Direct Insight into the Three-Dimensional Internal Morphology of Solid-Liquid-Vapor Interfaces at Microscale

Light and Magnetism Orchestrating Aquatic Pollutant-Degradation Robots in Programmable Trajectories

Interfacial floating robots have promising applications in carriers, environmental monitoring, water treatment, and so on. Even though, engineering smart robots with both precisely efficient navigation and elimination of water pollutants in long term remains a challenge, as the superhydrophobicity greatly lowers resistance for aquatic motion while sacrificing chemical reactivity of the surface. Here, a pollutant-removing superhydrophobic robot integrated with well-assembled iron oxide-bismuth sulfide heterojunction composite minerals, which provide both light and magnetic propulsion, and the ability of catalytic degradation, is reported. The motion velocity of the robot reaches up to 51.9 mm s-1 within only 300 ms of acceleration under the orchestration of light, and brakes rapidly (≈200-300 ms) once turn off the light. And magnetism extends the robot to work in broad range of surface tensions in any programmable trajectory. Besides, purification of polluted water is efficiently achieved in situ and the degradation efficiency exhibits eightfold enhancements under the effect of light-triggered photothermal behavior coupled with magnetic induction, overcoming the dilemma of efficient motion with catalytic superhydrophobicity. This strategy developed here provides guidelines for the explorations of high-performance smart devices.

Through-drop imaging of moving contact lines and contact areas on opaque water-repellent surfaces

A myriad of natural surfaces such as plant leaves and insect wings can repel water and remain unwetted inspiring scientists and engineers to develop water-repellent surfaces for various applications. Those natural and artificial water-repellent surfaces are typically opaque, containing micro- and nano-roughness, and their wetting properties are determined by the details at the actual liquid-solid interface. However, a generally applicable way to directly observe moving contact lines on opaque water-repellent surfaces is missing. Here, we show that the advancing and receding contact lines and corresponding contact area on micro- and nano-rough water-repellent surfaces can be readily and reproducibly quantified using a transparent droplet probe. Combined with a conventional optical microscope, we quantify the progression of the apparent contact area and apparent contact line irregularity in different types of superhydrophobic silicon nanograss surfaces. Contact angles near 180° can be determined with an uncertainty as low as 0.2°, that a conventional contact angle goniometer cannot distinguish. We also identify the pinning/depinning sequences of a pillared model surface with excellent repeatability and quantify the progression of the apparent contact interface and contact angle of natural plant leaves with irregular surface topography.

Prediction of the Lotus Effect on Solid Surfaces by Machine Learning

Superhydrophobic surfaces with the "lotus effect" have wide applications in daily life and industry, such as self-cleaning, anti-freezing, and anti-corrosion. However, it is difficult to reliably predict whether a designed superhydrophobic surface has the "lotus effect" by traditional theoretical models due to complex surface topographies. Here, a reliable machine learning (ML) model to accurately predict the "lotus effect" of solid surfaces by designing a set of descriptors about nano-scale roughness and micro-scale topographies in addition to the surface hydrophobic modification is demonstrated. Geometrical and mathematical descriptors combined with gray level cooccurrence matrices (GLCM) offer a feasible solution to the puzzle of accurate descriptions of complex topographies. Furthermore, the "black box" is opened by feature importance and Shapley-additive-explanations (SHAP) analysis to extract waterdrop adhesion trends on superhydrophobic surfaces. The accurate prediction on as-fabricated superhydrophobic surfaces strongly affirms the extensionality of the ML model. This approach can be easily generalized to screen solid surfaces with other properties.

Direct observation of wetting behavior of water drops on single micro-scale roughness surfaces of rose petal effect

HYPOTHESIS

It has been verified that a surface of single micro-scale structures with certain roughness could exhibit petal effect. That is, water drops with a contact angle larger than 150° would pin on the petal effect surface. It is conjectured that the water drop could pin on the single micro-scale roughness petal effect surface by totally infiltrating into spaces (or grooves) between micro-pillars.

EXPERIMENTS

An inverted optical microscopy system is synchronically applied in the process of advancing/receding contact angle (ACA/RCA) measurements to directly observe the wetting behavior of water droplets on hydrophobic patterned surfaces with regular arrays of square micro-pillars.

FINDINGS

A sequence of wetting behavior evolution, Wenzel → petal → pseudo-lotus → lotus, is observed on the hydrophobic patterned surfaces along with increasing surface roughness. It is interesting to observe a Cassie-Wenzel transition for water drops on a petal substrate during the ACA measurement (embedded needle method), leading to two ACAs, one before (in Cassie state) and one after the transition (in Wenzel state). Thus, the petal substrates have large contact angle hysteresis (CAH) (with both ACA and RCA in Wenzel state) to pin the water drop in Wenzel state. A Cassie-Wenzel transition is consistently observed during the evaporation process of water drops on pseudo-lotus substrates, leading to two RCAs: one in Cassie state and one in Wenzel state. The pseudo-lotus substrates have CAH (with both ACA and RCA in Cassie state) small enough to make water drops easily slide off.

What Can Probing Liquid-Air Menisci Inside Nanopores Teach Us About Macroscopic Wetting Phenomena?

