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 [STIX]x1D703∝ [STIX]x1D6FD1/3 , where [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 [STIX]x1D703 during retraction.