Wetting and spreading: Fundamental theories to cutting-edge applications

Ambient-mediated wetting on smooth surfaces

A consensus was built in the first half of the 20th century, which was further debated more than 3 decades ago, that the wettability and condensation mechanisms on smooth solid surfaces are modified by the adsorption of organic contaminants present in the environment. Recently, disagreement has formed about this topic once again, as many researchers have overlooked contamination due to its difficulty to eliminate. For example, the intrinsic wettability of rare earth oxides has been reported to be hydrophobic and non-wetting to water. These materials were subsequently shown to display dropwise condensation with steam. Nonetheless, follow on research has demonstrated that the intrinsic wettability of rare earth oxides is hydrophilic and wetting to water, and that a transition to hydrophobicity occurs in a matter of hours-to-days as a consequence of the adsorption of volatile organic compounds from the ambient environment. The adsorption mechanisms, kinetics, and selectivity, of these volatile organic compounds are empirically known to be functions of the substrate material and structure. However, these mechanisms, which govern the surface wettability, remain poorly understood. In this contribution, we introduce current research demonstrating the different intrinsic wettability of metals, rare earth oxides, and other smooth materials, showing that they are intrinsically hydrophilic. Then we provide details on research focusing on the transition from wetting (hydrophilicity) to non-wetting (hydrophobicity) on somooth surfaces due to adsorption of volatile organic compounds. A state-of-the-art figure of merit mapping the wettability of different smooth solid surfaces to ambient exposure as a function of the surface carbon content has also been developed. In addition, we analyse recent works that address these wetting transitions so to shed light on how such processes affect droplet pinning and lateral adhesion. We then conclude with objective perspectives about research on wetting to non-wetting transitions on smooth solid surfaces in an attempt to raise awareness regarding this surface contamination phenomenon within the engineering, interfacial science, and physical chemistry domains.

Spontaneous Transport Mechanics of Water Droplets under a Synergistic Action of Designed Pattern and Non-Wetting Gradient

The controllable spontaneous transport of water droplets on solid surfaces has a broad application background in daily life. Herein, a patterned surface with two different non-wetting characteristics was developed to control the droplet transport behavior. Consequently, the patterned surface exhibited great water-repellant properties in the superhydrophobic region, and the water contact angle reached 160° ± 0.2°. Meanwhile, the water contact angle on the wedge-shaped hydrophilic region dropped to 22° after UV irradiation treatment. On this basis, the maximum transport distance of water droplets could be observed on the sample surface with a small wedge angle of 5° (10.62 mm), and the maximum average transport velocity of droplets was obtained on the sample surface with a large wedge angle of 10° (218.01 mm/s). In terms of spontaneous droplet transport on an inclined surface (4°), both the 8 μL droplet and 50 μL droplet could move upward against gravity, which showed that the sample surface possessed an obvious driving force for droplet transport. Surface non-wetting gradient and the wedge-shaped pattern provided unbalanced surface tension to produce the driving forces in the process of droplet transport, and the Laplace pressure as well is produced inside the water droplet during this process. This work provides a new strategy to develop a patterned superhydrophobic surface for droplet transport.

Dynamic wetting of various liquids: Theoretical models, experiments, simulations and applications

Dynamic wetting is a ubiquitous phenomenon and frequently observed in our daily life, as exemplified by the famous lotus effect. It is also an interfacial process of upmost importance involving many cutting-edge applications and has hence received significantly increasing academic and industrial attention for several decades. However, we are still far away to completely understand and predict wetting dynamics for a given system due to the complexity of this dynamic process. The physics of moving contact lines is mainly ascribed to the full coupling with the solid surface on which the liquids contact, the atmosphere surrounding the liquids, and the physico-chemical characteristics of the liquids involved (small-molecule liquids, metal liquids, polymer liquids, and simulated liquids). Therefore, to deepen the understanding and efficiently harness wetting dynamics, we propose to review the major advances in the available literature. After an introduction providing a concise and general background on dynamic wetting, the main theories are presented and critically compared. Next, the dynamic wetting of various liquids ranging from small-molecule liquids to simulated liquids are systematically summarized, in which the new physical concepts (such as surface segregation, contact line fluctuations, etc.) are particularly highlighted. Subsequently, the related emerging applications are briefly presented in this review. Finally, some tentative suggestions and challenges are proposed with the aim to guide future developments.

