Shape Deformation of Liquid Droplets Falling in a Liquid Media

We experimentally investigate the hydrodynamics of initially spherical liquid droplets as they fall through a stationary continuous liquid phase under the action of gravity. Our study focuses on understanding the influence of droplet size, interfacial tension, particle concentration in droplets, and surfactant concentration in the continuous phase on droplet deformation and oscillation using an automated dosing system. Results reveal that increasing particle and surfactant concentrations led to greater droplet deformation but reduced oscillation. A viscoelastic emulsion interfacial layer is generated at the droplet interface at the high particle (above 4 wt.%) and surfactant (above 1 wt.%) concentrations and contribute to the stability of droplets, effectively minimizing their oscillation. Additionally, the oscillations decay over time where the droplets keep their flattened pancake shape due to the low interfacial tension, which cannot counteract the gravitational force. Interestingly, we observed that droplet deformation was independent of the Reynolds number, while the correlation with the Weber number varied with surfactant concentration. These findings have potential applications in various fields including microfluidics, emulsion formation, and industrial processes involving liquid-liquid interactions.