High Surface Tension Research Articles

Eco-friendly alkali lignin-assisted water-based graphene oxide ink and its application as a resistive temperature sensor

Inkjet-printable ink formulated with graphene oxide (GO) offers several advantages, including aqueous dispersion, low cost, and environmentally friendly production. However, water-based GO ink encounters challenges such as high surface tension, low wetting properties, and reduced ink stability over prolonged storage time. Alkali lignin, a natural surfactant, is promising in improving GO ink’s stability, wettability, and printing characteristics. The concentration of surfactant additives is a key factor in fine-tuning GO ink’s stability and printing properties. The current study aims to explore the detailed effects of alkali lignin concentration and optimize the overall properties of graphene oxide (GO) ink for drop-on-demand thermal inkjet printing. A meander-shaped temperature sensor electrode was printed using the optimized GO ink to demonstrate its practical applicability for commercial purposes. The sensing properties are evaluated using a simple experimental setup across a range of temperatures. The findings demonstrate a significant increase in zeta potential by 25% and maximum absorption by 84.3%, indicating enhanced stability during prolonged storage with an optimized alkali lignin concentration compared to the pure GO dispersions. The temperature sensor exhibits a remarkable thermal coefficient of resistance of 1.21 within the temperature range of 25 °C–52 °C, indicative of excellent sensitivity, response, and recovery time. These results highlight the potential of alkali lignin as a natural surfactant for improving the performance and applicability of inkjet-printable GO inks in various technological applications.

Flexible Finely and Directly Patternable Liquid Metal Electrodes via Selective Surface Wetting Technique

Eutectic gallium–indium (EGaIn) is an ideal material for preparing flexible electrodes, but its high surface tension poses a challenge during deposition and patterning. Herein, we propose a laser-induced selective surface wetting technique (SSWT) to enable the facile and straightforward fabrication of flexible finely and directly patternable EGaIn liquid metal electrodes. Our proposed technique selectively controls the wettability of EGaIn by establishing a perfluorinated self-assembled monolayer on a zinc oxide nanorod array to impart superhydrophobicity and then inducing specific sites on the hydrophilized surface by ultraviolet (UV) pulsed laser ablation, thereby enabling fine patterning (linewidth, ~50 μm). Surface analysis of the effect of laser ablation was also performed to elucidate the mechanism of SSWT. The patterned EGaIn liquid metal electrode fabricated by SSWT exhibited superior flexibility, with a resistance change (ΔR/R0) of only 18.6% compared with a Ag thin film electrode, which showed a dramatic increase in ΔR/R0 to nearly 500% after 50,000 folding cycles at a peak strain of 2.5%. The simple and easily implementable liquid metal patterning technique proposed in this study may potentially be applied in the field of wearable and stretchable electronics, which requires extreme flexibility.

Liquid metals nanotransformer for healthcare biosensors

Featuring low cost, low melting points, excellent biocompatibility, outstanding electrical conductivity, and mechanical properties, gallium-based liquid metals (LMs) have become a promising class of materials to fabricate flexible healthcare sensors. However, the extremely high surface tension hinders their manipulation and cooperation with substrates. To address this problem, the inspiration of nanomaterials has been adopted to mold LMs into LM nanoparticles (LMNPs) with expanded advantages. The transformability of LMNPs endows them with functionalities for sensors in multiple dimensions, such as intelligent response to specific molecules or strains, various morphologies, integration into high-resolution circuits, and conductive elastomers. This review aims to summarize the superior properties of LMs, transformability of LMNPs, and correlated advantages for sensor performance. Multidimensional functional sensing forms consisting of LMNPs and corresponding applications as healthcare sensors will be presented. In the end, the existing challenges and prospects in the processing and application of LMNPs will also be discussed.

Liquid-metal/NdFeB/Ni composites for high wettability, patternable, stretchable electronic wire

Room-temperature liquid-metal (LM) has high conductivity, which is an ideal material for manufacturing stretchable electronic wires. However, its high surface tension and fluidity make it extremely difficult to adhere to or encapsulate on flexible substrates. This work developed LM/NdFeB/Ni composites for stretchable electronic wires. The composites were composed of Ga68.5In21.5Sn10 (with a melting point at −19 °C) alloys, Nd2Fe14B (5 μm), and Ni (50–500 nm) microparticles. The SEM and EDS results showed the porous structure of the composites. Ni particles were added to improve the magnetic properties of LM/NdFeB composites, as well as the porous structure composed of magnetized NdFeB particles. Thereby the structure exhibited a heightened capacity for LM locking. Test results indicate that the liquid metal wires have good wettability and adhesion on different surfaces and could be manufactured using different patterning methods. Moreover, its anti-extrusion and anti-leakage characteristics were suitable for constructing stretchable circuits in open channels or on flat surfaces. The proposed LM/NdFeB/Ni composites are expected to manufacture low-cost, patternable, and anti-extrusion stretchable electronic wires.

