Self-Similarity in Surfactant-Driven Particle Dispersion at Air-Water Interfaces*

Many powders rapidly spread radially outward when they come into contact with the air-water interface. Previously, we showed that for non-Brownian (sub-millimetric) particles, this rapid spreading, typically at speeds of centimeters per second, can be significantly suppressed by appropriate cleaning. This suggests that fast and axisymmetric dispersion is primarily driven by surfactant-induced flow caused by common impurities [1].

In this work, we investigate the spreading behavior by varying the amount of surfactant mixed with clean particles. We track the particle spreading radius and the Marangoni-induced waves simultaneously. Our observations reveal that the particle spreading radius exhibits self-similar behavior across a wide range of surfactant concentrations. The Marangoni-induced wave fronts precede the particles, and the spreading dynamics can deviate from power laws once the wave hits the boundary. This indicates a strong coupling between Marangoni flow, capillary waves, and particles dispersion, which is crucial for understanding surfactant and particle transport at liquid interfaces.

[1] To, Kha-I., Rohit Vishwakarma, and Mahesh Bandi. "Effect of Particle Surface Properties on Dispersion at Air-Water Interface." Bulletin of the American Physical Society (2024).