Unraveling friction forces of droplets on a non-wetting surface

Understanding the friction forces acting on droplets moving across non-wetting surfaces is essential for advancing applications in fluid transport and microfluidics. In this study we quantify droplet friction on both superhydrophobic and superheated (Leidenfrost) substrates using two complementary experimental approaches: inclined plane dynamics and cantilever force sensing. We identify distinct friction regimes governed by contact-line pinning, viscous dissipation, and aerodynamic drag over a broad range of velocities and viscosities. A force-based scaling law captures the transition between these regimes as a function of the capillary number, integrating contact-line effects, internal dissipation, and aerodynamic contributions. Our model is further validated through white light interferometry and high-speed reflective interference contrast microscopy, which reveal the structure and evolution of the thin air or vapor layers beneath the droplets. These findings offer a comprehensive framework for predicting droplet behavior on non-wetting surfaces and inform the design of surfaces for reduced friction in engineering and microfluidic systems.