Effects of surface air injection on the air stability of superhydrophobic surface under partial replenishment of plastron

Abstract

Frictional drag reduction using superhydrophobic (SHPo) surface has attracted great attention due to its potential for practical application. Air plastron (i.e., pockets) trapped between structures in a submerged SHPo surface serves a crucial role in the drag reduction effect. However, the air plastron on SHPo surface can easily be depleted by various factors, resulting in the deterioration of the drag reduction performance. This study proposed a surface air injection method to resolve the depletion problem and explored its effect on the enhancement of the air stability of SHPo surface as a controllable strategy under partial replenishment conditions in which replenishment rate is less than the depletion rate. An air injection layer is added to a ridged multi-layered SHPo (ML-SHPo) surface to supply air through the surface. The dynamic behavior of the air plastron on the surface is directly visualized by an X-ray imaging technique. The temporal evolution of depletion length on the ridged ML-SHPo surface is monitored under laminar flows to understand the underlying basic physics of the enhanced air stability caused by surface air injection. The depletion rate of air plastrons on SHPo surfaces depended on the nondimensional air injection rate ( Cq). The dynamic behavior in the air depletion regime is closely associated with flow condition and Cq. The stability of air plastron is enhanced by 12%-87% as the Cq value increases. In addition, a simple scaling relationship between the depletion rate and Cq is proposed to predict the depletion rate of air plastrons (longevity) on ridged SHPo surfaces.

The Challenge of Superhydrophobicity: Environmentally Facilitated Cassie–Wenzel Transitions and Structural Design

“…Reproduced with permission. [ 39b ] Copyright 2022, American Chemical Society. d) Digital images demonstrating the regeneration of underwater superhydrophobicity through the generation of hydrogen gas bubbles in the dark and light.…”

“…[ 38 ] Once cavitation is depleted, the entire system enters a W state, leading to a loss in application performance. Cavitation can be recovered through active strategies, including air injection, [ 37 , 39 ] photocatalysis, [ 40 ] and electrolysis (Figure 1c–f ). [ 41 ] The passive strategy influences the presence time of cavitation by controlling the substrate surface morphology (e.g., Salvinia leaf can maintain a longer C state underwater due to its unique egg‐beater structure).…”

Self-Driven Gas Spreading on Mesh Surfaces for Regeneration of Underwater Superhydrophobicity

“…Under real-world application conditions, factors such as pressure, − gas diffusion, or flow shear , can deplete the plastron. To confront these challenges, active approaches have been designed to restore plastrons, including gas injection, ,− heat-induced boiling, electrolysis, − chemical reactions, − and control of gas solubility. − Nonetheless, the implementation of these techniques often complicates surface design and presents challenges for large-scale applications. Intermittent or continuous energy input also increases costs, as some methods involve integrating electrolysis or heating devices, ,, raising concerns about energy conversion efficiency.…”

“…In addition, the spreading of the gas on superhydrophobic surfaces is a prerequisite for plastron restoration. 12,32,36,43 The gas exists in four forms on the surface, namely, bubble, blister, hemiwicking blister, and plastron. 43 When existing in the form of plastron, the gas is more stable and less susceptible to being sheared away by the flow.…”

“…Intermittent or continuous energy input also increases costs, as some methods involve integrating electrolysis or heating devices, ,, raising concerns about energy conversion efficiency. In addition, the spreading of the gas on superhydrophobic surfaces is a prerequisite for plastron restoration. ,,, …”