Effects of reaction-induced viscosity variation on droplet impact dynamics

The dynamic mechanism of reactive droplet impact is significantly different from that of inert droplets. The impact and spreading dynamics of reactive polyurea droplets on a smooth hydrophilic glass surface is investigated, focusing on the interplay between fluid mechanics and in situ curing reactions. Using high-speed imaging, we identify two distinct stages in the droplet dynamics: a pinning stage and a spreading stage. During the pinning stage, the droplet's contact line remains stationary while internal flow and surface tension drive tip retraction. We show that the tip retraction process provides a sensitive probe of evolving viscosity, which is modeled by using third-order mass-action kinetics. In this stage, crucially, we demonstrate that the variation in viscosity drives enhanced viscous dissipation and results in the prolonged pinning. In the spreading stage, the chemically induced viscosity increase dominates, leading to a contact line radius evolution that follows a R ∼ t1/8 scaling law. Unlike inert droplets, which dissipate energy primarily through rebounds, receding-height increase, and oscillations, reactive polyurea droplets exhibit time-dependent dissipation driven by in-flight and post-impact curing. This work reveals the critical role of reaction-driven viscosity enhancement in shaping droplet impact behavior and offers new insight into the design and control of reactive spraying processes.