Thermodynamic Analysis of the Effect of the Hierarchical Architecture of a Superhydrophobic Surface on a Condensed Drop State

Condensed drops usually display a Wenzel state on a superhydrophobic surface (SHS) only with microrough architecture. while Cassie drops easily appear on a sui lace with micro-nano hierarchical roughness. The mechanism of this is not very clear It is important to understand how the hierarchical structure affects the states of condensation drops so that a good SHS can be designed to achieve the highly efficient dropwise condensation In this study, the interface free energy (I H) of a local condensate, which conies from the growth and combination of numerous initial condensation nuclei, was calculated during its shape changes from the early flat shape to a Wenzel or Cassie state The final state of a condensed drop was determined by whether the IFE continuously decreased or a minimum value existed The calculation results indicate that the condensation drops on the surface only with microroughness display a Wenzel state because the IFE curve of a condensed drop first decreases and then increases, existing at a minimum value col responding to a Wenzel drop On a surface with proper hierarchical roughness, however, the interface energy curve of a condensed drop will continuously decline until reaching a Cassie state. Therefore, a condensed chop on a hierarchical roughness surface can spontaneously change into a Cassie state Besides, the states and apparent contact angles of condensed drops on a SHS with different structural parameters published in the literature were calculated and compared with experimental observations The results show that the calculated condensed drop states are well-coordinated with experimental clarifications We can conclude that micro-nano hierarchical roughness is the key structural factor lot sustaining condensed drops in a Cassie state on a SHS