Confocal Microscopy for Capillary Film Measurements in a Flat Plate Heat Pipe

This paper aims to show how confocal microscopy can be useful for characterizing menisci in a flat plate heat pipe made of silicon. The capillary structure is made of radial microgrooves whose width decreases from the periphery to the center of the system. A transparent plate is used to close the system and allow visualizations. The confocal method allows measuring both the liquid film shape inside the grooves and the condensate films on the fins. The film thickness is lower than $10μm$⁠. The measurements show that the condensate film forms a drop connected to the meniscus in the grooves but their curvatures are reversed. As a result, a very thin region shall exist where the liquid formed by condensation is drained to the grooves. The drop curvature radius decreases from the condenser to the evaporator like the meniscus radius in the grooves. Therefore, a small part of the liquid is drained by the fins from the evaporator to the condenser. Furthermore, the condensate film covers a large part of the system and can also be in contact with the evaporator at high heat fluxes.