On the universality of Marangoni-driven spreading

When two liquids of different surface tensions come into contact, the liquid with lower surface tension spreads over the other. Here we measure the dynamics of this Marangoni-driven spreading in the drop-drop geometry, revealing universal behavior with respect to the control parameters as well as other geometries (such as spreading over a flat interface). The distance $L$ over which the low-surface-tension liquid has covered the high-surface-tension droplet is measured as a function of time $t$, surface tension difference between the liquids $\Delta\sigma$, and viscosity $\eta$, revealing power-law behavior $L(t)\sim t^{\alpha}$. The exponent $\alpha$ is discussed for the early and late spreading regimes. Spreading inhibition is observed at high viscosity, for which the threshold is discussed. Finally, we show that our results collapse onto a single curve of dimensionless $L(t)$ as a function of dimensionless time, which also captures previous results for different geometries, surface tension modifiers, and miscibility. As this curve spans 7 orders of magnitude, Marangoni-induced spreading can be considered a universal phenomenon for many practically encountered liquid-liquid systems.