Recent advances in slippery liquid-infused surfaces with unique properties inspired by nature

Smart materials for light control of droplets

Droplet manipulation plays a critical role in both fundamental research and practical applications, especially when combined with smart materials and external fields to achieve multifunctional droplet manipulation. Light control of droplets has emerged as a significant and widely used strategy, driven primarily by photochemistry, photomechanics, light-induced Marangoni effects, and light-induced electric effects. This approach allowing for droplet manipulation with high spatial and temporal resolution, all while maintaining a remote and non-contact mode of operation. This review aims to provide a comprehensive overview of the mechanisms underlying light control of droplets, the design of smart materials for this purpose, and the diverse range of applications enabled by this technique. These applications include merging, splitting, releasing, forwarding, backward movement, and rotation of droplets, as well as chemical reactions, droplet robots, and microfluidics. By presenting this information, we aim to establish a unified framework that guides the sustainable development of light control of droplets. Additionally, this review addresses the challenges associated with light control of droplets and suggests potential directions for future development.

Engineered Sustainable Omniphobic Coatings to Control Liquid Spreading on Food-Contact Materials

The adhesion of sticky liquid foods to a contacting surface can cause many technical challenges. The food manufacturing sector is confronted with many critical issues that can be overcome with long-lasting and highly nonwettable coatings. Nanoengineered biomimetic surfaces with distinct wettability and tunable interfaces have elicited increasing interest for their potential use in addressing a broad variety of scientific and technological applications, such as antifogging, anti-icing, antifouling, antiadhesion, and anticorrosion. Although a large number of nature-inspired surfaces have emerged, food-safe nonwetted surfaces are still in their infancy, and numerous structural design aspects remain unexplored. This Review summarizes the latest scientific research regarding the key principles, fabrication methods, and applications of three important categories of nonwettable surfaces: superhydrophobic, liquid-infused slippery, and re-entrant structured surfaces. The Review is particularly focused on new insights into the antiwetting mechanisms of these nanopatterned structures and discovering efficient platform methodologies to guide their rational design when in contact with food materials. A detailed description of the current opportunities, challenges, and future scale-up possibilities of these nanoengineered surfaces in the food industry is also provided.

Dependence of Slippery and Elastic Properties of Thin Polymer Films on the Grafted Flexible Sidechain Amount

In modern life, people face a wide number of sticky problems when adhesion is highly undesirable: water and dirt stick to clothes, useful materials stick to the walls of their containers and cannot be fully used, water sticking and freezing on airplane wings affects handling and can be dangerous, biological liquids can stick and form clots inside medical devices threatening patients' lives, etc. Slippery liquid-infused porous surfaces (SLIPSs) with pressure stable omniphobicity could help to solve these issues. Lubricant depletion from porous surface and subsequent degradation of omniphobic properties is the major problem for SLIPS. It could be resolved by attaching flexible, liquid-like sidechains to the polymer matrix. Understanding the relationship between the structure of such polymer films and wetting effects is therefore of great importance. The present work is devoted to the study of droplet pinning on crosslinked polydimethylsiloxane (PDMS) polymer films with varied amounts of attached flexible PDMS sidechains and clarification of the relationship between slippery and viscoelastic properties of the films. An one-stage approach to the synthesis of such slippery coatings on smooth and porous substrates in "eco-friendly" pressurized CO₂ solutions is proposed. Pinning force and Young's modulus (E) of the films on silicon substrates with variation of the grafted sidechains amount (x) are measured. The non-monotonic dependence of the pinning force on the amount of sidechains is obtained: the pinning force decreases at small x values (region I) and starts to increase at higher x (region II). The effects of the grafted sidechains amount, as well as matrix softening, are discussed for each case. It is demonstrated that the proposed method of film synthesis allows one to obtain thin, uniform coatings on fabrics without gluing the fibers. Such coatings with an optimal amount of PDMS sidechains demonstrate decreased sliding angles for droplets of water and aqueous alcohol solutions, as compared to PDMS coatings without grafted sidechains. The proposed technique may be of interest for deposition of coatings on porous surfaces having a complex morphology, such as textiles, aerogels, porous electrodes, etc.