Additive manufacturing of complex structures and flow channels using wire-arc thermal spray

The development of a cost-effective additive manufacturing method for millimeter-scale metal structures is essential for the fabrication of novel, high-efficiency heat exchangers and heat sinks. The millimeter-scale fluid channels produced by these structures provide a large surface area to promote heat transfer while minimizing system volume and mass. Traditional additive manufacturing methods are limited by their high cost, high energy consumption, and long processing times. This study presents an efficient additive manufacturing method by using wire-arc thermal spray through a polymer mask. Aluminum and copper structures with arbitrary shapes and a height of 1.3 mm were deposited on a plate 74 mm × 51 mm in size in 30 min and 15 min, respectively. Masks were fabricated by 3D-printing with a high-temperature photopolymer resin. All fabricated structures exhibited a dense center region, a porous side wall region, and a porous satellite particle region between deposited structures. The development of the deposited structures throughout the process is investigated for aluminum (having a low density, surface tension, and melting temperature) and copper (having a high density, surface tension, and melting temperature). The influence of the distance between the mask and the substrate on structure geometry and porosity is studied. The fabrication steps of a liquid-cooled mini-channel heat sink containing an enclosed flow channel are demonstrated using the proposed additive manufacturing method. The results suggest that using wire-arc thermal spray through a polymer mask can be used as a cost-efficient method to additively manufacture metal structures on the millimeter scale.

Pressureless sinter-joining of micron-Ag flake pastes at 160 °C enabled by solvent and interface engineering

Pressureless sinter-joining of cost-effective micron-Ag flake pastes at temperatures below 200 °C poses significant challenges due to their poor surface activity and low tapped density. Herein, we propose a novel solvent and interface engineering approach to enhance the contact among adjacent micron-Ag flakes and between the flakes and substrates. The solvent-enhanced sintering process is proposed, and its effect on the resistivity and shear strength of sintered Ag patterns and joints is studied in detail. Results show that the solvents with high wetting abilities on the Ag flakes and substrates are instrumental in achieving low resistivity and high shear strength in the sintered Ag patterns and joints. In particular, when the solvents possess high surface tension, they generate strong capillary adhesion among Ag flakes and substrates. This reduces the gap between Ag flakes and substrates, thereby promoting bonding among the Ag flakes by minimizing the distance of atom diffusion. Finally, mixed solvents of 50 % α-Terpineol and 50 % dipropyleneglycol methyl ether acetate show a synergistic effect due to the balance between wettability and surface tension, which enables the sintered joints to achieve an exceptional shear strength of 27 MPa even at an ultralow temperature of 160 °C. This achievement represents the state-of-the-art pressureless sinter-joining of Ag pastes and holds promising potential for practical use in power electronic packaging applications.

Lignin‐Based Encapsulation of Liquid Metal Particles for Flexible and High‐Efficiently Recyclable Electronics

AbstractRoom‐temperature liquid metals (RTLMs) have excellent shape reconfiguration capabilities, which make them ideal for flexible electrodes, sensors, and energy devices. However, due to the high surface tension and weak adhesion of RTLM, the types of printing substrates, patterning, and recovery processes are limited. It is essential to develop advanced encapsulation techniques for the patterning of RTLMs. Lignin has great potential for promotion as nanodispersants and nanocarriers because of its abundant hydroxyl groups and good self‐assembly properties. In this work, a green and facile encapsulation method using industrial lignin is reported for stable, uniform, and reproducible patterning of eutectic gallium–indium (EGaIn). Lignin‐encapsulated EGaIn particles exhibit good stability and can be patterned on the surface of various substrates with a simple ballpoint pen. The electrical resistance of the conductive tracks shows little change under bending and twisting (720°) conditions. More importantly, the lignin‐encapsulated system can be easily dissolved and regenerated, which is also supported by molecular dynamics simulations and density functional theory calculations. 96.9% of the EGaIn can be recovered from the system. These characteristics make it very environmentally friendly throughout the preparation process and find applications in flexible sensors, transient circuits, and many other areas.

