Liquid-Repellent Surfaces

Fluid Control on Bionics-Energized Surfaces

Engineered surfaces play a vital role in various fluid applications, serving specific functions such as self-cleaning, anti-icing, thermal management, and water energy harvesting. In nature, biological surfaces, particularly those displaying physiochemical heterogeneity, showcase remarkable fluid behaviors and functionalities, offering valuable insights for artificial designs. In this Review, we focus on exploring the fascinating fluid phenomena observed on natural biological surfaces and the manipulation of fluids on bioengineered surfaces, with a particular emphasis on droplets, liquid flows, and vapor flows. We delve into the fundamental principles governing symmetric fluid motion on homogeneous surfaces and directed fluid motion on heterogeneous surfaces. We discuss surface design strategies tailored to different fluid scenarios, outlining the strengths and limitations of engineered surfaces for specific applications. Additionally, the challenges faced by engineered surfaces in real-world fluid applications are put forward. By highlighting promising research directions, we hope to stimulate advancements in bioinspired engineering and fluid science, paving the way for future developments.

Shear Evolution and Slippage of the Liquid-Liquid Interface over a Liquid-Infused Surface: A Many-Body Dissipative Particle Dynamics Study

The liquid-liquid interface (LLI), which is the key to cause flow slippage and thus promote drag reduction of liquid-infused surfaces (LISs), does suffer from the action of flow shear. In the current study, the transverse many-body dissipative dynamics simulation method is applied to explore the shear evolution of LLI and the corresponding slippage over a periodically grooved LIS. Results show that a relatively small shear rate only induces a slight deformation of LLI and the corresponding effective slippage is dependent on the shear rate. With a further increase of the shear rate, LLI deforms apparently and then the downstream three phase contact line depins to move once the balance between the capillary force and the shear force is broken, which results in an apparent increase of the slippage, specifically for a convex LLI. Compared with a convex LLI or a concave LLI, a flat LLI remains relatively stable under the same shear action, and an increase of the viscosity ratio and a decrease of the LLI fraction can both strengthen the shear resistance of an LLI, while they are less effective to promote the slippage. Consequently, the current results not only indicate that the slippage is related to the interface deflection and the shear rate but also suggest that both the shear resistance and the slippage of LLI should be considered when designing an effective LIS.

Hydrophobicity of periodic structure with taper angle under pressure impact

Biomimetic periodic structures have garnered attention due to their excellent water repellency. The normal-taper angle, which is aspects of the cross-sectional structure, is important factor in achieving water repellency and pressure resistance; however, the underlying physical phenomenon has not been fully explained. Moreover, once a surface becomes hydrophobic, it is difficult to measure the apparent contact angle. The purpose of this paper is to clarify the taper angle that provides high water repellency under pressure impact conditions by formulating the relationship between the taper angle and the height of a droplet bouncing, instead of traditional contact angles, using experimental results. We fabricated multiple samples with different taper angles and groove width/tooth width ratios, through micro-processing using a femtosecond-pulsed laser and a control algorithm, and investigated their effects on water repellency. By using height of a droplet bouncing as an evaluation parameter, we were able to effectively differentiate between taper angles in terms of water repellency. Additionally, we suggested that in the bouncing phenomenon, where droplets are given velocity by falling, the sidewall of the periodic structure and the taper angle affect liquid repellency. To explain this phenomenon, we proposed a pressured-taper angle model where a droplet is pressed against the taper angle. Based on both experimental findings and the pressured-taper angle model, a relationship between the equilibrium contact angle, the taper angle, and the lifting force angle was revealed. Moreover, using this pressured-taper angle model, the taper angle of the periodic structure to achieve maximum liquid repellency was estimated from the equilibrium contact angle of the base material.

Recent advances of electrospun strategies in topical products encompassing skincare and dermatological treatments

As the potential applications of electrospinning in healthcare continue to be explored, along with advancements in industrial-scale solutions and the emergence of portable electrospinning devices, some researchers have explored electrospinning technology in topical products, including its application in skincare, such as facial masks, beauty patches, sunscreen, and dermatological treatments for conditions like atopic dermatitis, psoriasis, acne, skin cancer, etc. In this review, we first outline the fundamental principles of electrospinning and provide an overview of existing solutions for large-scale production and the components and functionalities of portable spinning devices. Based on the essential functionalities required for skincare products and the mechanisms and treatment methods for the aforementioned dermatological diseases, we summarize the potential advantages of electrospinning technology in these areas, including encapsulation, sustained release, large surface area, and biocompatibility, among others. Furthermore, considering the further commercialization and clinical development of electrospinning technology, we offer our insights on current challenges and future perspectives in these areas, including issues such as ingredients, functionality, residue concerns, environmental impact, and efficiency issues.

