Directional rebounding of a droplet impinging hydrophobic surfaces with roughness gradients. (May 2021)

• Record Type: Journal Article
• Authors: Yuan, Zhicheng; Matsumoto, Mitsuhiro; Kurose, Ryoichi
• Is Part Of: International journal of multiphase flow. Volume 138(2021)
• Publication Date: 2021-05
• Subjects: Hydrophobic surface -- Roughness gradient -- Direct numerical simulation -- Droplet impingement -- Directional rebound; Multiphase flow -- Periodicals; Écoulement polyphasique -- Périodiques; Multiphase flow; Periodicals; 620.1064
• DOI: 10.1016/j.ijmultiphaseflow.2021.103611
• Languages: English
• ISSNs: 0301-9322

Abstract

Highlights: The rebound direction of droplets is manipulated by surfaces with roughness gradients. Three types of rebound behavior are influenced by Weber number ( We ), groove width, and groove depth. The unbalanced Young's force and the wetting state dominate the droplet behavior. The rotating motion (counterclockwise) of the leaving droplet is observed. Abstract: Surfaces with a wettability gradient, which require no external energy input, play a significant role in controlling and manipulating the self-transport of liquid drops. This work numerically investigates the directional rebound behavior of water droplets on surfaces with wettability gradients obtained by gradually altering the groove width. The results show that three types of droplet rebound behavior (vertical bouncing, following, or against the roughness gradient) and the rotating motion (counterclockwise), which are dominated by the combined effects of the unbalanced Young's force and the wetting state, are affected by the Weber number ( We ), groove width, and groove depth. Droplets remain in the Cassie state and rebound following the roughness gradient for a small We, small groove width, and larger groove depth. By contrast, when the Cassie and Wenzel states are both present, droplets are partially arrested by the ridges and grooves and rebound against the roughness gradient. Additionally, vertical rebounding behaviors are observed at extremely small W e owing to the low contact-angle hysteresis, and at critical W e because the Young's force and capillary emptying process are balanced. Therefore, both the unbalanced Young's force and wetting state should be considered when investigating the bounce behavior of droplets impinging surfaces with a roughness gradient. The results provide important insight into the design of wettability gradient surfaces for controlling droplet transport. Graphical abstract: Weber number ( We ), groove width, and groove depth play vital roles in determining the dynamic pressure PD, the capillary pressure PC, and the effective water hammer pressure PWH, which together dominate the wetting state and rebound behaviors of droplets impinging textured surfaces. Detailly, three types of droplet rebound behavior (vertical bouncing and following or against the roughness gradient) and the rotating motion (counterclockwise) are demonstrated, which are driven by the combined effects of the unbalanced Young's force and wetting state such as Cassie state, Transition state, and Wenzel state. Image, graphical abstract