Breakup-Free and Colorful Liquid Metal Thin Films via Electrochemical Oxidation.

Thin-film metal conductors featuring high conductivity and stretchability are basic building blocks for high-performance conformable electronics. Gallium-based liquid metals are attractive candidates for thin-film conductors due to their intrinsic stretchability and ease of processing. Moreover, the phase change nature of liquid metal provides an opportunity to create conformal electronics in a substrate-free manner. However, thin liquid metal films tend to break during the solid-to-liquid transition due to the high surface tension of liquid metal. Here, we created breakup-free liquid metal thin films by the electrochemical oxidation of solid gallium films. We show that electrochemical oxidation can enhance the mechanical strength of the gallium oxide layer and its interfacial adhesion to the gallium core. When heated to the liquid state, the oxidized gallium films can maintain their structural integrity on various solid substrates, hydrogels, and even the water surface. The solid-liquid phase change-induced stiffness decrease allowed the gallium films to be conformably attached to various nonplanar surfaces upon heating or water transfer printing. Moreover, we also found that enhanced electrochemical oxidation can result in the formation of structure color due to nanoporous structures on the film surface. We also demonstrate the applications of oxidized liquid metal films in functional electronics, electrophysiological monitoring, and tattoo art.

Divide and Conquer: Design of Gallium‐Based Liquid Metal Particles for Soft and Stretchable Electronics

AbstractGallium‐based liquid metal (LM) has attracted considerable attention as a promising material for stretchable conductors due to its remarkable combination of deformability and metallic conductivity. However, LM inherently faces challenges such as high surface tension, resistance increase, electrical failure due to leakage, and limited mechanical stability. Recently, researchers have explored the concept of “dividing” bulk LM into microparticles (LMP) as a means of addressing these limitations. Nonetheless, the fabrication of LMP results in inherent electrical insulation, requiring additional activation steps to enable their use as stretchable conductors. In this review, the potential of LMP is discussed as an alternative to bulk LM and explore various methods to generate stable LMP‐based conductors and electrically activate LMP for their implementation in soft and stretchable electronics.

From Droplets to Devices: Recent Advances in Liquid Metal Droplet Enabled Electronics

AbstractLiquid metals, particularly non‐toxic gallium‐based alloys, have emerged as promising materials for future soft electronics due to their unique properties, including fluidity, excellent electrical and thermal conductivities, and surface reactivity. They demonstrate adaptability, responsivity, and self‐healing abilities, offering a platform for innovative electronic devices. Embodied in a droplet form factor, gallium‐based liquid metal droplets (LMDs) combine the traits of liquid metals with the advantages of miniaturized structures, including high surface tension, high surface area, high mobility, and surface functionalization. This review discusses the inherent properties of LMDs, which have driven substantial research interest across various fields, such as sensors, robotics, electronic circuits, energy harvesters, drug delivery, and microfluidics systems, among others. Effective fabrication and processing techniques are detailed for LMDs, illustrating their role in applications previously challenging with conventional materials, such as reconfigurable, self‐healing, and transient electronics. Existing challenges and future directions in this growing field are discussed. This extensive review seeks to further the understanding of LMDs and their potential, offering a roadmap for their journey from a niche interest to a key material in various electronic devices.

Scalable Electrodeposition of Liquid Metal from an Acetonitrile-Based Electrolyte for Highly Integrated Stretchable Electronics.

The advancement of highly integrated stretchable electronics requires the development of scalable sub-micrometer conductor patterning. Eutectic gallium indium (EGaIn) is an attractive conductor for stretchable electronics, as its liquid metallic character grants it high electrical conductivity upon deformation. However, its high surface tension makes its patterning with sub-micrometer resolution challenging. In this work, this limitation is overcome by way ofthe electrodeposition of EGaIn. A non-aqueous acetonitrile-based electrolyte that exhibits high electrochemical stability and chemical orthogonality is used. The electrodeposited material leads to low-resistance lines that remain stable upon (repeated) stretching to a 100% strain. Because electrodeposition benefits from the resolution of mature nanofabrication methods used to pattern the base metal, the proposed "bottom-up" approach achieves a record-high density integration of EGaIn regular lines of 300nm half-pitch on an elastomer substrate by plating on a gold seed layer prepatterned by nanoimprinting. Moreover, vertical integration is enabled by filling high-aspect-ratio vias. This capability is conceptualized by the fabrication of an omnidirectionally stretchable 3D electronic circuit, and demonstrates a soft-electronic analog of the stablished damascene process used to fabricate microchip interconnects. Overall, this work proposes a simple route to address the challenge of metallization in highly integrated (3D) stretchable electronics.

