Numerical Modelling of a Binary Droplet on Solid Surface

We extended the numerical evaporation model for pure liquid sessile droplets to include a binary droplet on hydrophilic and hydrophobic surfaces. The extended model comprises a diffusion-limited species transport equation to estimate the vapour diffusion from the liquid–vapour interface. The model employs an axis-symmetric cylindrical coordinate using the finite element method. The species’ volatility produces different saturation concentrations in the vicinity of liquid–vapour interface. The concentration at the interface depends on the composition and activity of species in binary droplets. The respective activity coefficient is estimated using the AIOMFAC model. The experimental study shows the nonlinear variation of volume with time for different ethanol concentrations. The nonlinearity is due to ethanol, the highly volatile species, controls the initial evaporation, and the latter stage is controlled by water, the less volatile species. The comparative results of the present model show a nonlinear variation of drying droplet with time, which is consistent with the published experimental findings of the evaporating binary droplet.