A validated method for contact angle measurement of low surface tension fluids using a modified Wilhelmy plate technique.

The wettability of a liquid on a solid surface is a critical parameter in numerous fields, from microfluidics to heat transfer applications. At the microscale, it directly governs droplet mobility on engineered surfaces, with implications for phase-change heat transfer, fluid transport, and interfacial phenomena. However, the characterization of wettability, commonly quantified through contact angle measurements, is particularly challenging for many fluids employed in thermal devices. These fluids usually have low surface tension resulting in small contact angles, and their normal boiling point is below ambient temperature, which precludes contact angles measurements in open environments. Here, we present a modified optical Wilhelmy method to address these problems by measuring contact angles inside a pressure vessel under saturated conditions. Particular attention was devoted to validating the technique against the standard sessile drop method at atmospheric conditions and to provide a rigorous uncertainty estimation. Results obtained with two low Global Warming Potential refrigerants, R1234ze(E) and R1233zd(E), demonstrate that the developed technique enables accurate and reproducible contact angle measurements, even below 10°. The technique provides a robust and practical tool for screening surface treatments and identifying those most effective for specific applications, including the promotion of dropwise condensation with refrigerants.