Droplet sliding modulated by Marangoni effect on an inclined miscible liquid film

Droplet sliding is widely exploited in applications such as anti-icing, drug delivery, and microscale printing. Although the effects of surface properties on sliding dynamics have been extensively investigated, the role of interfacial flow and mass transfer at miscible interfaces remains poorly understood. In this work, we demonstrate that an aqueous droplet deposited on an inclined solid surface precoated with an ethanol thin film slides while preserving its morphology, despite the miscibility between the droplet and the underlying film. This behavior is attributed to Marangoni-driven interfacial flow, which prevents immediate coalescence and continuously transports ethanol from the film to the droplet–air interface, followed by ethanol diffusion into the droplet. Experiments reveal that the sliding velocity is regulated by the coupled effects of surface tension difference and viscosity contrast between the droplet and film. Strong Marangoni convection suppresses droplet motion, whereas enhanced ethanol diffusion into the droplet promotes sliding. By considering the conservation of ethanol mass, we derive a scaling law that predicts the characteristic sliding velocity. Furthermore, the temporal evolution of droplet motion reveals two distinct modes. Low-viscosity droplets undergo continuous trailing-edge breakup, preserving droplets' morphology and enabling steady sliding. In contrast, high-viscosity droplets undergo shear-induced elongation due to strong viscous stresses, increasing capillary resistance and causing a gradual decay in sliding velocity. This work advances the understanding of droplet dynamics coupled with interfacial flow and mass transfer at miscible interfaces.