Mechanism Of Initial Droplet Formation In Dropwise Condensation

The mechanism of formation of initial condensate droplets for dropwise condensation isstill in suspense. In order to prove the mechanism of formation of initial condensate droplets,magnesium was applied as the condensation surfaces in this study since it can react with hotwater (condensate) and thus leave marks of initial condensate state on the surface. In theexperiment, the subcooling and reaction time were controlled to realized the initialcondensation on the surfaces, and then an electron probe microanalyzer (EPMA) andscanning electron microscope(SEM)were used to scan the variation of the chemicalcompositions on the surfaces before and after the initial condensation. These consequenceswere used to deduce whether the initial condensate state is in nuclei or in film.In this paper, weighing method was used to detect the reaction rate of magnesium withhotwater. The activation energy and the pre-exponential factor of this reaction is 75.24KJ/mol and 3.05×10~7 respectively. The reaction rate of magnesium and hotwater can becalculated based on Arrhenius equation. The reaction rate was proved to be fast enough forthe oxygen content on the surface to be detected after a few second reaction. Therefore it isfeasible to apply magnesium as the steam condensation surface to record the reaction markson the surface for the later deduction of initial condensate state.Mechanically polishing and magnetic-control sputtering (MCS) were used to prepare themagnesium surfaces. The topography and surface roughness of the magnesium surfaces werecharacterized with the atomic force microscope and the results show that the averageroughness of the mechanically polishing surface is about 100nm, while that of the MCSsurface is about 23nm. The thickness of the magnesium film plated with MCS method wasalso measured with the electron probe microanalyzer, and the thickness of the plated film is23μm, which meant that silicon substrates were covered by magnesium completely. So thefilms of magnesium were feasible for this condensation experiment, and they can meet therequirements of surface scan by electron probe microanalyzer.In the experiment, the initial condensation process was realized by controlling thesubcooling and reaction time, and then, EPMA and SEM were used to scan the variation ofthe chemical compositions on the magnesium surfaces. The results showed that the oxygencontents on the test surfaces increased with subcooling and condensation time obviously afterthe initial condensation, and the oxygen on the test surface distributed non-uniformly.Moreover the results of the two methods were nearly identical. It can be seen that the method presented in the paper is reliable, so the reaction marks on the surface are the true display ofinitial condensate state. Also, the reaction dynamic equation was used to calculate the areaoccupied by initial condensate, which was well agreed with the measured results of EPMA.All these consequences indicate that the initial condensate forms in the nucleus state on solidsurfaces, not in the state of thin film.