Controlling liquid spreading using microfabricated undercut edges

Recent Progress on Liquid Superspreading and Its Applications

The dynamic spreading of liquids on solid surfaces is essential across numerous daily and industrial processes. Surfaces that enable liquid superspreading, characterized by rapid or extensive spreading, are particularly valuable due to their implications in functional film fabrication, heat management, liquid/liquid separation, and more. Recently, significant research is conducted on liquid superspreading surfaces, with microstructure-regulated surfaces gaining increasing attention. However, the deeper correlations between microstructural physical factors and the superspreading behaviors, along with the relevant applications, remain inadequately understood. This review aims to consolidate the existing knowledge from published results and stimulate further investigation by detailing structures, functionalities, and principles for constructing liquid superspreading surfaces. Examining is began by the energy balance between input and dissipation that underpins droplet spreading dynamics. Then current designs are reviewed for superspreading surfaces, with a focus on chemical and physical aspects, giving greater emphasis to the physical perspective. Additionally, several typical applications are categorized based on liquid superspreading behaviors across various fields. Finally, the prevailing challenges are highlighted and provide insights into future research directions.

Fabrication and Applications of Nature-Inspired Surfaces with Selective Wettability

Inspired by the Stenocora beetle, selective wettability surfaces incorporate alternating wettable and nonwettable surface features that have received substantial attention over the past two decades. These surfaces are sought after for their very promising potential to drive progress in numerous application fields, including ecological protection, biomedical sciences, and industrial technologies. However, despite ongoing efforts to produce such surfaces in commercial quantities, understanding their basic fabrication concepts for practical applications can be challenging, especially for novices, given the vast technical literature in this area. This review, therefore, aims to elucidate the principles of wettability, along with the evolution of selective wettability surfaces and their uses. Beginning with a summary of the essential history and theory of wetting, we explore naturally occurring surfaces that have influenced wetting studies. We then detail state-of-the-art methods for fabricating these unique biwetting surfaces and show how contemporary science employs such designs in solving real-world problems. Finally, we offer an outlook for future research prospects on scalable, printing-based fabrication methods.

Barbed arrow-like structure membrane with ultra-high rectification coefficient enables ultra-fast, highly-sensitive lateral-flow assay of cTnI

Acute myocardial infarction (AMI) has become a public health disease threatening public life safety due to its high mortality. The lateral-flow assay (LFA) of a typical cardiac biomarker, troponin I (cTnI), is essential for the timely warnings of AMI. However, it is a challenge to achieve an ultra-fast and highly-sensitive assay for cTnI (hs-cTnI) using current LFA, due to the limited performance of chromatographic membranes. Here, we propose a barbed arrow-like structure membrane (BAS Mem), which enables the unidirectional, fast flow and low-residual of liquid. The liquid is rectified through the forces generated by the sidewalls of the barbed arrow-like grooves. The rectification coefficient of liquid flow on BAS Mem is 14.5 (highest to date). Using BAS Mem to replace the conventional chromatographic membrane, we prepare batches of lateral-flow strips and achieve LFA of cTnI within 240 s, with a limit of detection of 1.97 ng mL-1. The lateral-flow strips exhibit a specificity of 100%, a sensitivity of 93.3% in detecting 25 samples of suspected AMI patients. The lateral-flow strips show great performance in providing reliable results for clinical diagnosis, with the potential to provide early warnings for AMI.

Topological wetting states of microdroplets on closed-loop structured surfaces: Breakdown of the Gibbs equation at the microscale

Microdroplets are a class of soft matter that has been extensively employed for chemical, biochemical, and industrial applications. However, fabricating microdroplets with largely controllable contact-area shape and apparent contact angle, a key prerequisite for their applications, is still a challenge. Here, by engineering a type of surface with homocentric closed-loop microwalls/microchannels, we can achieve facile size, shape, and contact-angle tunability of microdroplets on the textured surfaces by design. More importantly, this class of surface topologies (with universal genus value = 1) allows us to reveal that the conventional Gibbs equation (widely used for assessing the edge effect on the apparent contact angle of macrodroplets) seems no longer applicable for water microdroplets or nanodroplets (evidenced by independent molecular dynamics simulations). Notably, for the flat surface with the intrinsic contact angle ~0°, we find that the critical contact angle on the microtextured counterparts (at edge angle 90°) can be as large as >130°, rather than 90° according to the Gibbs equation. Experiments show that the breakdown of the Gibbs equation occurs for microdroplets of different types of liquids including alcohol and hydrocarbon oils. Overall, the microtextured surface design and topological wetting states not only offer opportunities for diverse applications of microdroplets such as controllable chemical reactions and low-cost circuit fabrications but also provide testbeds for advancing the fundamental surface science of wetting beyond the Gibbs equation.