Improved Method to Characterize Leaf Surfaces, Guide Adjuvant Selection, and Improve Glyphosate Efficacy

Glyphosate, one of the most widely used herbicides, plays an important role in controlling weeds and ensuring crop production. While using glyphosate, adjuvants are commonly added to improve its deposition on weeds and control efficacy. However, changes in weed leaf surface characteristics may reduce glyphosate penetration and contribute to evolved glyphosate resistance. Therefore, it is significant to introduce an improved method for regularizing leaf surface characterization and guide adjuvant selection to improve glyphosate efficacy. In this work, surface characteristics of typical weed leaves have been systematically investigated by 3D surface analysis and scanning electron microscopy, finally quantified by apparent surface free energy (ASFE) due to its comprehensive and quantitative evaluation of leaf surfaces. Moreover, the relationship between the weed leaf surface characteristics and the retention of glyphosate on weeds was established, further related to the control efficacy against weeds. To maximize the utilization rate of glyphosate, the types and concentrations of adjuvants should be regulated according to the ASFE of weeds. Our findings not only regularize the surface properties of weed leaves but also reveal their influencing mechanism on the deposition and biological activity of glyphosate, which provide effective guidance for the use of glyphosate.

Introduction to the biomimetic design of interfacial materials for water overflow control

The overflow behavior of liquid at a curved surface or a surface border is a common phenomenon in many circumstances of daily life and industry. Accurate control of overflow is beneficial for improving the convenience of our daily life and efficiency of production, thus has great importance not only for enhancing existing applications but for creating new products. The effect of surface wettability on overflow has not been paid enough attention in the past, however, as the development of construction techniques, especially 3D printing, of superwettable surfaces and the understanding of the dynamic interfacial wetting properties, the potential of surface wettability and structure on the overflow control has gained great recognition in the recent decade. On this basis, the feature article will outline the understanding evolution of the overflow phenomenon, and summarise the current research on the control of overflow behavior from aspects including the bioinspired idea, the fabrication of superwettable surfaces, the development of control techniques, the exploration of control mechanisms, etc., and provide an outlook for the accurate control of overflow by surfactant and additives, along with challenges and perspectives.

On the Development of Ultradurable Extremely Water-Repellent and Oleophobic Soot-Based Fabrics with Direct Relevance to Sperm Cryopreservation

Nowadays, the tremendous progress of nanotechnologies and materials science facilitates the fabrication of universal and multifunctional superhydrophobic surfaces on a large scale. Yet, integrating icephobic and anti-bioadhesive properties in an individual water-repellent functional coating, for addressing the difficulties faced by cryobiologists, aircraft, and seacraft manufacturers, is quite tricky but feasible if using nonpolar soot nanoparticles, whose fragility, however, impedes their industrial applicability. Here, we advance the current state-of-the-art to an extent, permitting the introduction of economically affordable and ultradurable non-wettable soot-based coatings. The deposition of rapeseed oil soot, cyanoacrylate glue and fluorine compounds onto different fabrics confers the latter with superior tolerance to harsh mechanical and thermal interventions [e.g., scratching, blade scraping, liquid nitrogen immersion (T ∼ -196 °C), torsion and water jetting], while in the meantime retaining water repellency and oleophobicity. The as-prepared soot fabrics can stick continuously to the selected host surface and favor the recovery of ∼60% of the initial motility of human spermatozoa subjected to cryopreservation or being detached and utilized as standalone non-wettable membranes. Our invention may be considered as the first fundamental stage of safely (without any health concerns) transferring the soot in reproductive medicine and developing enhanced cryogenic and antibacterial medical devices.

Surface properties of dicationic ionic liquids and correlation with biological activity

The surface activity of dicationic ionic liquids is described in this paper. The basic interfacial parameters including critical micelle concentration (CMC), surface tension at the CMC (γ CMC), the adsorption efficiency (pC20), surface excess (Γmax), the minimum surface occupied by a single molecule (A min), and Gibbs energy (ΔG 0 ads) were investigated and compared. Basically, we wanted to extend our previous study on dicationic ionic liquids with bis-ammonium cation. Knowing that, the compounds obtained are effective in limiting the feeding of adult and larvae confused flour beetle (T. confusum), it was decided to correlate the deterrent activity with the surface properties of analyzed dicationic ionic liquids. Accordingly, it was found that the deterrent activity of the studied compounds increases with increasing wetting ability.

