The profile, apparent contact angle (ACA), contact angle hysteresis (CAH), and wetting state transmission energy barrier (WSTEB) are important static and dynamic properties of a large-volume droplet on the hierarchical surface. Understanding them can provide us with important insights into functional surfaces and promote the application in corresponding areas. In this paper, we establish three theoretical models (models 1-3) and the corresponding numerical methods, which were obtained by the free energy minimization and the nonlinear optimization algorithm, to predict the profile, ACA, CAH, and WSTEB of a large-volume droplet on the horizontal regular dual-rough surface. In consideration of the gravity, the energy barrier on the contact circle, the dual heterogeneous structures and their roughness on the surface, the models are more universal and accurate than the previous models. It showed that the predictions of the models were in good agreement with the results from the experiment or literature. The models are promising to become novel design approaches of functional surfaces, which are frequently applied in microfluidic chips, water self-catchment system, and dropwise condensation heat transfer system.
Double-roughness surfaces can be used to mimic lotus surfaces. The apparent contact angles (ACAs) of droplets on these surfaces were first calculated by Herminghaus. Then Patankar utilized the pillar model to improve the Herminghaus approach and put forward the formulas for ACAs calculation of the homogeneous double-roughness surfaces where the dual-scale structures and the bases were the same wettable materials. In this paper, we propose a numerical calculation method of ACAs on the heterogeneous double-roughness surfaces where the dual-scale structures and the bases are made of different wettable materials. This numerical calculation method has successfully enhanced the Herminghaus approach. It is promising to become a novel design approach of heterogeneous superhydrophobic surfaces, which are frequently applied in technical fields of self-cleaning, anti-icing, antifogging, and enhancing condensation heat transfer.
Engineering dropwise condensation on superhydrophobic surfaces (SHSs) has a wide range of applications from thermal management to water harvesting technologies. In this study, the hierarchical nanograssed micro‐V‐groove architectures are fabricated on copper surfaces for dropwise condensation and quick directional droplet departure. An optical microscope and a camera are adopted to investigate the spontaneous motions of condensate droplets on nanograssed micro‐V‐groove SHSs. The results show that the motions of condensate droplets on V‐groove slope surfaces appear quicker, more spontaneous, and more continuous, and reservoir droplets in V‐grooves appear more larger and more easy‐rolling. The phenomenon is explained by the critical radii of the rolling condensate droplets on V‐groove slope surfaces and maximum radii of the rolling reservoir droplets. In addition, water collection experiments are done, which shows that nanograssed V‐groove SHSs have better capacities of water collection than the nanograssed flat SHS. The smaller the V‐groove angle is, the better water collection capacity the SHS has. The synergistic cooperation between micro‐V‐grooves and nanograss contributes directly to the dropwise condensation process, which enhances vapor–liquid phase change efficiency. Exploiting such multiscale coupling effects can help us design optimal condensation surfaces for high performances of phase‐change cooling and water self‐catchment systems.
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