were completely baffled to witness how these quasi-particles swirl within active matter, behaving like individual super-particles with surprising properties including not moving with acceleration when force is applied, and coalescing upon collision to form swirlons of a larger mass.

"These patterns have previously been observed for animals at different evolution stages, ranging from plant-animal worms and insects to fish, but rather as singular structures, not as a phase which borders other phases, resembling gaseous and liquid phases of "normal" matter." 

It is comprised of swirlons, formed by groups of active particles orbiting their common center of mass. These quasi-particles demonstrate a surprising behavior: 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. In addition to the swirlonic state, we observe gaseous, liquid and solid states, depending on the inter-particle and self-driving forces. Interestingly, in contrast to molecular systems, liquid and gaseous states of active matter do not coexist. We explain this unusual phenomenon by the lack of fast particles in active matter. We perform extensive numerical simulations and theoretical analysis. The predictions of the theory agree qualitatively and quantitatively with the simulation results.