Closely Packed Conductive Droplets with Polygon-Like Patterns Confined at the Interface in Ternary Polymer Blends

This work reports on the formation of closely packed conductive droplets demonstrating polygon-like patterns at the interface in partially wetted ternary polymer systems prepared by melt blending and annealing treatment. The low-density polyethylene/poly(ether-block-amide)/poly(butylene-adipate-co-terephthalate) (LDPE/PEBA/PBAT) blend showed an intermediate partial wetting tendency where the interfacially localized conductive PEBA phase developed connected structure after blending but transformed into dispersed droplets upon annealing. The coalescence of the PEBA droplets appeared to be initiated by the Rayleigh-type instability in the thin PBAT film separating PEBA. However, the intrinsic coalescence rate of the PEBA droplets was very low due to the low interfacial tension of PEBA/PBAT. This slow coalescence of PEBA combined with the fast reduction in the interfacial area during annealing and the intermediate partial wetting state of the LDPE/PEBA/PBAT system resulted in a unique morphology of closely packed PEBA droplets with polygon-like patterns at a volume fraction of 50/10/40. Two other representative ternary polymer blends, LDPE/PEBA/polypropylene (PP) and compatibilized LDPE/PEBA/polystyrene (PS), with strong and weak partial wetting morphologies were also examined to highlight the mechanism for the morphology development in the LDPE/PEBA/PBAT blend.

Unveiling the Atomic Structure and Growth Dynamics of One-Dimensional Water on ZnO(10-10)

The adsorption and organization state of water on the metal oxide surface is of critical importance for wide fields where interface chemistry dominates. On the technically important ZnO(10-10) surface, we found water assembles into an one-dimensional (1D) chain structure at submonolayer coverage instead of the well-known half-dissociated two-dimensional (2D) island. With a combination of high resolution scanning tunneling microscopy (STM) and density functional theory (DFT) calculations, we clearly distinguished the single and double water chains, which are composed of dissociated monomers and half-dissociated dimers, respectively. Moreover, we unambiguously determined that single water molecules dissociate spontaneously before agglomerating into ordered phase, which is contrary to the proposition of previous studies. These results have deepened our understandings of the adsorbed water species on the ZnO surface, which may bring new insights into the mechanisms of water-stimulated surface reactions.

Novel Hydrophobic Nanostructured Antibacterial Coatings for Metallic Surface Protection

A simple and cost-efficient method to modify different surfaces in order to improve their bioactivity, corrosion and wear resistance proved to be sol-gel coatings. The silane layers have been shown to be effective in the protection of steel, aluminum or magnesium alloys and copper and copper alloys. Moreover, it has been found that the adding of different inorganic nanoparticles into silica films leads to increasing their performance regarding corrosion protection. In this study, we fabricated, a simple sol-gel method, transparent mono- and bi-layered hydrophobic coatings with simultaneous antibacterial, hydrophobic and anti-corrosive properties for the protection of metallic surfaces against the action of air pollutants or from biological attacks of pathogens. The first layer (the base) of the coating contains silver (Ag) or zinc oxide (ZnO) nanoparticles with an antibacterial effect. The second layer includes zinc oxide nanoparticles with flower-like morphology to increase the hydrophobicity of the coating and to improve corrosion-resistant properties. The second layer of the coating contains a fluorinated silica derivative, 1H,1H,2H,2H-perfluorooctyl triethoxysilane (PFOTES), which contributes to the hydrophobic properties of the final coating by means of its hydrophobic groups. The mono- and bi-layered coatings with micro/nano rough structures have been applied by brushing on various substrates, including metallic surfaces (copper, brass and mild steel) and glass (microscope slides). The as-prepared coatings showed improved hydrophobic properties (water CA > 90°) when compared with the untreated substrates while maintaining the transparent aspect. The corrosion resistance tests revealed significantly lower values of the corrosion rates recorded for all the protected metallic surfaces, with the lowest values being measured for the bi-layered coatings containing ZnO particles, both in the first and in the second layers of the coating. Considering the antibacterial activity, the most effective were the AOAg-II and AOZnO-II coatings, which exhibited the highest reduction of microbial growth.

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.

Study on the Wettability of a Composite Solution Based on Surface Structures of Coal

In order to research the influence of composite surfactant solutions on the wetting ability of coal, the wettability parameters, pore structure parameters, and surface microscopic morphology were determined in this study. The results show that the wettability of the surfactant can be improved by adding NaCl. As the concentration of NaCl increased, the spreading coefficient of the NaCl-SDS composite solution increases; the spreading coefficient increased from -10.838 to -3.7624 mN/m. The surface free energy decreased from -2.031 to 3.670 J/mol × 10³, and the adhesion work decreased from 113.802 to 55.058 mJ/m². In terms of the pore structure and surface morphology of coal, the pore size will affect the wettability of coal. The contact angle reduces with pore size increase (R ² = 0.955). The surface pores of coal treated with the NaCl-SDS composite solution increase and the fracture connectivity is improved, which facilitated the solution to penetrate coal for wetting. The results of this paper have great practical significance for mining at the face to reduce coal dust pollution.

