In Roman mythology, the god Janus had two faces that allowed him to see the past and the future. Therefore, scientists decided to use this deity to name a new type of particle composed of two sides, each with different chemical properties. The interactions between the two sides create a self-propelled movement, which allows the particle to violate some of the most famous laws of physics - Newton's second law, known as the fundamental principle of dynamics.
When we look at the world around us, we realize that things seem to work in a simple way. When we push an object, it moves. As we increase the force applied to that object, it gains speed. If the object gains mass, its acceleration decreases, and so on. These are some of the laws that Isaac Newton observed and even today few things found in nature are capable of violating them. Well, that in the world that we can see. In the microscopic world of particles, however, the rules can be quite different.
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| Diagram illustrating Janus nanoparticle organization (Image: Reproduction / Yifan Li) |
The rules described above apply to passive matter, and not to active matter, which is living beings. Both we humans, animals, and even very small beings like bacteria, are able to move on their own, without the need for an external force. Of course, this does not mean that energy is not necessary, but that energy is contained in the active matter itself - in our case, the human body. What scientists did not expect until recently is that a passive, that is, non-living matter, could also be self-propelling.
Nikolai Brilliantov, a mathematician at the Skolkovo Institute of Science and Technology in Russia, wants to better understand this type of particle, which is named Janus. The most common Janus particle model is to have one hydrophilic side (attracted to water) and the other hydrophobic side (water repellent). There are others that are even more complex, and we already have some interesting applications, such as micro vehicles driven by chemical reactions that only happen on one side of the Janus particles.
However, it may be that there are many other possible technologies, just better understand how these particles work. To explore this active matter, Brilliantov and his colleagues used a computer capable of simulating the particles, and the result revealed some surprises. The simulations were simple, with particles that do not consciously interact with the environment, resembling simple bacteria, or even nanoparticles with internal sources of energy. In addition, they lacked the capacity to process information.
One of the interesting results was to note that this matter does not coexist in different states, such as passive matter - a glass of liquid water, for example, can evaporate into the gaseous state, leaving behind a reminder of the water in the liquid state. The active matter, however, did not present this possibility, remaining entirely in the solid, liquid, or gaseous state, without a gradual transition.
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| Janus particle composed of polystyrene surface and rough gold surface (Image: Reproduction / LY Wu / BM Ross / S. Hong / LPLee) |
Another curiosity is that the particles were grouped as large conglomerates, which mixed in a circular pattern around a central void, similar to what we see in a school. The researchers dubbed these particle conglomerates "whirlpools" and named the new state of that matter - a specific condition in which Janus' particles formed a "whirlwind state". And it is in this state that the particles begin to behave in an even stranger way: when a force was applied, they did not accelerate, that is, they violate Newton's second law. They just “move at a constant speed,” said Brilliantov.
The next step in understanding these particles is to conduct experiments with the real matter. The team of scientists still intends to do more complex simulations, this time using particles of active matter with the ability to process information. They will behave more like insects, which may help to reveal the physical laws that govern these particles. The research by Brilliantov and the team was detailed in October 2020 in the journal Scientific Reports.
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