Numerical study of droplet impingement on surfaces with hierarchical structures

Decorating a surface with pillars and grooves has been proved effective in enhancing its hydrophobicity. However, the poor stability of the non-wetting state is the main reason for limiting the practical applications of textured surfaces. In this study, a textured surface, with primary structures (taller) to improve hydrophobicity and secondary structures (shorter) to stabilize the non-wetting state, is proposed. The effects of the secondary structure, impingement position, and Weber number (We) on the deformation, penetration, and wetting state of a droplet on this surface are investigated by means of a three-dimensional direct numerical simulation (DNS). The results show that decorating the surface with hierarchical structures enhances both the surface hydrophobicity and the non-wetting stability due to the large breakthrough pressure brought by the secondary structures. The penetration depth and the emptying time decrease with the increase in the height of secondary structures. The impingement position, at the groove or the ridge, also plays a vital role in the non-wetting stability because of the difference in conversion between kinetic energy and surface energy. Surfaces with hierarchical structures require a significantly higher We to trigger the liquid penetration at the secondary structures We also influences the contact time and emptying time due to the abrupt change of surface roughness when liquid slides down the sidewall of the primary groove and then touches the secondary ridge. The two-step wetting transition found on the structured surface is well explained by the correlation of the Laplace's law, the Young's equation, and the Gibbs extension. It is seen that such hierarchical structures offer significant possibilities for the development of hydrophobic surfaces.