Eco-Friendly Castor Oil-Based Delivery System with Sustained Pesticide Release and Enhanced Retention

The deposition of pesticides and their retention on plant surfaces are critical challenges for modern precision agriculture, which directly affect phytosanitary treatment, bioavailability, efficacy, and the loss of pesticides. Herein, a novel and eco-friendly waterborne polyurethane delivery system was developed to enhance the spray deposition and pesticide retention on plant surfaces. More specifically, biobased cationic and anionic waterborne polyurethane dispersions were synthesized from castor oil. Both cationic and anionic polyurethane dispersions exhibited remarkable microstructural, amphiphilic, and nanoparticle morphologies with a core-shell structure that served to encapsulate a biopesticide (azadirachtin) in their hydrophobic cores (WPU-ACT). The results indicated that the cationic WPU-ACT carriers exhibited a better sustained release behavior and a better protective effect from light and heat for azadirachtin. In addition, the simultaneous spray of anionic and cationic WPU-ACT significantly enhanced the spray deposition and prolonged the retention of pesticides due to the reduced surface tension and surface precipitation induced by the electrostatic interaction when two droplets with opposite charges come into contact with each other. A field efficacy assessment also indicated that the simultaneous spray of anionic and cationic WPU-ACT could control the infestation of brown planthopper in rice crops. Castor oil-based waterborne polyurethanes in this study work as an efficient pesticide delivery system by exhibiting enhanced deposition, rainfastness, retention ability, protection, and sustained release behavior, holding great promise for spraying pesticide formulations in modern and environmentally friendly agricultural applications.

Bioinspired Smart Liquid Directional Transport Control

Developments in bioinspired superwetting materials have triggered technological revolutions in many disciplines. One representative area is liquid directional transport dominated by interface properties, which has experienced rapid progress recently. To improve the controllability, scientists try to use the external field, such as light, electricity, thermal, and so on, to assist or achieve controllable smart, responsive liquid directional transport. However, there are still some intractable problems and challenges behind prosperity. Here, we summarize the relevant basic theory of surface wettability and the processes of the development of bioinspired superwetting materials. We discuss the different essential mechanisms of liquid directional transport. Furthermore, smart external field-controlled fluid directional transport is the primary focus of this feature article. We briefly put forward our views on some outstanding problems, existing challenges, and trends in this field.

The crystal packing, morphology and hydrophobicity of polyoxometalate-based amphiphilic materials

Crystal structures of polyoxometalate-based amphiphiles with multiple alkyl tails were characterized by single crystal X-ray diffraction to reveal the relationship between morphologies and manner of packing.

3D Organic Nanofabrics: Plasma-Assisted Synthesis and Antifreezing Behavior of Superhydrophobic and Lubricant-Infused Slippery Surfaces

Herein, we present the development of supported organic nanofabrics formed by a conformal polymer-like interconnection of small-molecule organic nanowires and nanotrees. These organic nanostructures are fabricated by a combination of vacuum and plasma-assisted deposition techniques to generate step by step, single-crystalline organic nanowires forming one-dimensional building blocks, organic nanotrees applied as three-dimensional templates, and the polymer-like shell that produces the final fabric. The complete procedure is carried out at low temperatures and is compatible with an ample variety of substrates (polymers, metal, ceramics; either planar or in the form of meshes) yielding flexible and low solid-fraction three-dimensional nanostructures. The systematic investigation of this progressively complex organic nanomaterial delivers key clues relating their wetting, nonwetting, and anti-icing properties with their specific morphology and outer surface composition. Water contact angles higher than 150° are attainable as a function of the nanofabric shell thickness with outstanding freezing-delay times (FDT) longer than 2 h at -5 °C. The role of the extremely low roughness of the shell surface is settled as a critical feature for such an achievement. In addition, the characteristic interconnected microstructure of the nanofabrics is demonstrated as ideal for the fabrication of slippery liquid-infused porous surfaces (SLIPS). We present the straightforward deposition of the nanofabric on laser patterns and the knowledge of how this approach provides SLIPS with FDTs longer than 5 h at -5 °C and 1 h at -15 °C.

