Self-lubricated spreading of nonpolar, surfactant-laden droplets

While surfactants are known to affect fluid–fluid interfaces, their impact on solid–liquid interfaces is an open problem. Here, we show that surfactants carried by a spreading nonpolar droplet can dynamically alter the solid–liquid interfacial energy, leading to a new spreading regime beyond the classical Tanner’s law and known Marangoni regimes. We develop a theoretical framework that combines the new spreading mechanism, governed by the solid–liquid interfacial energy gradient, together with capillarity. Experiments across twelve distinct combinations of nonpolar solvents, surfactants, and substrates confirm our theoretical predictions for the transition from Tanner’s law to the newly uncovered spreading regime. Our findings provide predictive control for applications in coatings, printing, microfluidics and surface engineering.