Tribological performance of electrically conductive and self-lubricating polypropylene-ionic-liquid composites

In this work, self-lubricating and electrically conductive polymers on a polypropylene (PP) matrix were prepared and investigated. These properties were obtained by additivating PP with carbon black (CB) and multi-walled carbon nanotubes (MWCNTs), in combination with a surface active ionic liquid (IL, trihexyltetradecylphosphonium docusate [P₆₆₆₁₄][DOC]). These polymeric composites are expected to achieve coefficients of friction (COFs) comparable to lubricated systems. Combined with electrical conductivity, these materials could be applied in electrically loaded tribosystems. The COF was reduced by up to 25% compared to that of plain PP, and high electrical conductivity and self-lubrication were achieved. Fundamental differences between the carbon-based fillers in their interaction with IL were investigated with high-resolution surface analysis (TEM, AFM) and Raman and ATR-FTIR spectroscopy. By varying the tribological test parameters, the application limits of self-lubrication were identified. It was demonstrated that the contact pressure has a strong influence on the COF. Therefore, this work points to potential applications in (e.g. 3D-printed) bearings and electrically loaded bearings where electrical conductivity and relatively low COFs are required.

Recent Advances on the Abrasion Resistance Enhancements and Applications of Superhydrophobic Materials

Researches on superhydrophobicity have been overwhelming and have shown great advantages in various fields. However, the abrasion resistance of superhydrophobic structures was usually poor, and they were easily damaged by external force or harsh environment, which greatly limited the applications of superhydrophobic surfaces. Much attention has been paid to improving the abrasion resistance of superhydrophobic materials by researchers. In this review, aimed at the advances on improving the abrasion resistance of superhydrophobic surfaces, it was summarized and compared three enhancement strategies including the reasonably design of micro-nano structures, the adoption of adhesives, and the preparation of self-healing surface. Finally, the applications of typical superhydrophobic materials with abrasion resistance were reviewed in various fields. In order to broaden the application fields of superhydrophobic materials, the abarasion resistance should be further improved. Therefore, we proposed the ideas for the future development of superhydrophobic materials with higher abrasion resistance. We hope that this review will provide a new approach to the preparation and development of stable superhydrophobic surfaces with higher abrasion resistance.

Oil-Grafted Track-Assisted Directional Transport of Water Droplets and Submerged Air Bubbles on Solid Surfaces

Wettability-tailored tracks are emerging as an efficient approach to collecting and transporting underwater air bubbles as well as water from the mist. However, tailoring the surface wettability by modifying the surface structural features via physiochemical methods to create superhydrophilic-superhydrophobic contrast tracks suffers from long-term durability issues, while the emerging liquid-infused slippery surface has inherent design engineering limitations and issues from infused oil depletion. Herein, we demonstrate that by selective silicone oil grafting onto the glass substrate, it is possible to create a wettability contrast of ∼ 43°. Further, we illustrate the application of such tracks for underwater air bubble capturing and transportation in an aqueous medium with surface tension ranging from 72 to 43.5 mN/m. In addition, the potential of these nonadhesive and adhesive tracks for water collection from the mist is shown and the critical effect of the track dimension and intertrack spacing on the water harvesting rate is investigated in detail. The study illustrates that the nonadhesive nature of the oil-grafted region enables the easy transport of underwater air bubbles as well as water from the flow medium and thus offers an easy and facile approach to creating substrates for underwater air bubble collection and water harvesting.