Directional Superspreading of Water Droplets on Grooved Hydrogel Surfaces for Open Microfluidic Platforms

Directional liquid spreading has an irreplaceable role in applications such as microfluidic devices, disposable biosensors, and point-of-care diagnostics. However, how to achieve directional, rapid, and complete spreading (i.e., superspreading) of liquids without external energy input is a great challenge. Herein, inspired by the peristome surface of Nepenthes pitcher, the directional superspreading of water droplets on hydrogel surfaces with predesigned microchannels by using the synergistic effect of the liquid-like property of hydrogels and the guidance of anisotropic microstructures is reported. Compared with the smooth ones, hydrogel surfaces with isotropic microstructures can facilitate the superspreading of water droplets, which can be realized within 500 ms in the absence of external forces. Furthermore, directional superspreading and the flow of water droplets are realized under the guidance of anisotropic microgrooves. Such a unique spreading behavior can also be observed on the hydrogel surfaces with various shaped microchannels, such as periodic, bent, shunted, divergent, and confluent morphologies, which have potential for the development of open microfluidic platforms for various healthcare-related applications.

Active Role of Vapor Clouds around Evaporating Sessile Droplets in Microgravity: Marangoni Jets and Electroconvection

A benchmark microgravity experiment (dubbed "ARLES") is analyzed. It concerns evaporation of several-μL sessile droplets with a pinned millimetric circular contact line on a flat substrate into a vast calm (here nitrogen) atmosphere at nearly normal conditions. Hydrofluoroether (HFE-7100) is used as a working liquid whose appreciable volatility and heavy vapor accentuate the contrast between the micro- and normal gravity. A possibility of switching on a DC electric field (EF) of several kV/mm orthogonally to the substrate is envisaged. We here focus on the findings intimately associated with the visualization of the vapor cloud by means of interferometry and rationalized by means of extensive simulations. In particular, with different degrees of unexpectedness, we discover and explore a Marangoni jet (without EF) and electroconvection (with EF) in the gas, which would otherwise be masked by buoyancy convection. Using the same tools, we examine some malfunctions of the space experiment.

Permanent Anticoagulation Blood-Vessel by Mezzo-Sized Double Re-Entrant Structure

Having a permanent omniphobicity on the inner surface of the tube can bring enormous advantages, such as reducing resistance and avoiding precipitation during mass transfer. For example, such a tube can prevent blood clotting when delivering blood composed of complex hydrophilic and lipophilic compounds. However, it is very challenging to fabricate micro and nanostructures inside a tube. To overcome these, a wearability and deformation-free structural omniphobic surface is fabricated. The omniphobic surface can repel liquids by its "air-spring" under the structure, regardless of surface tension. Furthermore, it is not lost an omniphobicity under physical deformation like curved or twisted. By using these properties, omniphobic structures on the inner wall of the tube by the "roll-up" method are fabricated. Fabricated omniphobic tubes still repels liquids, even complex liquids like blood. According to the ex vivo blood tests for medical usage, the tube can reduce thrombus formation by 99%, like the heparin-coated tube. So, it is believed the tube can be soon replaced typical coating-based medical surfaces or anticoagulation blood vessel.

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.

Overflow Control for Sustainable Development by Superwetting Surface with Biomimetic Structure

Liquid flowing around a solid edge, i.e., overflow, is a commonly observed flow behavior. Recent research into surface wetting properties and microstructure-controlled overflow behavior has attracted much attention. Achieving controllable macroscale liquid dynamics by manipulating the micro-nanoscale liquid overflow has stimulated diverse scientific interest and fostered widespread use in practical applications. In this review, we outline the evolution of overflow and present a critical survey of the mechanism of surface wetting properties and microstructure-controlled liquid overflow in multilength scales ranging from centimeter to micro and even nanoscale. We summarize the latest progress in utilizing the mechanisms to manipulate liquid overflow and achieve macroscale liquid dynamics and in emerging applications to manipulate overflow for sustainable development in various fields, along with challenges and perspectives.