Armored Superhydrophobic Surfaces with Excellent Drag Reduction in Complex Environmental Conditions

Superhydrophobic surfaces (SHSs) have possibilities for achieving significantly reduced solid-liquid frictional drag in the marine sector due to their excellent water-repelling properties. Although the stability of SHSs plays a key role in drag reduction, little consideration was given to the effect of extreme environments on the ability of SHSs to achieve drag reduction underwater, particularly when subjected to acidic conditions. Here, we propose interconnected microstructures to protect superhydrophobic coatings with the aim of enhancing the stability of SHSs in extreme environments. The stability of armored SHSs (ASHSs) was demonstrated by the contact angle and bounce time of droplets on superhydrophobic surfaces treated by various methods, resulting in an ASHS surface with excellent stability under extreme environmental conditions. Additionally, inspired by microstructures protecting superhydrophobic nanomaterials from frictional wear, the armored superhydrophobic spheres (ASSPs) were designed to explain from theoretical and experimental perspectives why ASSPs can achieve sustainable drag reduction and demonstrate that the ASSPs can achieve drag reduction of over 90.4% at a Reynolds number of 6.25 × 104 by conducting water entry experiments on spheres treated in various solutions. These studies promote a fundamental understanding of what drives the application of SHSs under extreme environmental conditions and provide practical strategies to maximize frictional drag reduction.

Superomniphobic and Photoactive Surface Presents Antimicrobial Properties by Repelling and Killing Pathogens

Healthcare-acquired infections place a significant burden on the cost and quality of patient care in hospitals. Reducing contamination on surfaces within healthcare environments is critical for halting the spread of these infections. Herein, we report a bifunctional─repel and kill─surface developed using photoactive TiO2 nanoparticles integrated into a hierarchical scaffold (OmniKill). To quantify the repellency of OmniKill, we developed a touch-based assay, capable of simulating the transfer of individual pathogens, multiple pathogens, or pathogen-latent fecal matter from hands to surfaces. OmniKill repels bacterial pathogens by at least 2.77-log (99.8%). The photoactive material within OmniKill further reduces the viability of transferred pathogens on the surface by an additional 2.43-log (99.6%) after 1 h of light exposure. The antipathogenic effects─repel and kill─remain robust under complex biological contaminates such as feces. These findings show the potential use of OmniKill in reducing the physical transmission of bacterial pathogens in healthcare settings.

Stable food grade wax/attapulgite superhydrophobic coatings for anti-adhesion of liquid foods

Adhesion of liquid foods on their packaging materials has caused significant food wastes and environment pollution, which has attracted great attention. Food grade superhydrophobic coatings are very promising to solve the issue but suffer from low mechanical stability and complex preparation methods. Herein, a food grade superhydrophobic coating for anti-adhesion of liquid foods was prepared by combining edible paraffin wax, polydimethylsiloxane-modified attapulgite natural nanorods and a food grade silicone adhesive. The concentration of polydimethylsiloxane-modified attapulgite, ultrasonication time and the volume ratio of the paraffin wax/attapulgite suspension to the silicone adhesive solution have great influences on wettability and morphology of the coatings. The coatings exhibit good static and dynamic superhydrophobicity due to their hierarchical micro-/nanostructure and low surface energy of the polydimethylsiloxane-modified attapulgite and paraffin wax. Moreover, the coatings exhibit good mechanical and chemical stability. The coatings are also highly repellent towards various liquid foods including the hot ones. Furthermore, the coatings are applicable onto various frequently used flexible and hard food packing materials including polypropylene, polyethylene terephthalate, aluminium alloy and paper, etc. Thus, the superhydrophobic coatings have great application potential in the food packing industry for anti-adhesion of liquid foods.