Current Status and Outlook of Low‐Melting‐Point Metals in Biomedical Applications

AbstractIn recent years, low‐melting‐point metals including liquid metals, exhibiting outstanding physical and chemical properties such as excellent thermal and electrical conductivity, high surface tension, and biocompatibility, have garnered increasing attention from researchers. The melting point of such metals profoundly influences their properties and determines their range of applications, and comprehending the characteristics and properties of low‐melting‐point metals is crucial for their future applications. Although studies related to liquid metals are growing exponentially in particular, reports exploring the properties and applications of low‐melting‐point metals from the perspective of the melting point are still in their early stages. This review aims to comprehensively summarize the key properties and relevant applications of current low‐melting‐point metals described in recent studies, focusing on gallium‐ and bismuth‐based metal alloys. In addition, this review discusses the opportunities and challenges associated with low‐melting‐point metals, and it is anticipated that this review will contribute to the advancement of low‐melting‐point materials in the fields of flexible electronics and biomedicine, particularly for biomedical applications.

MXene-Derived Na+ -Pillared Vanadate Cathodes for Dendrite-Free Potassium Metal Batteries.

Cation-intercalated vanadates, which have considerable promise as the cathode for high-performance potassium metal batteries (PMBs), suffer from structural collapse upon K+ insertion and desertion. Exotic cations in the vanadate cathode may ease the collapse, yet their effect on the intrinsic cation remains speculative. Herein, a stable and dendrite-free PMB, composed of a Na+ and K+ co-intercalated vanadate (NKVO) cathode and a liquid NaK alloy anode, is presented. A series of NKVO with tuneable Na/K ratios are facilely prepared using MXene precursors, in which Na+ is testified to be immobilized upon cycling, functioning as a structural pillar. Due to stronger ionic bonding and lower Fermi level of Na+ compared to K+ , moderate Na+ intercalation could reduce K+ binding to the solvation sheath and favor K+ diffusion kinetics. As a result, the MXene-derived Na+ -pillared NKVO exhibits markedly improved specific capacities, rate performance, and cycle stability than the Na+ -free counterpart. Moreover, thermally-treated carbon paper, which imitates the microscopic structure of Chinese Xuan paper, allows high surface tension liquid NaK alloy to adhere readily, enabling dendrite-free metal anodes. By clarifying the role of foreign intercalating cations, this study may lead to a more rational design of stable and high-performance electrode materials.

Machine learning (ML)-assisted surface tension and oscillation-induced elastic modulus studies of oxide-coated liquid metal (LM) alloys

Pendant drops of oxide-coated high-surface tension fluids frequently produce perturbed shapes that impede interfacial studies. Eutectic gallium indium or Galinstan are high-surface tension fluids coated with a ∼5 nm gallium oxide (Ga2O3) film and falls under this fluid classification, also known as liquid metals (LMs). The recent emergence of LM-based applications often cannot proceed without analyzing interfacial energetics in different environments. While numerous techniques are available in the literature for interfacial studies- pendant droplet-based analyses are the simplest. However, the perturbed shape of the pendant drops due to the presence of surface oxide has been ignored frequently as a source of error. Also, exploratory investigations of surface oxide leveraging oscillatory pendant droplets have remained untapped. We address both challenges and present two contributing novelties- (a) by utilizing the machine learning (ML) technique, we predict the approximate surface tension value of perturbed pendant droplets, (ii) by leveraging the oscillation-induced bubble tensiometry method, we study the dynamic elastic modulus of the oxide-coated LM droplets. We have created our dataset from LM’s pendant drop shape parameters and trained different models for comparison. We have achieved >99% accuracy with all models and added versatility to work with other fluids. The best-performing model was leveraged further to predict the approximate values of the nonaxisymmetric LM droplets. Then, we analyzed LM’s elastic and viscous moduli in air, harnessing oscillation-induced pendant droplets, which provides complementary opportunities for interfacial studies alternative to expensive rheometers. We believe it will enable more fundamental studies of the oxide layer on LM, leveraging both symmetric and perturbed droplets. Our study broadens the materials science horizon, where researchers from ML and artificial intelligence domains can work synergistically to solve more complex problems related to surface science, interfacial studies, and other studies relevant to LM-based systems.

Probing Acid-Induced Compaction of Denatured Proteins by High-Pressure Electrospray Mass Spectrometry.

