Spreading dynamics and contact angle of completely wetting volatile drops

The spreading of evaporating drops without a pinned contact line is studied experimentally and theoretically, measuring the radius $R(t)$ of completely wetting alkane drops of different volatility on glass. Initially the drop spreads ( $R$ increases), then owing to evaporation reverses direction and recedes with an almost constant non-zero contact angle $\unicode[STIX]{x1D703}\propto \unicode[STIX]{x1D6FD}^{1/3}$ , where $\unicode[STIX]{x1D6FD}$ measures the rate of evaporation; eventually the drop vanishes at a finite-time singularity. Our theory, based on a first-principles hydrodynamic description, well reproduces the dynamics of $R$ and the value of $\unicode[STIX]{x1D703}$ during retraction.