Molecular dynamics study of water vapor condensation on a composite wedge-shaped surface with multi wettability gradients
B Xu and ZQ Chen, INTERNATIONAL COMMUNICATIONS IN HEAT AND MASS TRANSFER, 105, 65-72 (2019).
To find out whether the composite wedge-shaped surface with multi wettability gradients could accelerate the condensate drainage in a micro view, the model of water vapor condensation on such surface was built and studied by molecular dynamics (MD). The vertex angle and wettability gradient of wedge-shaped surface were investigated to obtain the best condition for accelerated removal of condensate. The wettability gradient of two surfaces was divided into three groups: Group A (hydrophilic - super hydrophilic), Group B (hydrophobic hydrophilic) and Group C (hydrophobic - super hydrophobic). The vertex angle was changed from 7 degrees, 10 degrees, 14 degrees to 20 degrees. Filmwise condensation (FWC) appeared in Group A and dropwise condensation (DWC) formed in Group B. However, there was no condensation in Group C. Although the FWC in Group A could drain, the drainage rate was slow. Only in Group B, the DWC could form water droplet and the movement of droplet could be controlled by the wedge-shaped surface, which was helpful for condensate drainage. When comparing the effect of different vertex angle, the smaller vertex angle could finish the FWC more quickly in Group A due to the smaller area of the super hydrophilic surface. In group B, small wedge-shaped surface would take more time to form main nuclei, while large wedge-shaped surface forming two nuclei also took more time to form a whole nanoscale droplet. In that case, DWC rate was quicker and condensation drainage was better in vertex angle of 14 degrees in these four conditions.
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