Light-Triggered Manipulations of Droplets All in One: Reversible Wetting, Transport, Splitting, and Merging

Here, we establish different ways of light-triggered droplet manipulation such as reversible wetting, splitting, merging, and transport. The unique features of our approach are that the changes in the wetting properties of microscopic droplets of isotropic (oil) or anisotropic (liquid crystalline) liquids adsorbed on photoswitchable films can be triggered just by application of soft optical stimuli, which lead to dynamical, reversible changes in the local morphology of the structured surfaces. The adaptive films consist of an azobenzene-containing surfactant ionically attached to oppositely charged polymer chains. Under exposure to irradiation with light, the azobenzene photoisomerizes between two states, nonpolar trans-isomer and polar cis-isomer, resulting in the corresponding changes in the surface energy and orientation of the surfactant tails at the interface. Additionally, the local increase in the surface temperature due to absorption of light by the azobenzene groups enables diverse processes of manipulation of the adsorbed small droplets, such as the reversible increase of the droplet basal area up to 5 times, anisotropic wetting during irradiation with modulated light, and precise partition of the droplet into many small pieces, which can then be merged on demand to the desired number of larger droplets. Moreover, using a moving focused light spot, we experimentally demonstrate and theoretically explain the locomotion of the droplet over macroscopic distances with a velocity of up to 150 μm·s^-1. Our findings could lead to the ultimate application of a programmable workbench for manipulating and operating an ensemble of droplets, just using simple and gentle optical stimuli.

Flexible and Stable Omniphobic Surfaces Based on Biomimetic Repulsive Air-Spring Structures

In artificial biological circulation systems such as extracorporeal membrane oxygenation, surface wettability is a critical factor in blood clotting problems. Therefore, to prevent blood from clotting, omniphobic surfaces are required to repel both hydrophilic and oleophilic liquids and reduce surface friction. However, most omniphobic surfaces have been fabricated by combining chemical reagent coating and physical structures and/or using rigid materials such as silicon and metal. It is almost impossible for chemicals to be used in the omniphobic surface for biomedical devices due to durability and toxicity. Moreover, a flexible and stable omniphobic surface is difficult to be fabricated by using conventional rigid materials. This study demonstrates a flexible and stable omniphobic surface by mimicking the re-entrant structure of springtail's skin. Our surface consists of a thin nanohole membrane on supporting microstructures. This structure traps air under the membrane, which can repel the liquid on the surface like a spring and increase the contact angle regardless of liquid type. By theoretical wetting model and simulation, we confirm that the omniphobic property is derived from air trapped in the structure. Also, our surface well maintains the omniphobicity under a highly pressurized condition. As a proof of our concept and one of the real-life applications, blood experiments are performed with our flat and curved surfaces and the results including contact angle, advancing/receding angles, and residuals show significant omniphobicity. We hope that our omniphobic surface has a significant impact on blood-contacting biomedical applications.

Tuning Surface Wettability through Hot Carrier Initiated Impact Ionization in Cold Plasma

Advanced surface engineering aims to produce surfaces with well-controlled wettabilities; however, precise control over water imbibition (WI) and liquid spreading on patterned surfaces remains a challenge. Nonthermal atmospheric plasma (NAP) treatment can dramatically change wettability; however, for coated biological objects, such as seeds, plasma interaction is not entirely understood. Herein, we employed atmospheric hybrid cold plasma to elucidate how NAP fundamentally interacts with seed surfaces. We show that NAP can control WI and liquid spreading on seeds. By investigating two distinct seed surface structures and their permeabilities, we show that the modified-surface properties are primarily due to the combined effects of enhanced physical etching and chemical functionalization. We propose the tunable surface functionalization model based on electric field-assisted electron ion-initiated impact ionization enhancing the reactive species generation. Importantly, rice seeds are not damaged by plasma treatment, and 90% of treated seeds germinate upon artificial aging. The ability to control the wettability and liquid spreading of seed surfaces can help achieve seedlings of better quality, especially in difficult-to-grow regions, including those affected by drought. Well-controlled wettability and related attributes open up new avenues for the NAP treatment of a broad range of surfaces.

Microfluidic chips for plasma flow chemistry: application to controlled oxidative processes

A novel biphasic gas/liquid plasma microreactor performed controlled oxidation of cyclohexane into “KA oil” with more than 70% selectivity and more than 10% conversion.

Mapping microscale wetting variations on biological and synthetic water-repellent surfaces

Droplets slip and bounce on superhydrophobic surfaces, enabling remarkable functions in biology and technology. These surfaces often contain microscopic irregularities in surface texture and chemical composition, which may affect or even govern macroscopic wetting phenomena. However, effective ways to quantify and map microscopic variations of wettability are still missing, because existing contact angle and force-based methods lack sensitivity and spatial resolution. Here, we introduce wetting maps that visualize local variations in wetting through droplet adhesion forces, which correlate with wettability. We develop scanning droplet adhesion microscopy, a technique to obtain wetting maps with spatial resolution down to 10 µm and three orders of magnitude better force sensitivity than current tensiometers. The microscope allows characterization of challenging non-flat surfaces, like the butterfly wing, previously difficult to characterize by contact angle method due to obscured view. Furthermore, the technique reveals wetting heterogeneity of micropillared model surfaces previously assumed to be uniform.

