Top-View Method as a Robust Alternative for Contact Angle Measurement.

This study validates the top-view method for contact angle measurement, based on the spherical cap assumption, as a practical alternative to conventional side-view techniques. It is particularly useful when side-view imaging is challenging, such as on rough, deformable, irregular, or confined areas where baseline visibility, meridian clarity, or drop symmetry is compromised. To address these challenges, this study details the simultaneous acquisition of top and side-view images of sessile drops along with precise drop volume via drop mass measurements. The contact angles were then deduced from drop volume and contact area based on the spherical cap assumption. Contact angles of water and formamide (1-40 μL) on poly(methyl methacrylate) (PMMA) and Teflon surfaces, covering both wetting (θs < 90°) and nonwetting (θs > 90°) regimes, were measured. The results demonstrate that the top-view method yields statistically comparable contact angle values to side-view methods for small drops (1-10 μL). The study also introduces a drop projection index (DPI), a dimensionless parameter analogous to the bond number, to quantify gravity-induced effects in top-view imaging. The results showed that for θs < 90°, DPI was ∼0.15 for both formamide and water on PMMA, while for θs > 90°, DPI ranged from ∼0.2 (formamide on Teflon) to ∼0.3 (water on Teflon). Findings in this study provide a foundation for refining the applicability of the top-view method based on the spherical cap assumption and adapting it to a wide range of systems.