Aluminum-Based Heterogeneous Surface for Efficient Solar Desalination and Fog Harvesting Processed by a Picosecond Laser

Solar desalination and fog harvesting are two common ways to obtain fresh water, and both are promising methods to solve the water shortage problem. However, through either the fabrication of interfacial evaporators for solar desalination or the preparation of superwetting surfaces for fog harvesting, current methods suffer from long preparation times, high costs, and low efficiency. Herein, we report an efficient and simple method to process heterogeneous surfaces (HSs) on aluminum (Al) by picosecond laser processing combined with chemical treatment used for fog harvesting and seawater desalination. The as-prepared HS simultaneously consists of regular periodic stripe structures with superhydrophilicity and superhydrophobicity. The spacing of the superhydrophilic and superhydrophobic regions can be adjusted through the processing path. This surface has a 44% improvement in fog harvesting efficiency compared to a pristine Al sheet, which is 0.53 kg·m-2·h-1. Furthermore, it shows a high evaporation rate of 2.35 kg·m-2·h-1 under one sun irradiation with an energy efficiency of 52.39%. Such functional surfaces can be applied to obtain fresh water resources in both coastal regions and arid areas, where water mist is relatively abundant, providing reference and guidance for fresh water collection, and being a promising way to solve the water shortage problem.

Friction force-based measurements for simultaneous determination of the wetting properties and stability of superhydrophobic surfaces

HYPOTHESIS

Contact angle and sliding angle measurements are widely used to characterize superhydrophobic surfaces because of the simplicity and accessibility of the technique. We hypothesize that dynamic friction measurements, with increasing pre-loads, between a water drop and a superhydrophobic surface is more accurate because this technique is less influenced by local surface inhomogeneities and temporal surface changes.

EXPERIMENTS

A water drop, held by a ring probe which is connected to a dual-axis force sensor, is sheared against a superhydrophobic surface while maintaining a constant preload. From this force-based technique, static and kinetic friction forces measurements are used to characterize the wetting properties of the superhydrophobic surfaces. Furthermore, by applying increased pre-loads to the water drop while shearing, the critical load at which the drop transitions from the Cassie-Baxter to Wenzel state is also measured.

FINDINGS

The force-based technique predicts sliding angles with reduced standard deviations (between 56 and 64%) compared to conventional optical-based measurements. Kinetic friction force measurements show a higher accuracy (between 35 and 80%) compared to static friction force measurements in characterizing the wetting properties of superhydrophobic surfaces. The critical loads for the Cassie-Baxter to Wenzel state transition allows for stability characterization between seemingly similar superhydrophobic surfaces.

Water Spider-Inspired Nanofiber Coating with Sustainable Scale Repellency via Air-Replenishing Strategy

To survive underwater even in severely hypoxic water for a long period, the water spider has to periodically collect and replenish air into the diving bell. Inspired by this natural air-replenishing strategy, a water spider-inspired nanofiber (WSN) coating with underwater superaerophilicity displaying excellent and sustainable scalephobic capability is prepared. Air film on the WSN coating can be well-kept and further employed as the barrier layer for scale repellence. Significantly, scalephobic capability of the WSN coating mainly originates from two aspects: inhibiting interfacial nucleation and reducing interfacial adhesion of scale. Compared with previous studies, this WSN coating achieves excellent and sustainable scale repellence (≈ 98% reduction in scale deposition) even after a one-month dynamic scaling test. Thus, this air-replenishing strategy may raise a new avenue for advanced long-term scalephobic materials.

Tribological Behavior of Bioinspired Surfaces

Energy losses due to various tribological phenomena pose a significant challenge to sustainable development. These energy losses also contribute toward increased emissions of greenhouse gases. Various attempts have been made to reduce energy consumption through the use of various surface engineering solutions. The bioinspired surfaces can provide a sustainable solution to address these tribological challenges by minimizing friction and wear. The current study majorly focuses on the recent advancements in the tribological behavior of bioinspired surfaces and bio-inspired materials. The miniaturization of technological devices has increased the need to understand micro- and nano-scale tribological behavior, which could significantly reduce energy wastage and material degradation. Integrating advanced research methods is crucial in developing new aspects of structures and characteristics of biological materials. Depending upon the interaction of the species with the surrounding, the present study is divided into segments depicting the tribological behavior of the biological surfaces inspired by animals and plants. The mimicking of bio-inspired surfaces resulted in significant noise, friction, and drag reduction, promoting the development of anti-wear and anti-adhesion surfaces. Along with the reduction in friction through the bioinspired surface, a few studies providing evidence for the enhancement in the frictional properties were also depicted.