Scanning Electron-microscopy of Liquid-metal Infiltration of Capillaries

The infiltration front of liquid metals (tin, lead, or copper) inside capillaries (iron, copper, or tungsten) after solidification of the infiltrant metal was examined using scanning electron microscopy. The existence of a precursor film ahead of the bulk liquid front was invariably revealed. The precursor film was seen to advance parallel to the solid walls of the capillaries, which indicates that the microscopic wetting angle of the film tends to zero. The microstructures observed at different stages indicate that the precursor film forms principally under the influence of the infiltrant vapour by a chain of elementary processes suggested to be adsorption, condensation, and surface migration of the condensed atoms on the solid walls. The vapour condensation observed in all the systems investigated is thus an important factor in the mechanism of capillary rise of liquid metals and the amount of condensation was observed to increase with increasing solubility of the solid in the liquid.