Direct measurement of selective evaporation of binary mixture droplets by dissolving materials

We investigate experimentally and theoretically how a droplet of a binary mixture evaporates when placed on a solid substrate. Our focus is the limit at which the two liquid components have different vapour pressures. Using physicochemical effects, we directly visualize the selective evaporation of the more volatile component and so document the space and time dependence of the chemical distribution in the droplet. In particular, we observe that a mixture consisting of an organic solvent and deionized water dissolves suspended fluorescent polystyrene particles if the lower volatility organic solvent reaches a critical concentration. Consequently, we show that for a small contact angle (theta < pi/2) the suspended polystyrene particles begin to disappear from near the contact line, which indicates that the volatile component, here water, evaporates rapidly compared to the other component(s). Finally, we show that a diffusion-dominated model for evaporation of a binary mixture can predict well the experimental results where convective and diffusive mixing effects are negligible, in which case there is significant chemical segregation in the drop.