Optical Ring and Integral AUC Analysis for Determining the Apparent Contact Angle of Sessile Water Drops with FT-IR and XPS Analyses

A previous study by the authors established and experimentally validated a geometrical–optical method for determining the apparent contact angle of sessile water drops from the dark optical ring formed in a water–Mylar–glass system. The present work extends this approach to a practical comparison between two BoPET surfaces with different surface states: untreated Mylar 1 and factory corona-treated Mylar 2. Distilled water from the same batch was used on both surfaces, and the apparent contact angle was monitored at 5 min intervals throughout complete evaporation. The time-dependent contact-angle curves were evaluated by full-process area-under-the-curve (AUC) analysis and by a dimensionless apparent wettability parameter, H(t) = [1 + cos(θ)]/2, calculated from the contact angles. For Mylar 1, complete evaporation lasted 270 min and yielded an AUC of 11,023.06 ± 60.06 °·min, whereas for Mylar 2, the corresponding values were 245 min and 9322.70 ± 20.1 °·min. The initial apparent contact angle decreased from 67.2° for Mylar 1 to 61.1° for Mylar 2, while the normalized wettability parameter increased from 0.694 to 0.742. The reported XPS O/C atomic ratios were approximately 0.32 for Mylar 1 and 0.47 for Mylar 2, suggesting a higher relative surface oxygen content in Mylar 2. The results demonstrate the applicability of contact-angle monitoring combined with AUC analysis over the common observation interval for distinguishing the evaporation-driven apparent wetting behavior of untreated and corona-treated BoPET surfaces.