Researchers at Case Western Reserve University have uncovered previously overlooked physical mechanisms that control how microscopic self-propelled particles move near surfaces— a discovery that could influence the future design of targeted drug delivery systems, environmental cleanup technologies and microscopic robots.
In a study published in Physical Review E, Muhammad Haroon—a third-year chemical engineering PhD student—and Christopher Wirth, PhD—associate professor of chemical and biomolecular engineering at Case School of Engineering—investigated catalytic “Janus particles,” tiny spheres coated with platinum on one side that propel themselves through liquid by chemically decomposing hydrogen peroxide. These particles are widely studied as model systems for active matter and materials capable of autonomous motion.
By systematically varying the thickness of the platinum coating and observing particle motion near confining boundaries, the team discovered that propulsion is strongest only within a narrow design window. Particles with intermediate metal thickness moved fastest, while heavier coatings suppressed motion because gravity subtly reoriented the particles toward nearby surfaces.
They also identified an unexpected effect: plasma cleaning, a routine laboratory preparation technique, significantly slowed particle motion throughout the entire system, even far from the treated surface. The results suggest that surface-generated charges diffuse into the surrounding fluid, altering the electrochemical environment that drives propulsion.
The findings reveal that microscopic motion depends not only on chemical reactions but also on a complex coupling between particle design, gravity, and interfacial chemistry. Understanding these interactions provides a new framework for engineering controllable active materials and microscale machines, which have potential application in biomedical devices, drug-delivery, and environmental remediation.
