Comparative Study of PVDF- and PMHS-Modified ZnO Coatings: Effect on Leidenfrost Vapor-Film Stability and Falling Velocity

This study examines the effects of PVDF- and PMHS-modified ZnO coatings on vapor-film formation and persistence around heated copper spheres immersed in water, together with the cooling behaviour and average falling velocities of the specimens. Coating thermal behaviour was assessed by thermogravimetric analysis; surface morphology and topography were characterized by scanning electron microscopy and three-dimensional contact profilometry, with the maximum peak-to-valley height difference (Sz) used as a descriptive topographic parameter. Vapor films were observed on coated specimens from an initial temperature of 180 °C. For the Cu + ZnOPMHS specimen, the vapor film persisted for 16 s at 220 °C and more than 20 s at 240 °C, with locally measured film thicknesses of 0.88 and 0.89 mm, respectively. At 240 °C, Cu + ZnOPMHS exhibited an average falling velocity of 2.17 m/s, compared with 2.06 m/s for uncoated copper at the same initial temperature (approximately 5.3% higher). The difference relative to unheated uncoated copper was 17.29%, but this comparison combines temperature and coating effects. Vapor films were not monitored during the falling-velocity tests; consequently, the results do not establish that vapor-film persistence caused the observed velocity differences.