A sandwiched patch toward leakage-free and anti-postoperative tissue adhesion sealing of intestinal injuries

Ideal repair of intestinal injury requires a combination of leakage-free sealing and postoperative antiadhesion. However, neither conventional hand-sewn closures nor existing bioglues/patches can achieve such a combination. To this end, we develop a sandwiched patch composed of an inner adhesive and an outer antiadhesive layer that are topologically linked together through a reinforced interlayer. The inner adhesive layer tightly and instantly adheres to the wound sites via -NHS chemistry; the outer antiadhesive layer can inhibit cell and protein fouling based on the zwitterion structure; and the interlayer enhances the bulk resilience of the patch under excessive deformation. This complementary trilayer patch (TLP) possesses a unique combination of instant wet adhesion, high mechanical strength, and biological inertness. Both rat and pig models demonstrate that the sandwiched TLP can effectively seal intestinal injuries and inhibit undesired postoperative tissue adhesion. The study provides valuable insight into the design of multifunctional bioadhesives to enhance the treatment efficacy of intestinal injuries.

Study of the Droplet Pinning Force in the Transition from Dry to Liquid-Infused Thin Polymer Films

The change in the pinning force during the transition from dry to oil-impregnated thin polymer films is studied for droplets of water and hexadecane. A careful variation of the oil amount in the films is performed by means of supercritical impregnation. The film thickness dependence on the oil content is measured using ellipsometry and compared to gel swelling theory estimates. Depending on the oil content, two cases of pinning force behavior have been identified. For each case, the factors that determine the pinning force are discussed. The pinning force in the transition from dry to equilibrium swollen gel films is well approximated by the Joanny and de Gennes hysteresis model of dilute defects.

A robust surface with superhydrophobicity and underwater superoleophobicity for on-demand oil/water separation

Superhydrophobic and underwater superoleophobic surface combine the superiorities of the two opposite wettabilities. Generally, such a surface is constructed by hydrophilic areas and hydrophobic areas treated with fluorine-containing modifiers. However, the surface energy in a narrow range and poor bonding force between water-loving and water-repelling components make the surface fragile and its wettability unstable. Herein, we present a strategy to fabricate a robust surface with superhydrophobicity and underwater superoleophobicity. Hydrophilic aluminum phosphate as a binder can strongly interact with superhydrophobic titanium dioxide nanoparticles. Mixing the two ingredients to accurately control the surface energy in a narrow range and then spray coating, the superhydrophobic and underwater superoleophobic surfaces are conveniently prepared on diverse substrates. Under acid/base aqueous solution conditions, O₂-plasma etching, and sand impingement, the coatings remain superhydrophobic and underwater superoleophobic. Taking advantage of the wettability and robustness of coatings, the as-prepared membranes realize on-demand and multicycle separation under gravity without continuous external stimulus. Importantly, even after 100 sand impingement cycles, the treated membranes still maintain prominent separation performance.

External-field-induced directional droplet transport: A review

Directional transport of fluids is crucial for vital activities of organisms and numerous industrial applications. This process has garnered widespread research attention due to the wide breadth of flexible applications such as medical diagnostics, drug delivery, and digital microfluidics. The rational design of functional surfaces that can achieve the subtle control of liquid behavior. Previous studies were mainly dependent on the special asymmetric structures, which inevitably have the problem of slow transport speed and short distance. To improve controllability, researchers have attempted to use external fields, such as thermal, light, electric fields, and magnetic fields, to achieve controllable droplet transport. On the fundamental side, much of their widespread applicably is due to the degree of control over droplet transport. This review provides an overview of recent progress in the last three years toward the transport of droplets with different mechanisms induced by various external stimuli, including light, electric, thermal, and magnetic field. First, the relevant basic theory and typical induced gradient for directional liquid transport are illustrated. We will then review the latest advances in the external-field-induced directional transport. Moreover, the most emerging applications such as digital microfluidics, harvesting of energy and water, heat transfer, and oil/water separation are also presented. Finally, we will outline possible future perspectives to attract more researchers interest and promote the development of this field.

