  Droplet spreading is ubiquitous and plays a significant role in liquid-based
energy systems, thermal management devices, and microfluidics. While the
spreading of non-volatile droplets is quantitatively understood, the spreading
and flow transition in volatile droplets remains elusive due to the complexity
added by interfacial phase change and non-equilibrium thermal transport. Here
we show, using both mathematical modeling and experiments, that the wetting
dynamics of volatile droplets can be scaled by the spatial-temporal interplay
between capillary, evaporation, and thermal Marangoni effects. We elucidate and
quantify these complex interactions using phase diagrams based on systematic
theoretical and experimental investigations. A spreading law of evaporative
droplets is derived by generalizing Tanner's law (valid for non-volatile
liquids) to a full range of liquids with saturation vapor pressure spanning
from 10^1 to 10^4 Pa and on substrates with thermal conductivity from 10^{-1}
to 10^3 W/m/K. Besides its importance in fluid-based industries, the
conclusions also enable a unifying explanation to a series of individual works
including the criterion of flow reversal and the state of dynamic wetting,
making it possible to control liquid transport in diverse application
scenarios.
