Wetting Effect on Patterned Substrates

Abstract

A droplet deposited on a solid substrate leads to the wetting phenomenon. A natural observation is the lotus effect, known for its superhydrophobicity. This special feature is engendered by the structured microstructure of the lotus leaf, namely, surface heterogeneity, as explained by the quintessential Cassie-Wenzel theory (CWT). In this work, recent designs of functional substrates are overviewed based on the CWT via manipulating the contact area between the liquid and the solid substrate as well as the intrinsic Young's contact angle. Moreover, the limitation of the CWT is discussed. When the droplet size is comparable to the surface heterogeneity, anisotropic wetting morphology often appears, which is beyond the scope of the Cassie-Wenzel work. In this case, several recent studies addressing the anisotropic wetting effect on chemically and mechanically patterned substrates are elucidated. Surface designs for anisotropic wetting morphologies are summarized with respect to the shape and the arrangement of the surface heterogeneity, the droplet volume, the deposition position of the droplet, as well as the mean curvature of the surface heterogeneity. A thermodynamic interpretation for the wetting effect and the corresponding open questions are presented at the end.

Sliding Behavior of Droplets on a Tilted Substrate with a Chemical Step

“…On the other hand, the equilibrium contact angle on a heterogeneous surface can be described by Cassie’s equation. However, the range of applicability and the accuracy of Cassie’s equation have remained controversial for many years, especially in the case that the droplet size is comparable to the chemical heterogeneity . Gao and McCarthy proved by experiment that the local contact angle is determined by interactions of the liquid and solid in the vicinity of the triple-phase contact line, not by the overall contact area of liquid–solid. , Based on this knowledge, different modified Cassie equations in which the contact angles are determined by considering the local area fraction in the vicinity of the triple-phase contact line have been proposed, such as in the stripped or checkboard pattern surface. − Hence, when the gas–liquid transition zone is located on a heterogeneous surface, the equilibrium contact angle is described by the modified Cassie–Baxter equation cos nobreak0em0.25em⁡ θ normale normalq = r φ d nobreak0em0.1em⁡ cos nobreak0em0.25em⁡ θ normals 1 + ( 1 − φ normald ) cos nobreak0em0.25em⁡ θ normals 2 where θ s1 and cos θ s2 are the inherent static contact angles of each surface component and r is the “roughness” of the surface ( r = 1 for chemically inhomogeneous surfaces), φ d and 1 – φ d are the local area ratios of each surface component on the triple-phase contact area, respectively.…”

“…However, the range of applicability and the accuracy of Cassie's equation have remained controversial for many years, especially in the case that the droplet size is comparable to the chemical heterogeneity. 49 phase contact line, not by the overall contact area of liquid− solid. 50,51 Based on this knowledge, different modified Cassie equations in which the contact angles are determined by considering the local area fraction in the vicinity of the triplephase contact line have been proposed, such as in the stripped or checkboard pattern surface.…”

A thermodynamically consistent diffuse interface model for the wetting phenomenon of miscible and immiscible ternary fluids

“…2016; Wang et al. 2022; Wang, Wu & Nestler 2023). The scientific significance of the wetting phenomenon lies in its natural importance, such as lotus effect (Marmur 2004) and coffee-ring pattern (Deegan et al.…”

Bioinspired Flexible Wearable Sensor with High Self-Cleaning and Antibacterial Performance for Human Motion Sensing

“…Superhydrophobic materials are developed through in-depth observation and exploration of the microsurface structures of the “lotus effect” in nature, and possess hydrophobic, self-cleaning, and antibacterial characteristics. − The integration of superhydrophobic materials with flexible electronic devices in superhydrophobic flexible strain sensors has greatly overcome the limitations of sensor performance in various environments, particularly in applications related to human motion monitoring. Graphene, carbon nanotubes, carbon black, and Ag nanowires are frequently utilized in the construction of superhydrophobic micro/nano rough structures and conductive networks. − These nanomaterials possess high electrical conductivity and adhesion with the substrate while providing rough structures for superhydrophobicity.…”