Sub-Nanometer Scale Investigation of In Situ Wettability Using Environmental Transmission Electron Microscopy

Abstract Hypothesis Contact angle measurements alongside Young’s equation have been frequently used to quantitatively characterize the wettabilities of solid surfaces. In the literature, the Wenzel and Cassie-Baxter models have been proposed to account for surface roughness and chemical heterogeneity, while precursor film models have been developed to account for stress singularity. However, the majority of these models were derived based on theoretical analysis or indirect experimental measurements. We hypothesize that sub-nanometer-scale in situ investigations will elucidate additional complexities that impact wettability characterization. Experiments To develop further insights into in situ wettability, we provide the first direct experimental observation of fluid-solid occupancies at three-phase contacts at sub-nanometer resolution, using environmental transmission electron microscopy. Findings Considering the partially spreading phenomenon and capillarity, we provide an improved physics-based interpretation of measuring the sub-nanometer-scale contact angle at the inflection point of the fluid-fluid interface. The difference between this angle and the commonly-used apparent one at a lower resolution is also discussed. Furthermore, we provide direct experimental evidence for the density differences between the adsorbed wetting film and the bulk wetting phase. For the effect of surface roughness, the applicability of the Wenzel model is discussed based on the observed in situ solid-fluid occupancies.