Facile preparation of a superamphiphilic nitrocellulose membrane enabling on-demand and energy-efficient separation of oil/water mixtures and emulsions by prewetting

A membrane with superamphiphilicity presents many advantages in various oil/water separation applications due to its switchable wettability by prewetting. However, it is still a great challenge to switch between two types of superwettability on a single cellulose surface by switching between different liquid media. Herein, in order to obtain in-air superamphiphilic and under-liquid dual superlyophobic membranes, dopamine-modified nitrocellulose membranes (with a pore size of 0.22 μm) were prepared via a facile immersion modification approach. Under 0.08 MPa, the as-prepared NC membrane switches wettability by prewetting to achieve on-demand oil/water separation, and the separation efficiency is more than 99.9%. Futhermore, the membrane prepared in this work can also be applied to high-efficiency on-demand separation of surfactant-stabilized emulsions with a separation efficiency greater than 99.0%. Hence, the PDA-modified NC membrane is a promising controllable oil/water separation material in terms of repeatable cycles, separation efficiency, flux, prominent long-term durability and anti-oil fouling.

Review on the recent development of durable superhydrophobic materials for practical applications

Biomimetic superhydrophobic surfaces show great potential in oil-water separation, anti-icing and self-cleaning. However, due to the instability caused by its fragile structure and non-durable superhydrophobicity, it is difficult to apply them in the actual field. Here, by introducing surface wettability and analysing the mechanism of superhydrophobic failure, it is concluded that the reason for the failure of the superhydrophobic surface comes from the transition of the surface energy and the hysteresis of the contact angle (CA). On the basis of this analysis, it is concluded that the principle of designing a durable superhydrophobic surface is to satisfy one of the following three points: improving the binding force between molecules, introducing durable materials and improving chemical durability. On this basis, a variety of preparation methods are proposed, such as assembly method and spray/dip coating method, and the design and preparation of a self-healing surface inspired by nature will also be included in the introduction. Last but not least, the preparation and application of a durable super-hydrophobic surface in oil-water separation, anti-icing and self-cleaning are also introduced in detail. This review reveals the conclusions and prospects of durable superhydrophobic surfaces, and aims to inspire more researchers to invest in this research.

A combined structural and wettability gradient surface for directional droplet transport and efficient fog collection

HYPOTHESIS

The droplet manipulation behavior is affected by chemical structural driving force (including the superposition of electric, magnetic, optical and thermal fields), which directly determine transportation velocity. A lot of research has focused on a single driving force that induces the directional transportation behavior, which affects its performance.

EXPERIMENTS

A simple method for preparing wettability gradient conical copper needles (WGCCN) combining structural gradient and chemical gradient was formulated. The effect of droplet volume and tilt angles on droplet transport velocity was systematically studied. The process of droplet transport was revealed through theoretical model and mechanical analysis. Finally, the application of WGCCN and its array model in fog collection were explored.

FINDINGS

A continuous chemical gradient in the conical structure gradient induces the droplet directional transportation, and the transportation velocity depends on the droplet volume. In addition, under the cooperation effect of multiple driving force, the droplet can still be transported in a directional orientation even if it is tilted at a certain angle. The simple droplet manipulation behavior portends that the droplets directional transport behavior can be applied in microfluidic manipulation by cooperation of effective multiple driving force with satisfactory results.

Bioinspired textile with dual-stimuli responsive wettability for body moisture management and signal expression

A smart bioinspired loofah textile with biosafe wettability shows high directional liquid transport capacity and the ability to identify liquids with different pH values.

Cellulose acetate/fiber paper composite membrane for separation of an oil-in-water emulsion

The cellulose composite membrane combines the advantages of cellulose acetate and cellulose filter paper with good antifouling performance and excellent mechanical properties.

Stable and biocompatible slippery lubricant-infused anode-oxidated titanium nanotube surfaces via a grafted polydimethylsiloxane brush

This work aims at optimizing the structure and enhancing the interaction force between the substrate and the lubricant to prepare a slippery lubricant-infused TiO2 NT surface that maintains stable omniphobic performance in extreme environments.