Gravity‐driven double emulsion droplet formation: Interfacial dynamics and stability control

Abstract

Abstract Gravity‐driven double emulsion droplet formation in a hydrocarbon‐ammonia column is industrially important yet poorly understood. By combining high‐speed imaging with adaptive‐mesh‐refinement volume‐of‐fluid simulations, a distinct oil‐coating route is revealed: a preformed hydrosol droplet crosses the macroscale oil–aqueous interface and entrains oil to form the outer shell, whose trailing oil bridge undergoes necking and pinch‐off. During oil‐coating, reserved droplet/wake momentum and constant gravity promote penetration, whereas capillarity, buoyancy, and viscous dissipation resist it. Successful encapsulation requires penetration to persist until sustained necking begins; earlier reversal causes rebound‐type failure. By affecting driving contribution or resistance, increasing hydrosol density or decreasing oil viscosity facilitates oil‐coating, while droplet size reduction narrows hydrodynamic margin for success. As practical tuning approaches, replacing paraffin with lower‐viscosity kerosene raised consecutive successes from <30 to >500; locally enriching surfactant to further mitigate capillary constraint enabled 212 consecutive formations for size‐reduced inner droplets (2.5 mm equivalent diameter).