Spontaneous robot dances emphasize a new kind of order in active matter

Predicting when and how the collection of particles, robots or animals will become orderly remains a challenge for science and engineering.

In the 19th century, scientists and engineers developed the discipline of statistical mechanics, which predicts how groups of simple particles transition between order and disorder, such as when a collection of randomly colliding atoms freezes to form a uniform crystal lattice.

More challenging to predict is the collective behavior that can be achieved when the particles become more complex so that they can move under their own power. This type of system – observed in bird herds, bacterial colonies and robotic worms – has the name “active substance”.

As reported in the magazine of January 1, 2021 Science, a team of physicists and engineers proposed a new principle by which active matter systems could order spontaneously, without requiring higher instructions or even programmed interaction between the substances. And they have demonstrated this principle in a variety of systems, including groups of time-changing robots called ‘smarticles’ – smart, active particles.

The theory, developed by dr. Pavel Chvykov at the Massachusetts Institute of Technology, while a student of prof. Jeremy England, who is now a researcher at the School of Physics at the Georgia Institute of Technology, claims that certain types of active substances are sufficiently nasty. dynamics will spontaneously find what the researchers refer to as ‘low rattling’ states.

“Rattle is when matter takes up energy flowing in it and turns it into random motion,” England said. “The rattlesnake can be larger if the motion is more intense, or more random. Conversely, low rattlesnakes are very small or very organized – or both. “randomly rearranges until it finds that state and then gets stuck there. If you supply energy by forces with a particular pattern, it means that the selected state will find a way to move the matter that closely matches that pattern.”

To develop their theory, England and Chvykov inspired them from a phenomenon – called baptism – discovered in the late 19th century by the Swiss physicist Charles Soret. In Soret’s experiments, he discovered that subjecting an initially uniform salt solution in a tube to a temperature difference would spontaneously lead to an increase in salt concentration in the colder region – which corresponds to an increase in the order of the solution.

Chvykov and England developed numerous mathematical models to demonstrate the low rattle principle, but only with Daniel Goldman, professor of physics at the Dunn family at the Georgia Institute of Technology, could they test their predictions.

Goldman said: ” A few years back I saw England hold a seminar and thought that some of our robust robots might be valuable in testing this theory. ‘Collaborates with Chvykov, who runs Goldman’s laboratory, Ph.D. students William Savoie and Akash Vardhan used three fluttering cords enclosed in a ring to compare experiments with theory. The students noted that instead of displaying intricate dynamics and fully exploring the container, the robots would spontaneously organize into several dances – for example, one dance consists of three robots clapping each other’s arms in sequence. These dances can hold hundreds of flaps, but suddenly lose stability and are replaced by a dance of a different pattern.

After first showing that these simple dances were indeed low rattles, Chvykov, together with engineers at the North-West University, prof. Todd Murphey and Ph.D. student Thomas Berrueta, who developed more refined and better controlled smarticles. The improved slime tools enabled the researchers to test the limits of the theory, including how the types and number of dances differ for different arm-fluttering patterns, as well as how these dances can be controlled. “By controlling the sequence of low rattling conditions, we were able to achieve system configurations that deliver useful work,” Berrueta said. The researchers from the North-West University say that these findings may have broad practical implications for micro-robotic swarms, active materials and metamaterials.

As England noted: “For robot worms, it’s about getting a lot of adaptable and smart group behaviors that you can design to accomplish in a single swarm, even though the individual robots are relatively inexpensive and calculations simple. For living cells and new materials, it’s can be about the understanding that the ‘swarm’ of atoms or proteins can bring you, in terms of new material or computational properties. ‘