Modeling of the finite boundary limit of evaporation flux in the contact line region using the surface plasmon resonance imaging

The present study aims to visualize a contact line region including evaporating thin film (ETF) and adsorbed film, and to suggest a new approach for estimating a finite evaporation flux based on a diffusion-limited model. The profiles that cover part of the ETF and the adsorbed film were obtained using the surface plasmon resonance imaging (SPRi) technique. The contact line region film profiles were obtained using an extrapolation method to find the inflection point, as the inward boundary condition. We successfully visualized the ETF at the sub-micron scale that exists near the droplet edge at which the fastest evaporation occurred. The width and thickness of the contact line region were determined. Also, the present study suggested a mathematical expression for the average value of evaporation flux over the area of the contact line region. The results showed that the average evaporation flux (AEF) decreases with an increase of droplet volume; this is because the local evaporation becomes greater as droplets become smaller. Moreover, this approach could be used to estimate the finite evaporation rate that occurs in the contact line region, as well as to impose a finite boundary limit instead of a singular value at the droplet edge.