Mechanisms of fast particle dispersion at air-water interfaces

Kha-I To^1*, Shreyas Mandre², Mahesh Bandi¹

¹Nonlinear and Non-equilibrium Physics Unit, Okinawa Institute of Science and Technology, Okinawa, Japan
²Department of Engineering, University of Cambridge, UK

* Presenter:Kha-I To, email:tokhai@protonmail.com

Many powders rapidly spread radially outward when introduced to an air-water interface. In experiments with submillimeter-sized hollow glass spheres, we observed initial spreading typically at speeds of centimeters per second, with the radius following a self-similar behavior over time. While capillary waves generated upon particle impact were previously suggested as a driver for this dispersion, our experiments show that the behavior is primarily influenced by a surfactant-driven mechanism. Using systematic cleaning and particle-wettability variations based on silica surface chemistry, we found that dispersion does not significantly depend on particle wettability; rather, impurities and surfactants play a critical role.

In further experiments, we varied the surfactant concentration among cleaned particles, tracking both the particle spreading radius and the Marangoni-induced waves. Our findings reveal that the particle spreading exhibits self-similarity across a range of surfactant concentrations, with Marangoni wave fronts preceding particle movement. The dynamics deviate from power laws upon the wave reaching the boundary, indicating a strong coupling between Marangoni flow, capillary waves, and particle dispersion. We further discovered that interstitial water can independently induce similar particle dispersion, highlighting the importance of wetting dynamics upon contact. These insights are crucial for understanding surfactant and particle transport behaviors at liquid interfaces.

Keywords: Marangoni flow, Self-similarity, Particle-laden interfaces, Interfacial flows