(90a) On Modeling Evaporation of Sessile Drops

We consider an evaporating sessile drop with non-pinned contact line. Although this problem is seemingly trivial and of considerable importance in a number of applications, it is still not well understood. In particular, there is a question of an appropriate evaporative mass flux model. One group of models considers that gas/vapor phase is the most important in determining the evaporative flux, while the other one concentrates on the liquid phase. We discuss the theoretical motivation for these models and show that they may lead to contradictory results regarding the temperature gradient along gas/liquid interface and correspondingly to Marangoni stresses which can be acting in the opposing directions. We then describe our experiments carried out with the goal of distinguishing between these models. These experiments are carried out with 4 different liquid/solid configurations (dionized water and isopropyl alcohol on silicon and copper substrates). The experimental results are compared with the numerical simulations of evaporating drops which include appropriate evaporative fluxes and material parameters. For each considered configuration we discuss the appropriateness of the considered evaporative models and show that, indeed, the calculated temperature gradients are model-dependent. The numerical simulations are then used to consider in more detail the instabilities which occur during evaporation, such as so-called ``octopus-shaped'' patterns that appear ahead of the contact line (Phys. Rev. Lett. 97, 186101 (2006)).