High-Temperature Surface Energy and Interfacial Energy of the Molten FLiNaK-Highly Oriented Pyrolytic Graphite (HOPG) System from Selected Contact Angle Measurement.

Accurate high-temperature surface and interfacial energies are essential for understanding molten salt interactions with nuclear graphite. This study examined how initial pellet geometry and temperature affect the wetting of FLiNaK on highly oriented pyrolytic graphite (HOPG). Sessile-drop experiment using 1.5 m chunk and 3 mm pellet showed that precursor geometry influences transient contact angles and the time required to reach a common long-time value. Capillary forces dominated, while minor gravity effects for 3 mm pellets could not be excluded. Observed variations were mainly attributed to droplet shape evolution and contact-line pinning. The Neumann equation of state was used to estimate HOPG surface energy, yielding approximately 145 mN/m at 500 °C and 120 mN/m at 800 °C. Solid-liquid interfacial energies were consistently 30-40 mN/m below solid-gas values, indicating partial wetting dominated by physical interactions. These findings establish a reproducible method and show that contact angle convergence must be demonstrated before interfacial energy analysis.