Solid surfaces with excellent nonwetting ability have drawn significant interest from interfacial scientists and engineers. While much effort was devoted to investigating macroscopic wetting phenomena on nonwetting surfaces, the otherwise microscopic wetting has received less attention, and the surface/interface properties at the microscopic scale are not well resolved and correlated with the macroscopic wetting behavior. Herein, we first characterize the nanoscopic morphology and effective stiffness of liquid-air interfaces inside nanopores (nanomenisci) on diverse nonwetting nanoporous surfaces underneath water droplets using atomic force microscopy. Detailed three-dimensional imaging of the droplet-surface contact region reveals that water only slightly penetrates into the nanopores, allowing for quantitative prediction of the macroscopic contact angle using the Cassie-Baxter model. By gradually increasing the scanning force, we observe incrementally wetting of nanopores by water, and dewetting occurs when the force is lowered again, exhibiting reversible wetting-dewetting transitions. Further, nanoindentation measurements demonstrate that the nanomenisci show apparent elastic deformation and size-dependent effective stiffness at small indenting forces. Finally, we correlate the effective stiffness of the nanomenisci with the transition from complete rebound to partial rebound for impinging droplets on nanoporous surfaces. Our study suggests that probing the physical properties of the liquid-air menisci at the nanoscale is essential to rationalize macroscopic static and dynamic wetting phenomena on structured surfaces.

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

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 instruments-especially useful for large samples and field tests.

Direct Imaging of Superwetting Behavior on Solid-Liquid-Vapor Triphase Interfaces

A solid-liquid-vapor interface dominated by a three-phase contact line usually serves as an active area for interfacial reactions and provides a vital clue to surface behavior. Recently, direct imaging of the triphase interface of superwetting interfaces on the microscale/nanoscale has attracted broad scientific attention for both theoretical research and practical applications, and has gradually become an efficient and intuitive approach to explore the wetting behaviors of various multiphase interfaces. Here, recent progress on characterizing the solid-liquid-vapor triphase interface on the microscale/nanoscale with diverse types of imaging apparatus is summarized. Moreover, the accurate, visible, and quantitative information that can be obtained shows the real interfacial morphology of the wetting behaviors of multiphase interfaces. On the basis of fundamental research, technical innovations in imaging and complicated multiphase interfaces of the superwetting surface are also briefly presented.

Rutile TiO2 Flocculent Ripples with High Antireflectivity and Superhydrophobicity on the Surface of Titanium under 10 ns Laser Irradiation without Focusing

We report on the formation of rutile TiO₂ flocculent laser-induced periodic surface structures (LIPSSs) with high antireflectivity and superhydrophobicity on the surface of titanium under 10 ns 1064 nm laser irradiation without focusing. The center part of the Gaussian laser beam is used to deposit flocculent structure and the edge part used to produce LIPSSs. The melt and modification thresholds of titanium were determined first, and then, the melt and modification spot-overlap numbers, several responsible for the formation of flocculent structure and LIPSSs, were introduced. It is found that both the melt and modification spot-overlap numbers increase with an increase in laser fluence and spot-overlap number, contributing to the production of flocculent LIPSSs. LIPSSs are obtained with the modification spot-overlap number above 300, and the amount of flocculent structures increases with an increase in the peak laser fluence and spot-overlap number. Then, considering that the fine adjustment of the melt and modification spot-overlop numbers in one-time line scanning is quite difficult, the composite structure, of which both LIPSSs and flocculent structures are distinct, was optimized using laser line scanning twice. On this basis, a characterization test shows the sample full of the flocculent LIPSSs represents best antireflectivity with the value around 10% in the waveband between 260 and 2600 nm (advance 5 times in infrared wavelengths compared to the initial titanium surface), and shows the no-stick hydrophobicity with the contact angle of 160° and roll-off angle of 25° because of the pure rutile phase of TiO₂.

Peristome-Mimetic Curved Surface for Spontaneous and Directional Separation of Micro Water-in-Oil Drops

Separation of micro-scaled water-in-oil droplets is important in environmental protection, bioassays, and saving functional inks. So far, bulk oil-water separation has been achieved by membrane separation and sponge absorption, but micro-drop separation still remains a challenge. Herein we report that instead of the "plug-and-go" separation model, tiny water-in-oil droplets can be separated into pure water and oil droplets through "go-in-opposite ways" on curved peristome-mimetic surfaces, in milliseconds, without energy input. More importantly, this overflow controlled method can be applied to handle oil-in-oil droplets with surface tension differences as low as 14.7 mN m^-1 and viscous liquids with viscosities as high as hundreds centipoises, which markedly increases the range of applicable liquids for micro-scaled separation. Furthermore, the curved peristome-mimetic surface guides the separated drops in different directions with high efficiency.

In Situ Observation of Dynamic Wetting Transition in Re-Entrant Microstructures

Re-entrant microstructures exhibit excellent wetting stability under different pressure levels, but the underlying mechanism determined by wetting transition behavior at the microscale level remains unclear. We propose the "wetting chip" method for in situ assessment of the dynamic behavior of wetting transition in re-entrant microstructures. High sag and transverse depinning were observed in re-entrant microstructures. Analysis indicated that high sag and transverse depinning mainly influenced the stability of the structures. The threshold pressure and longevity of wetting transition were predicted and experimentally verified. The design criteria of wetting stability, including small geometry design, hydrophobic material selection, and sidewall condition, were also presented. The proposed method and model can be applied to different shapes and geometry microstructures to elucidate wetting stability.