Swirlon: a New State of Active Matter

March 29, 2021

Researchers discovered a new state of active matter that bends the laws of physics. This state called “swirlon” behaves similarly to how fishes swim in schools and birds fly in murmurations.

A group of Russian researchers discovered a new state of active matter that bends the laws of physics. This new state, which they call “swirlon,” behaves similarly to a swirling school of fish or murmuration of birds.

In this recent paper, entitled “Swirlonic state of active matter” and published in the journal “Scientific Reports”, researchers investigated the properties of non-living active matter, discovering that it behaves differently from non-living passive matter, and revealed “a new state of active matter, which we call a swirlonic state, where all active particles belong to swirlons, comprised of particles orbiting around common center of mass.”

The laws of physics apply to non-living passive matter, ranging from planets to the smallest atoms. Yet, much of the matter in the world is active matter (i.e. living things such bacteria, fish and humans can interact with the forces upon them), and there are also examples of non-living active matter (i.e. certain nanoparticles made up of two sides with different chemical properties, where the interactions between the two sides create self-propelled movement).

Reseachers explored the properties of non-living active matter using a computer to simulate particles that could self-propel. These particles were akin to nanoparticles or bacteria having internal sources of energy but not having the ability to process information about the environment around them.

When they ran the simulation, they found a number of surprises:

1) The first surprise was that the active matter behaved differently than passive matter. Different states of passive matter can coexist at the same time. The active matter, on the contrary, did not coexist in different phases: it was all solid, all liquid or all gas.

2) They also noted that the particles all grouped together in large conglomerates, or quasi-particles, that milled together in a circular pattern around a central void, similar to a swirling school of fish or swarm of insect. Researchers named these particle conglomerates “swirlons,” and called the new state of matter a “swirlonic state.”

3) In this “swirlonic state”, the particles behaved rather strangely. They violated Newton’s second law, 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. The swirlons, on the contrary, did not accelerate when force was applied to them, rather they moved with a constant velocity.

Authors write: “We report a novel state of active matter – a swirlonic state. 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.”

They conclude: “The dimension of swirlons is almost independent on the number of particles inside it and determined by the intensity of the self-driven and inter-particle forces. These quasi-particles demonstrate an astonishing property – under an applied external force they move with a constant velocity, proportional to the force, just as the objects in viscous medium.”

“Swirlons attract to each other and coalesce upon a collision, forming a joint swirlon. Therefore evolution of a swirlonic state of matter corresponds to continuous aggregation of particles when fewer and fewer swirlons of larger and larger mass are formed. This process being fast in the beginning, becomes extremely slow afterwards, leading to the formation of a rarified state of massive immobile swirlons; the evolution at this stage is practically frozen.”

Resarchers’ next step is to do experimental work with real-world active matter. They plan to do more complex simulations using active-matter particles with information-processing abilities. These particles will more closely resemble animals and insects, and will help to reveal the physical laws that govern schooling, swarming and flocking.

Full Study “Swirlonic state of active matter”