Light-Driven Wettability Tailoring of Azopolymer Surfaces with Reconfigured Three-Dimensional Posts

The directional light-induced mass migration phenomenon arising in the photoresponsive azobenzene-containing materials has become an increasingly used approach for the fabrication of controlled tridimensional superficial textures. In the present work we demonstrate the tailoring of the superficial wettability of an azopolymer by means of the light-driven reconfiguration of an array of imprinted micropillars. Few simple illumination parameters are controlled to induce nontrivial wetting effects. Wetting anisotropy with controlled directionality, unidirectional spreading, and even polarization-intensity driven two-dimensional paths for wetting anisotropy are obtained starting from a single pristine pillar geometry. The obtained results prove that the versatility of the light-reconfiguration process, together with the possibility of reversible reshaping at reduced costs, represents a valid approach for both applications and fundamental studies in the field of geometry-based wettability of solid surfaces.

Surface tension-driven self-alignment

Surface tension-driven self-alignment is a passive and highly-accurate positioning mechanism that can significantly simplify and enhance the construction of advanced microsystems. After years of research, demonstrations and developments, the surface engineering and manufacturing technology enabling capillary self-alignment has achieved a degree of maturity conducive to a successful transfer to industrial practice. In view of this transition, a broad and accessible review of the physics, material science and applications of capillary self-alignment is presented. Statics and dynamics of the self-aligning action of deformed liquid bridges are explained through simple models and experiments, and all fundamental aspects of surface patterning and conditioning, of choice, deposition and confinement of liquids, and of component feeding and interconnection to substrates are illustrated through relevant applications in micro- and nanotechnology. A final outline addresses remaining challenges and additional extensions envisioned to further spread the use and fully exploit the potential of the technique.

Bio-inspired hierarchically structured polymer fibers for anisotropic non-wetting surfaces

A rice leaf-like hierarchically textured polymer fiber arrays for anisotropic non-wetting surfaces.

A Novel Bioinspired Continuous Unidirectional Liquid Spreading Surface Structure from the Peristome Surface of Nepenthes alata

A novel unidirectional liquid spreading surface with an inclined arc pitted groove, inspired by the continuous unidirectional liquid spreading mechanism on the peristome surface of N. alata, is explored and fabricated by two-step UV lithography. Its superior unidirectional liquid spreading capability to that of other surface patterns is demonstrated, and its unidirectional liquid spreading mechanism is investigated.

Oil droplet self-transportation on oleophobic surfaces

Directional liquid transportation is important for a variety of biological processes and technical applications. Although surface engineering through asymmetric chemical modification or geometrical patterning facilitates effective liquid manipulation and enables water droplet self-transportation on synthetic surfaces, self-transportation of oil droplets poses a major challenge because of their low surface tension. We report oil droplet self-transportation on oleophobic surfaces that are microtextured with radial arrays of undercut stripes. More significantly, we observe three modes of oil motion on various sample surfaces, namely, inward transportation, pinned, and outward spreading, which can be switched by the structure parameters, including stripe intersection angle and width. Accompanying theoretical modeling provides an in-depth mechanistic understanding of the structure-droplet motion relationship. Finally, we reveal how to optimize the texture parameters to maximize oil droplet self-transportation capability and demonstrate spontaneous droplet movement for liquids down to a surface tension of 22.4 mN/m. The surfaces presented here open up new avenues for power-free liquid transportation and oil contamination self-removal applications in various analytical and fluidic devices.

Maskless, High-Precision, Persistent, and Extreme Wetting-Contrast Patterning in an Environmental Scanning Electron Microscope

A maskless and programmable direct electron beam writing method is reported for making high-precision superhydrophilic-superhydrophobic wetting patterns with 152° contact angle contrast using an environmental scanning electron microscope (ESEM). The smallest linewidth achieved is below 1 μm. The reported effects of the electron beam induced local plasma may also influence a variety of microscopic wetting studies in ESEM.

