Synthetic Particles Mimic Bacteria Movement

Northwestern Engineering researchers have developed a synthetic system that mimics how bacteria move and self-organize, paving the way for new research.

For years, scientists and engineers have studied self-organizing systems, like flocks of birds or swarms of bacteria, to determine how they organize without a central administrator. Collective, emergent systems are difficult to study in nature — but they hold huge potential.

By understanding how these systems work, engineers could design their own — such as creating microrobots that move autonomously in the body to deliver drugs, or that swim through the ocean to clean up an oil spill.

“Our artificial self-propelled particles that mimic bacterial locomotion could be a new testbed for studying collective dynamics,” said Petia Vlahovska, associate professor of engineering sciences and applied mathematics at the McCormick School of Engineering and lead author of the research. The results were published November 14 in the journal Physical Review Letters.

Looking to bacteria to understand swarms

Though single bacteria are just micrometers in size and move seemingly randomly through space — through movements called “run and tumble” that allow them to explore, search for food, and escape predators — when they get together in large groups, they move much faster, and in coordinated swarms.

As an engineer, Vlahovska became interested in finding potential applications for such collective behavior, but found that studying bacterial systems was complicated: bacteria require food and upkeep, and the bacteria themselves have varying levels of activity, making it difficult to measure.

Although systems that use synthetic particles to mimic bacteria exist, they are complicated, using advanced chemistry or magnetic set-ups and most involve particles that travel in a straight line — not in the erratic, stop-and-go way that bacteria move.

Creating a synthetic system

Vlahovska and postdoctoral fellow Hamid Karani developed a simple synthetic environment to study this movement: polystyrene particles suspended in oil. To stimulate the particles to move, the researchers applied an electric field, causing the particles to roll through a phenomenon called the Quincke effect. When the electric field was turned off, the particles stopped, and when it was applied again, they moved again, but in a different direction — just as bacteria do.

When many particles were placed together and energized with an electric field, they began to move in swarms, just like bacteria. The duration and speed of the pulses caused the particles to swarm into different configurations.

“This is the first experimental realization of self-propelled particles that mimic bacterial locomotion,” Vlahovska said. “We now have a very clean way to characterize how individual particles affect the collective behavior.”

Teasing out collective dynamic rules

Vlahovska hopes to use this system to mimic more kinds of bacterial locomotion, as well as to suspend particles in heterogeneous systems that are more like the natural habitats, like soil, where bacteria also move.

The ultimate goal is to tease out the physical basis for rules that are used in mathematical models of flocking behavior, which can in turn lay the groundwork for rational design of self-organizing systems.

“We can take inspiration from bacteria to design autonomous systems and materials,” Vlahovska said. “We are excited to keep working with this system to see what we can find.”