Uniform Spread of High-Speed Drops on Superhydrophobic Surface by Live-Oligomeric Surfactant Jamming

Inkjet printing of water-based inks on superhydrophobic surfaces is important in high-resolution bioarray detection, chemical analysis, and high-performance electronic circuits and devices. Obtaining uniform spreading of a drop on a superhydrophobic surface is still a challenge. Uniform round drop spreading and high-resolution inkjet printing patterns are demonstrated on superhydrophobic surfaces without splash or rebound after high-speed impacting by introducing live-oligomeric surfactant adhesion. During impact, the live-oligomeric surfactant molecules aggregate into dynamic, wormlike micelle networks, which jam at the solid-liquid interface by entangling with the surface micro/nanostructures to pin the contact line and jam at the spreading periphery to keep the uniform spreading lamellar shape. This efficient uniform spreading of high-speed impact drops opens a promising avenue to control drop impact dynamics and achieve high-resolution printing.

Bubble shapes and their changes on slippery surfaces during directional transportation

HYPOTHESIS

Understanding bubble shapes on narrow slippery stripes and how they change during manipulated processes is significant for both industrial and daily life applications. Moreover, bubble shapes are mainly affected by bubble volume, stripe width and substrate wettability. Revealing the relationship between those factors for bubble shapes is crucial for further study. While there are many studies of bubble manipulation underwater on slippery surfaces, the detailed evolution of bubble shapes on those substrates is still worthy of further study. In this research, the details of bubbles shapes in different conditions were studied and analyzed.

EXPERIMENTS

Air bubbles were continuously released on substrates with different chemistries and different geometric parameters to study their effect on bubble shape evolution.

FINDINGS

Bubble shape can be categorized into four states according to the relationship between bubbles and the stripe widths. Bubble shapes have considerable differences in their top-view and side-view under each state. Furthermore, during the directional transportation of bubbles on a wedge-shaped slippery surface, bubble shapes change from the tip to the end by the width gradients. The outcomes of this work broaden the horizon for the understanding of bubble shapes on slippery surfaces and their changes during transportation.

Chemical Reaction-Driven Spreading of an Organic Extractant on the Gas-Water Interface: Insight into the Controllable Formation of a Gas Bubble-Supported Organic Extractant Liquid Membrane

The extraction and recovery of low-concentration valuable metals from various complex aqueous solutions or industrial waste waters have attracted extensive interests in recent years. In our previous works, we suggested a novel technique called bubbling organic liquid membrane extraction by spreading and covering an organic extractant with extremely small volume on the surface of gas bubbles to form a layer of the gas bubble-supported organic liquid membrane for selective extraction and enrichment of low-concentration targets from dilute aqueous solutions. It was found that for successfully performing the bubbling organic liquid membrane extraction, a prerequisite is knowing how to control the formation of a stable organic liquid membrane covered on the surface of gas bubbles. However, once the organic extractant starts to spread on the surface of gas bubbles, the extraction chemical reaction at the interface between the organic extractant liquid membrane and the rare-earth aqueous solution will occur. In the present work, the spreading behavior of the organic extractant P507 on the surface of rare-earth aqueous solutions was investigated and was compared with the behaviors on the surface of deionized water. It was revealed that the spreading of the organic extractant P507 on the surface of aqueous solutions containing rare-earth ions was accelerated because of the occurrence of the chemical reactions at the gas-water interface. The difference in the spreading rate of organic extractant P507 liquid droplets on the surface of deionized water and on that of Er(III) aqueous solutions with an increase in the P507 concentration, the saponification degrees of the P507 extractant, and the preloading amount of Er(III) in the P507 extractant revealed that the chemical reaction at the interface between the spreading P507 thin liquid membrane and the Er(III) aqueous solution would result in the Marangoni convection along the interface, which is in favor of overcoming the resistance from the viscous force when the surface tension gradient replaces gravity as a dominant driving force for the spreading. The present work provides an experimental foundation toward understanding the effect of the interfacial chemical reaction on the spreading behavior of an organic oil droplet on the gas-water interface. It is beneficial for the development of our suggested new technique of bubbling organic liquid membrane extraction and to achieve a controllable generation of a stable gas bubble-supported organic liquid membrane for performing solvent extraction at large aqueous-to-oil phase ratios.

