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 theta proportional to beta(1/3), where beta 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 theta during retraction.