Brightness of Microtrench Superhydrophobic Surfaces and Visual Detection of Intermediate Wetting States

Abstract

For a superhydrophobic (SHPo) surface under water, the dewetted or wetted states are easily distinguishable by the bright silvery plastron or lack of it, respectively. However, to detect an intermediate state between the two, where water partially intrudes the surface roughness, a special visualization technique has been needed. Focusing on SHPo surfaces of parallel microtrenches and considering drag reduction as a prominent application, we (i) show the reliance on surface brightness alone may seriously mislead the wetting state, (ii) theorize how the brightness is determined by water intrusion depth and viewing direction, (iii) support the theory experimentally with confocal microscopy and CCD cameras, (iv) present how to estimate the intrusion depth using optical images taken from different angles, and (v) showcase how to detect intermediate states slightly off the properly dewetted state by simply looking. The proposed method would allow monitoring SHPo trench surfaces without bulky instrumentsespecially useful for large samples and field tests.

Superhydrophobic drag reduction in turbulent flows: a critical review

“…Generally, sharp edges, including reentrant edges (Xu et al 2020b), help keep the interface pinned on top of the roughness feature longer under unfavorable conditions until the onset of wetting starts to compromise the slip effect. Recently, Yu et al (2021) developed a convenient (only with naked eyes) observation technique that can detect the depinning of interface, not just the existence of plastron, on microgrooves, making the monitoring of the plastron status throughout flow experiments more practical.…”

Estimation of surface free energy at microstructured surface to investigate intermediate wetting state for partial wetting model

“…Intermediate-like wetting states have been qualitatively investigated through theoretical analyses 36,37 and experimental observations. [38][39][40][41][42] However, few studies have quantitatively evaluated intermediate wetting states, owing to difficulties in performing precise measurement. To overcome the challenges faced in performing a quantitative investigation, we have previously employed electrochemical impedance spectroscopy (EIS) to estimate the effective wetting area for the intermediate wetting state.…”

Sustainability of the plastron on nano-grass-covered micro-trench superhydrophobic surfaces in high-speed flows of open water

“…In other words, the original notion of SHPo drag reduction is valid as far as the plastron remains in a good shape. The tortuous path to the current state of knowledge also indicates how difficult yet important it is to accurately monitor the state of the plastron during experimental studies of SHPo drag reduction, leading to the two-camera observation technique by Yu et al (2021). Focusing on longitudinal micro-trench SHPo surfaces, which have been the most effective for drag reduction (Park et al 2021) and to help the design of SHPo surfaces capable of reducing the drag for watercraft, this paper aims to understand the range of trench geometries that can maintain a pinned or slightly degraded plastron, which has its air-water interfaces pinned or slightly depinned at the top edges over the entire or nearly entire trench length so that much of the pristine slip capability is preserved, in high-speed open-water flows.…”