Quantitative analysis of contact line behaviors of evaporating binary mixture droplets using surface plasmon resonance imaging

1Contact line behavior of evaporating binary mixture droplet are examined quantitatively.2The surface plasmon resonance imaging technology is used for in-situ measurement of concentration of binary mixture droplets.3The weighted ethanol flux is quantitatively correlated with the receding velocity of the contact line. A sessile binary mixture droplet (BMD) has a relatively simple geometry; it is in the shape of a spherical cap. However, the contact line motion of a BMD during evaporation shows complex transient behavior that cannot be modeled using the current droplet evaporation model. The present study aims to quantitatively investigate the influence of initial ethanol concentrations on the contact line dynamics of an ethanol-water BMD. We use surface plasmon resonance imaging (SPRi) to simultaneously measure contact line motion and ethanol concentration during BMD evaporation. The visualization results show that non-monotonic contact line motions during evaporation can be subdivided into three stages: I) an initial spreading stage, II) a rapid sliding stage, and III) a moderate sliding stage. Results show behavior that is quite different from the typical motion of an evaporating DI water droplet, i.e., a simple pinning-depinning contact line motion. A weighted ethanol flux is newly introduced to consider the effect of spatial and temporal evaporative flux on the complex sliding motion of a BMD. Moreover, the weighted ethanol flux is quantitatively correlated with the receding velocity of the contact line in stage II.