Effect of Surface Protrusions on Gravitational Settling of Particles at Immiscible Liquid Interfaces

The gravitational settling of particles through an immiscible liquid-liquid interface is ubiquitous in industrial and environmental applications. However, the influence of particle surface protrusions on interfacial dynamics remains unclear. In this study, we investigated how sharp spikes affect the gravitational settling of a sphere in an interface of silicone oil and aqueous solution. We varied the Bond number and relative particle density ratio for different spike lengths and packing densities. High-speed imaging recorded sphere motion and interfacial deformation. It was found that a smooth sphere may either float at the interface or sink and entrain a column of lighter silicone oil into the heavier aqueous solution. When spikes are present, the sphere entraps more oil, creating a thicker interstitial oil film and exerting a greater upward force, extending the floating domain of particles. Interestingly, sharp spikes do not enable an immediate contact between the sphere and lower fluid upon impact as one might expect. The oil film thickness is proportional to spike length and persists longer during floating conditions. Scaling analysis establishes a new floating-sinking transition boundary for spiky spheres, consistent with experimental results.