Evaporating drops of alkane mixtures

Alkane mixtures are model systems where the influence of surface tension gradients during the spreading and the evaporation of wetting drops can be easily studied. The surface tension gradients are mainly induced by concentration gradients, mass diffusion being a stabilising process. A remarkable observation is that, at given initial volume, a drop of mixture often evaporates faster than one of the most volatile alkane. Depending on the relative concentration of the mixture, a rich pattern of behaviours is obtained, which is qualitatively the same for the three mixtures investigated (heptane–octane, hexane–octane and heptane–nonane). There is an intermediate range of concentration where a dry ring develops inside the drop. For lower concentrations of the most volatile alkane, it disappears completely before the end of the evaporating process. For larger concentrations, the drop is still a mixture till the end of its life. The coupling between evaporation, local concentration and drop shape will probably require a numerical treatment of the dynamic equations.