Flow near the contact line of an evaporating drop

Evaporation of water droplets with a pinned contact line is investigated in the entire range of possible contact angles (up to 150$^\circ$). We measured the evolution of the mass and the contact angle of droplets on superhydrophobic carbon nanofiber substrates. The experimental data are in quantitative agreement with a diffusion-based model, and can be collapsed onto a single universal curve for all droplet sizes and initial contact angles. However, the nature of the flow inside the drop near the contact line has remained unclear: it has been argued that the lubrication approximation does not apply in the contact-line region, and that the Marangoni effect can overcome the evaporation-driven outward flow. Here, we resolve these questions by analytically solving the Stokes flow field in a wedge geometry, imposing the evaporative flux as a boundary condition.