Modeling Dropwise Condensation underneath Unidirectional Wettability   Graded Surfaces
  The dropwise condensation underneath a horizontal super-hydrophobic surface having unidirectional wettability gradient is modeled with implication to enhance the rate of condensation. The mathematical model includes nucleation, growth by vapor condensation and coalescence, and drop instability arising from force imbalances at the three-phase contact line. The wettability graded surfaces, formed by a variable surface energy coating, allow the micro-drop of condensate to slide from the hydrophobic to the hydrophilic region without the aid of gravity. The resulting decrease in the drop sliding size shifts the drop size distribution to smaller radii. Furthermore, this decrease in sliding size enhances the heat transfer rate during dropwise condensation. Simulation data has been compared with condensation patterns for a horizontal surface and inclined surface, both with zero wettability gradients. Results obtained show that a wettability graded surface can effectively control the condensation process by decreasing the cycle time of nucleation, growth and removal. 
Basant Singh Sikarwar and Abdelwadood Adil Daoud
cond-mat.mtrl-sci