Drop Cargo Transfer via Unidirectional Lubricant Spreading on Peristome-Mimetic Surface

To promote drop mobility, lubricating the gap between liquid drop and solid surface is a facile method which has been widely exploited by nature. Examples include lotus and rice leaves using entrapped air to "lubricate" water and Nepenthes pitcher plant using a slippery water layer to trap insects. Inspired by these, here, we report a strategy for transporting drop cargoes via the unidirectional spreading of immiscible lubricants on the peristome-mimetic surface. Oleophilic/hydrophobic peristome-mimetic surfaces were fabricated through replicating three-dimensional printed samples. The peristome-mimetic surface, via unidirectional immiscible hexadecane spreading, can transport a wide diversity of drop cargoes over a long distance with no loss with controllable drop volumes and velocities, hence mixing multiphase liquids and even reacting liquids. We anticipate this unidirectional drop cargo transport technique will find use in microfluidics, microreactors, water harvesting systems, etc.

Bioinspired Designs of Superhydrophobic and Superhydrophilic Materials

Bioinspired designs of superhydrophobic and superhydrophilic materials have been an important and fascinating area of research in recent years for their extensive potential application prospects from industry to our daily life. Despite extensive progress, existing research achievements are far from real applications. From biomimetic performance to service life, the related research has faced serious problems at present. A timely outlook is therefore necessary to summarize the existing research, to discuss the challenges faced, and to propose constructive advice for the ongoing scientific trend. Here, we comb the process of development of bioinspired superhydrophobic and superhydrophilic materials at first. Then, we also describe how to design artificial superhydrophobic and superhydrophilic materials. Furthermore, current challenges faced by bioinspired designs of superhydrophobic and superhydrophilic materials are pointed out, separately, and the possible solutions are discussed. Emerging applications in this field are also briefly considered. Finally, the development trend within this field is highlighted to lead future research.

Liquids Unidirectional Transport on Dual-Scale Arrays

Liquids unidirectional transport has cutting-edge applications ranging from fog collection, oil-water separation, to microfluidic devices. Despite extensive progresses, existing man-made surfaces with asymmetric wettability or micro/nanoscales structures are still limited by complex fabrication techniques or obscure essential transport mechanisms to achieve unidirectional transport with both high speeds and large volumes. Here, we demonstrate the three-dimensional printed micro/macro dual-scale arrays for rapid, spontaneous, and continuous unidirectional transport. We reveal the essential directional transport mechanism via a Laplace pressure driven theory. The relationship between liquid unidirectional transport and surface morphology parameter is systematically explored. Threshold values to achieve unidirectional transport are determined. Significantly, dual-scale arrays even facilitate liquid's uphill running, microfluidics patterning, and liquid shunting in target directions without external energy input. Free combination of dual-scale island arrays modules, just like LEGO bricks, achieves fast liquid transport on demand. This dual-scale island array can be used to build smart laboratory-on-a-chip devices, printable microfluidic integration systems, and advanced biochemistry microreactors.

Spontaneous directional transportations of water droplets on surfaces driven by gradient structures

Spontaneous directional transportation of droplets on solid surfaces driven by structure gradients has attracted much attention due to its large-scale applications, such as heat transfer, microfluidic devices, water collection, and separation. It also provides new insight for theoretical research into the interactions between droplets and solid surfaces. This review article summarizes recent progress in the spontaneous directional transportation of droplets on surfaces with structure gradients. Currently, surfaces with structure gradients can be divided into three types: wedge corners with a gradient opening angle, wedge-shaped surfaces, and conical substrates. This review focuses on their basic theory, detailed transport processes, fabrication methods, influence factors and application development. Finally, a perspective of this mode of transportation for future development is proposed.

Time-Dependent Liquid Transport on a Biomimetic Topological Surface

Liquid drops impacting on a solid surface is a familiar phenomenon. On rainy days, it is quite important for leaves to drain off impacting raindrops. Water can bounce off or flow down a water-repellent leaf easily, but with difficulty on a hydrophilic leaf. Here, we show an interesting phenomenon in which impacting drops on the hydrophilic pitcher rim of Nepenthes alata can spread outward to prohibit water filling the pitcher tank. We mimic the peristome surface through a designed 3D printing and replicating way and report a time-dependently switchable liquid transport based on biomimetic topological structures, where surface curvature can work synergistically with the surface microtextures to manipulate the switchable spreading performance. Motived by this strange behavior, we construct a large-scaled peristome-mimetic surface in a 3D profile, demonstrating the ability to reduce the need to mop or to squeegee drops that form during the drop impacting process on pipes or other curved surfaces in food processing, moisture transfer, heat management, etc.