Tunable Self‐Emulsification Via Viscoelastic Control of Marangoni‐Driven Interfacial Instabilities

Interfacial instabilities in multicomponent fluidic systems are widespread in nature and in industrial processes, yet controlling their dynamics remains a challenge. Here, a strategy is presented to actively tune Marangoni-driven self-emulsification at liquid-liquid interfaces by harnessing fluid viscoelasticity. When a water-alcohol droplet spreads on an oil bath, a radial surface tension gradient induced by selective alcohol evaporation drives an interfacial instability, leading to the spontaneous formation of a dense 2D array of "daughter" droplets. It is demonstrated that introducing trace amounts of high-molecular-weight polymers, which introduces viscoelasticity, provides a robust means of controlling this process. Increasing viscoelasticity systematically suppresses the instability, resulting in a delayed onset of fragmentation and longer spreading fingers. By combining high-resolution experimental visualization and theoretical analysis, a quantitative relationship between the polymer concentration and the finger length prior to breakup is uncovered. These findings establish a predictive framework for designing viscoelastic interfacial materials with programmable dynamics and offer new opportunities for surface-tension-mediated patterning, emulsification, and fluidic control in soft material systems.