The laws of physics apply to passive, nonliving matter, ranging from planets all the way down to the smallest atoms. But according to Nikolai Brilliantov, a mathematician at England's University of Leicester and Russia's Skolkovo Institute of Science and Technology, much of the matter in the world is active matter. Living things such bacteria, fish and humans can interact with the forces upon them.
In addition, there are also examples of non-living active matter. Certain nanoparticles, called "Janus particles," are made up of two sides with different chemical properties. The interactions between the two sides allow these particles to self-propel.
The researchers published their findings in the journal Scientific Reports.
Exploring active matter and swirlons
Brilliantov and his colleagues explored the properties of non-living active matter using a computer to simulate particles that could self-propel. He explained these particles were not consciously interacting with their environment. Rather, they were more akin to nanoparticles or some bacteria that had internal sources of energy but did not have the ability to process information about the environment around them.
When the researchers ran the simulation, they were confronted with a number of surprises. The first was that the active matter behaved differently from passive matter. (Related: Scientists use nanoparticles as an alternative propulsion method for small spacecraft.)
Different states of passive matter can coexist at the same time. The example used was how a glass of water could slowly evaporate, but still leave some liquid water behind. But Brilliantov noted that active matter did not coexist in different phases; it was all solid, all liquid or all gas.
In addition, they 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.
The team named these particle conglomerates "swirlons," and dubbed the new state of matter they formed a "swirlonic state."
In this state, the particles behaved rather strangely. For one, they violated Newton's second law. The latter states that as force is applied to an object, its acceleration increases, and that as its mass increases, its deceleration decreases. But the swirlons did not accelerate when force was applied to them.
"[They] just move with a constant velocity, which is absolutely surprising," said Brilliantov.
Simulation results still need real-life verification
While the discoveries that the researchers made were exciting, they were only done with basic simulations. The next step then would be to do experimental work with real-world active matter. These should show whether or not the results seen in the computer actually occur in real life.
In addition, Brilliantov and his team are also looking to do more complex simulations with active-matter particles with information-processing abilities. These particles will more closely resemble animals and insects, and will hopefully reveal the physical laws that govern schooling, swarming and flocking.
According to Brilliantov, their ultimate goal is to create self-assembling swirlon materials out of active matter.
"It's quite important that we see the nature of active matter" is much richer than that of passive matter, Brilliantov said.
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