Research on mechanism of initial droplet formation for dropwise condensation

The mechanism of formation of initial droplets for dropwise condensation is still in suspense.An electron probe microanalyzer(EPMA)and scanning electron microscope(SEM)were used to scan the variation of the chemical compositions on the magnesium surfaces before and after the initial condensation.The results show that the oxygen content on the test surfaces increases with subcooling and condensation time obviously after the initial dropwise condensation.In order to further analyze the variation of oxygen content occurring on the whole surface or only at the partial locations,the reaction dynamic relationship of magnesium and condensate was founded.And the calculated result indicates that the area ratio occupied by condensate is less than 1.At the same time,the detected results of EMPA and SEM show that oxygen on the test surface distributes non-uniformly,the area ratios occupied by initial condensate calculated with the reaction dynamic equation agree well with those from the image analysis.Therefore,the initial condensate forms only in partial positions and doesn't cover the whole surface in the form of film.As a result,the mechanism of formation of initial droplets for dropwise condensation accords with the hypothesis of nucleation sites in nano-meter scale.