The wetting transition of low surface tension droplet on the special-shaped microstructure surface

The hydrophobic surfaces have been widely used in many field due to their excellent properties. However, the droplet with low surface tension can easily undergo wetting transition and highly infiltrate on a surface with conventional structure, so it is difficult to achieve the hydrophobic state. Preliminary bionics studies showed that the microstructure surface with a reentrant angle could suspend the droplet with low surface tension to achieve the hydrophobic property. But, the wetting mechanism of droplet on the reentrant microstructure surface was not clear. The wettability of the droplet on the reentrant microstructure surface was studied with the numerical method of VOF-CSF in this paper. It was found that the reentrant structure could effectively inhibit the occurrence of wetting transition compared with the conventional columnar-shaped structure, so that the hydrophilic droplet remained suspended and showed a large apparent contact angle. The mechanical model and the relative surface free energy of the droplet in the wetting process on the reentrant microstructure surface were analyzed theoretically. It was found that the geometrical parameters of reentrant structure affected the breakthrough pressure and energy barrier, and then affected the wetting transition. The results showed that the performance of doubly-reentrant structure was better than that of singly-reentrant structure in inhibiting wetting transition. For the reentrant structure with larger width and smaller spacing, the advantage of inhibiting wetting transition was more obvious, and the effect of width gradually disappeared with the increase of spacing. For the droplet that has been suspended on the reentrant structure surface, a slight change of structure width will greatly affect the stability of the small droplet.