Particle Dispersion at Air–Water Interfaces: Role of Cleaning and Wettability

Many powders spread rapidly and axisymmetrically when deposited at an air-water interface, a behavior whose physicochemical origin may involve multiple contributing mechanisms, including thermal diffusion, capillary wave-driven motion, and surfactant-induced flows. Here, we experimentally investigate this nonequilibrium collective dispersion behavior at air-water interfaces using floating submillimetric particles. Our results show that rapid spreading, typically occurring at speeds on the order of centimeters per second, is suppressed once the particles are properly cleaned. This suppression is robust over a wide range of particle wettabilities, demonstrating that the rapid spreading is not driven by the release of interfacial energy once the particles are trapped at the interface. Instead, our results show that the fast spreading is caused by Marangoni stresses caused by the common presence of trace impurities in many powder substances. Reintroducing controlled amounts of surfactant restores the rapid dispersion, confirming the impurity-mediated nature of the flow. These findings clarify the transport mechanism for submillimetric particles and provide a foundation for future studies of Marangoni-mediated particle motion at interfaces.