Researchers studied the swarming behavior of microswimmers – Zoo House News

Researchers studied the swarming behavior of microswimmers – Zoo House News

  • Science
  • January 15, 2023
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The apparently spontaneously coordinated swarm behavior of large groups of animals is a fascinating and striking collective phenomenon. Experiments by researchers from the University of Leipzig on laser-controlled synthetic microswimmers now show that supposed swarm intelligence can sometimes be the result of simple and generic physical mechanisms. A team of physicists led by Professor Frank Cichos and Professor Klaus Kroy found that swarms of synthetically manufactured Brownian microswimmers appear to spontaneously decide to orbit their target rather than head directly for it. They have just published their results in the journal Nature Communications.

“Scientific investigations into herd and swarm behavior are usually based on field observations. In such cases, it is usually difficult to reliably record the internal states of the herd animals,” says Kroy. The interpretation of observations is therefore often based on plausible assumptions about which individual rules of behavior are necessary for the complex collective groups to be observed. Researchers at the University of Leipzig have therefore developed an experimental model system of microswimmers that evokes properties of natural swarm intelligence and enables complete control over the internal states, strategies and the conversion of signal perception into a navigational response of the individuals.

Thanks to an ingenious laser heating system (see picture), the colloidal swimmers, which can only be seen under the microscope, can actively move themselves in a water tank by a kind of “thermophoretic self-propulsion”, while their locomotion is permanently disturbed by random Brownian motion. “Apart from random Brownian motion, which is ubiquitous in microphysics, the experimental setup provides complete control over the physical parameters and navigation rules of each colloidal float, allowing for long-term observations of swarms of different sizes,” said Cichos.

According to Cichos, if only a very simple and generic navigation rule is followed identically by all swimmers, a surprisingly complex shoal behavior results. If, for example, the swimmers aim for the same fixed point, a kind of carousel can form instead of a meeting at the same place. Similar to satellites or atomic electrons, the swimmers then circle their attractive center on circular orbits of different heights. The only “intelligent” rule of conduct required for this is that the self-propulsion system reacts to the perception of the environment with a certain time delay, which is common anyway with natural swarming phenomena, from mosquito dances to road traffic. It turns out that such a “delayed” effect alone is enough to form complex dynamic patterns like the carousel described above. “Physically, any individual swimmer can spontaneously break the radial symmetry of the system and transition into circular motion if the product of delayed time and swimming speed is large enough,” Kroy said. In contrast, the orbits of larger swarms, as well as their synchronization and stabilization, depend on other details such as the steric, phoretic, and hydrodynamic interactions between the individual swimmers.

Since all signal-response interactions in the living world are time-delayed, these findings should also advance the understanding of dynamic pattern formation in natural swarm ensembles. The researchers deliberately chose primitive and uniform navigation rules for their experiment. This allowed them to develop a stringent mathematical description of the observed phenomena. Analyzing the lagged stochastic differential equations used for this, the lag-induced effective synchronization of the swimmers with their own past turned out to be the key mechanism for the spontaneous circular motion. The theory largely allows us to mathematically predict the experimental observations. “Overall, we have succeeded in creating a laboratory for swarms of Brownian microswimmers. This can serve as a building block for future systematic studies of the increasingly complex and possibly still unknown swarming behavior and may also explain why puppies often circle around their food bowl when fed,” said Cichos.

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