Wettability and Stability of Wetting States for the Surfaces with Reentrant Structures

Controlling surface wettability has important application prospects in many aspects. Both the Cassie-Baxter equation and Wenzel equation explain the effects of surface chemical composition and roughened texture on surface wettability. Recently, the third factor - reentrant surface curvature - has been proposed; nonetheless, its impact based on theoretical calculation has never been investigated. Based on the theory proposed by Marmur, herein, the overlap fraction φr was introduced to explain the effect, and three novel necessary conditions for the existence of the Cassie state were drawn. This theory allows us to quantitatively evaluate the influence of various reentrant structures on the wettability and stability of the Cassie state. In addition, the simulation results of two typical structures of paraboloid and square T-shaped were demonstrated. For the paraboloid structures, the influence law of spatial period on the apparent contact angle of the Cassie state (CAC) and GT∗ was found to be opposite. In contrast, for the square T-shaped structures, the GT∗ increased gradually, but the CAC remained constant with the increase of φr. Although the reentrant structures provide us with the possibility to construct the superamphiphobic surface, the stability of the Cassie state of the square T-shaped structure is extremely poor, and a larger φr carries a risk of structural mechanical strength reduction. Finally, three reentrant structures reported in the literature were simulated to verify the correctness of this theory. Excessive pursuit of one of the two properties results in a decrease in the combination property. These results may be potentially helpful for the design of superamphiphobic surfaces in real applications.