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Microscopic particles of 'active matter' dance to the tune of electrochemical reactions

A new University of Maryland study has revealed a coordinated dance of microscopic particles—breaking up and clustering back together in just seconds—after receiving electrical and chemical stimuli. This work represents a new class of materials that mimic the behaviors of living organisms, known as "active matter."

Like the skins of chameleons and octopuses, which respond to external stimuli by changing colors, active matter can display dynamic and autonomous behavior including motility, assembly and swarming. The study, led by Associate Professor Taylor Woehl and in Nature Communications, revealed a new mechanism to activate these properties within seconds.

Practical applications of the discovery could include national defense or sustainability technology—think windows in which smart materials array themselves to automatically block light, or active camouflage that constantly helps troops blend with their environment.

Woehl and colleagues in the Department of Chemical and Biomolecular Engineering demonstrated a method that consists of shocking microscopic particles in liquid with an electrical current, driving the particles to assemble into crystals that disassemble and reassemble in a repeating cycle, essentially dancing to the tune of electrically stimulated chemical reactions, Woehl said.

While previous research has shown similar behavior in such particles, it required actively changing the stimulus over time—a process similar to programming a robot. This new method causes this cycle to occur independently by applying a constant stimulus.

"In that way, it's more similar to a biological system, but what's really happening in the background are chemical reactions telling these particles what to do," said Woehl.

Additionally, biological cells use chemical signals to replicate, move and perform functions autonomously and within time scales of minutes. Previous attempts at using chemical signals to entice active matter to exhibit similar dynamic behavior have resulted in systems with very slow response times of hours to days. Likewise, prior methods using electric voltage to coordinate clustering of microscopic particles have lacked control over how long the particles cluster.

Woehl's method overcomes these hurdles by combining electric voltage and chemical signals to enable coordinated clustering of microscopic particles with precise control over response time.

The study was a collaboration between Woehl and Associate Professor Paul Albertus' research group, with Albertus contributing theoretical modeling to understand how the electrical voltage controlled the chemical stimulus. This enabled predictions of how changes in the electric stimulus would impact solution acidity and thus the response of the microscopic particles.