Effects of surface free energy and nanostructures on dropwise condensation

Effects of surface free energy and nanostructures on dropwise condensation (DWC) were investigated experimentally. The oxidation and etching methods were applied to prepare the nanostructures on the copper substrates. Self-assembled monolayers coatings of n-octadecyl mercaptan were prepared on mirror-polished (SAM-2) and the nanostructured (SAM-1) copper substrates to promote the DWC. Experimental data presented that the nanostructure surface SAM-1 did not improve the dropwise condensation heat-transfer performance so much as to be expected for increasing the possible condensing surface area, compared to the mirror-polished SAM-2. This may be caused from the nanostructure's retardance to the condensate film. However, the incorporating effects of surface free energy and nanostructures of the condensing surface were found to play a really important role in the condensation heat-transfer enhancement. The fractal-like structures and the voids on SAM-1 surface were filled with condensate in the condensing process which resulted in a composite condensing surface of condensate and copper regions. Thus the average surface free energy of this composite condensing surface is larger than that of SAM-2 surface. The surface free energy difference between the condensate and the condensing surface of SAM-1 is less than that of SAM-2, so are the heat-transfer coefficients. The condensation heat-transfer is enhanced by a factor of 3 for SAM-2 surface, due to an increase of surface free energy difference between the condensate and condensing surface.