Local analysis of the contact region of an evaporating sessile drop

In experiments by Gu\'ena et al. (2007), a drop of perfectly wetting pure liquid evaporates from a non--heated substrate at a rate controlled by vapour diffusion. The drop spreads until reaching a radius $a$ determined by initial drop volume; the apparent contact line then reverses direction. The apparent contact angle measured at reversal was found experimentally to vary as $a^{-1/6}$ for $a<1$ mm (about); for larger drops $\theta$ decreases more strongly. Local analysis (Morris {\it J. Fluid Mech.} 739: 308--337. 2014) predicts that $\theta\propto a^{-1/6}$; for the smaller drops obeying the 1/6th rule, predicted values agree with experiment to within 10--30\%. Though the behaviour of drops smaller than the capillary length thus appears to be understood, that of larger drops is not.