The Fluid Joint: The Soft Spot of Micro- and Nanosystems

Fluid bridges are ubiquitous soft structures of finite size that conform to and link the surfaces of neighboring objects. Fluid joints, the specific type of fluid bridge with at least one extremity constrained laterally, display even more pronounced reactivity and self-restoration, which make them remarkably suited for assembly, actuation, and manipulation purposes. Their peculiar surface and bulk properties place fluid joints at the rich intersection of diverse scientific interests, and foster their widespread use throughout micro- and nanotechnology. A critical survey of the mechanics and of the manifold applications of fluid bridges and joints in micro- and nanosystems is presented here, along with current challenges and multidisciplinary perspectives.

Controlling flow behavior of water in microfluidics with a chemically patterned anisotropic wetting surface

We report the flow behavior of water in microfluidic systems based on a chemically patterned anisotropic wetting surface. When water flows inside a microchannel on top of a micropatterned surface with alternating hydrophilic/hydrophobic stripes, it exhibits an anisotropic flowing characteristic owing to the anisotropic wettability; thus, the patterned surface acts as a microvalve for the microfluidic system. The anisotropic flow of water is influenced by the microscale features of the patterns and the dimensions of the microchannels. Furthermore, by reasonably combining the patterned surface and microchannel together, we realize the transportation of water in a microchannel along a "virtual" wall, which is the boundary of the hydrophilic and hydrophobic area. We believe that the chemically patterned surfaces could be an alternative strategy to control the flow behavior of water in microfluidic channels.

Lateral flow through a parallel gap driven by surface hydrophilicity and liquid edge pinning for creating microlens array

This letter proposes a surface-energy driven process for economically creating polymer microlens array (MLA) with well controllable curvatures. When a UV-curable prepolymer flows into a cell constructed by multiple holes on a top template and a flat substrate, since the edge pinning of the contact line, an array of curved air/prepolymer interface forms around each microhole of the template. Then a UV-radiation of the bulk prepolymer leads to a solid microlens array. The curvature of the air/prepolymer interface can be controlled by choosing materials with different interface free energy or varying the gap height mechanically.

Plasma-assisted interface engineering of boron nitride nanostructure films

Today many aspects of science and technology are progressing into the nanoscale realm where surfaces and interfaces are intrinsically important in determining properties and performances of materials and devices. One familiar phenomenon in which interfacial interactions play a major role is the wetting of solids. In this work we use a facile one-step plasma method to control the wettability of boron nitride (BN) nanostructure films via covalent chemical functionalization, while their surface morphology remains intact. By tailoring the concentration of grafted hydroxyl groups, superhydrophilic, hydrophilic, and hydrophobic patterns are created on the initially superhydrophobic BN nanosheet and nanotube films. Moreover, by introducing a gradient of the functional groups, directional liquid spreading toward increasing [OH] content is achieved on the films. The resulting insights are meant to illustrate great potentials of this method to tailor wettability of ceramic films, control liquid flow patterns for engineering applications such as microfluidics and biosensing, and improve the interfacial contact and adhesion in nanocomposite materials.

Manipulating and dispensing micro/nanoliter droplets by superhydrophobic needle nozzles

There is rapidly increasing research interest focused on manipulating and dispensing tiny droplets in nanotechnology and biotechnology. A micro/nanostructured superhydrophobic nozzle surface is one promising candidate for the realization of tiny droplet manipulating applications. Here, we explore the feasibility of using superhydrophobicity for guided dispensing of tiny water droplets. A facile dip-coating method is developed to prepare superhydrophobic needle nozzles (SNNs) based on commercial needle nozzles with reduced inner diameter. The SNNs can manipulate tiny droplets of different volumes by only changing the inner diameter of the nozzle, rather than reducing the nozzle size as a whole. Different from the previous electric-field-directed process or pyroelectrodynamic-driven technique, quasi-stable water drops down to the picoliter scale can be produced by SNNs without employing any extra driving mechanisms. Due to their intrinsic superhydrophobic nature, the SNNs also possess the properties of reducing sample liquid retention, improving sample volume transfer accuracy, and saving expensive reagents. In addition, this kind of dip-coating method can also be applied to micropipet tips, inkjet or bio-printer heads, etc. As the issues of reducing drop size and increasing drop volume accuracy are quite important in the laboratory and industry, this facile but effective superhydrophobic nozzle-coating method for manipulating tiny droplets could be of great help to make breakthroughs in next-generation liquid transport and biometric and inkjet printing devices.

Directional oil sliding surfaces with hierarchical anisotropic groove microstructures

A robust directional oil sliding surface is presented by utilizing re-entrant micro-groove arrays inspired from the microgrooves of rice leaf. The overhang micro-groove arrays are shown to provide two primary goals of "omniphobicty" and "anisotropic sliding" with DI water (γlv = 72.1 mN/m) as well as mineral oil (γlv = 28 mN/m) and conventional photoresist.