Further increase in the acidity in the most denaturing acidic solution is known to induce compaction of the fully unfolded protein into a compact molten globule. The phenomenon of "acid-induced folding of proteins" takes place at pH ∼1 in strong acid aqueous solutions with high electrical conductivity and surface tension, a condition that is difficult to handle using conventional electrospray ionization methods for mass spectrometry. Here, high-pressure electrospray ionization (HP-ESI) is used to produce well-resolved mass spectra for proteins in strong acids with pH as low as 1. The compaction of protein conformation is indicated by a large shift in the charge state from high charges to native-like low charges. The addition of salt to the protein in the most denaturing condition also reproduces the compaction effect, thereby supporting the role of anions in this phenomenon. Similar compaction of proteins is also observed in organic solvent/acid mixtures. The charge state of the compacted protein depends on the type of anions that formed ion pairs with a positive charge on the protein. The dissociation of ion pairs during the ionization process forms neutral acids that can be observed by HP-ESI using a soft ion introduction configuration.

Oppositely charged surfactants and nanoparticles at the air-water interface: Influence of surfactant to nanoparticle ratio

HypothesisThe interactions between oppositely charged nanoparticles and surfactants can significantly influence the interfacial properties of the system. Traditionally, in the study of such systems, the nanoparticle concentration is varied while the surfactant concentration is kept constant, or vice versa. However, we believe that a defined variation of both components' concentration is necessary to accurately assess their effects on the interfacial properties of the system. We argue that the effect of nanoparticle-surfactant complexes can only be properly evaluated by keeping the surfactant to nanoparticle ratio constant. ExperimentsZeta potential, dynamic light scattering, high amplitude surface pressure and surface tension measurements are employed synergistically to characterize the interfacial properties of the nanoparticle-surfactant system. Interferometric experiments are performed to highlight the effect of surface concentration on the stability of thin liquid films. FindingsThe interfacial properties of surfactant/nanoparticle mixtures are primarily determined by the surfactant/nanoparticle ratio. Below a certain ratio, free surfactant molecules are removed from the solution by the formation of surfactant-nanoparticle complexes. Surprisingly, even though the concentration and hydrophobicity of these complexes do not seem to have a noticeable impact on the surface tension, they do significantly affect the rheological properties of the interface. Above this ratio, free surfactant monomers and nanoparticle-surfactant complexes coexist and can co-adsorb at the interface, changing both the interfacial tension and the interfacial rheology, and thus, for example, the foamability and foam stability of the system.

Thermocapillary convection in a laser-heated Ni melt pool: A molecular dynamics study

Thermocapillary convection was investigated in a metallic system of pure Ni, at the nanoscale, by molecular dynamics. The system interface was irradiated by a heat flux, mimicking a focused laser source. The melt pool was submitted to a large temperature gradient that modified the surface tension along the interface. In liquid metal, because surface tension typically decreases with increasing temperature, the result is a gradient of surface tension along the free surface. The liquid metal, therefore, started to flow in the direction of high surface tension. Two counter-rotating convection cells developed, characteristic of those observed in welding and other material processing. A systematic estimation of relevant parameters in hydrodynamics allowed us to interpret the results in terms of Prandtl, Marangoni, and Péclet numbers. This study demonstrates the influence of laser power and system size on pool shape and flow characteristics.

Experimental Concerns of Current Collector Interference and Electrolyte Creep During Zinc-Air Battery Testing

Evaluating the electrochemical performance of catalysts towards the oxygen reduction and evolution reactions in zinc-air batteries is a routine process often conducted using a two-electrode cell. At the air electrode, a current collector is necessary if a carbon paper-based gas diffusion layer (GDL) is employed. The catalytic properties of the current collector may interfere with the studied catalyst on the GDL if flaws in cell design allow electrolyte contact to be made with the current collector. At the zinc electrode, highly alkaline electrolytes (e.g., KOH), with high surface tension, can easily climb up the electrode and accumulate at the interface between the cell and the surrounding atmosphere. An oxygen concentration cell is then enabled by the deposited electrolyte and the zinc electrode is rapidly corroded until failure, prematurely ending a long-term cycling test.

Drop rise and interfacial coalescence initiation in Bingham materials

Drop coalescence in viscoplastic materials is present in various industrial applications and the environment. The plasticity of the surrounding material affects drop rise, collision, and film drainage dynamics, and knowledge about these is essential for designing and operating industrial mixer and separator units. Despite its importance, the coalescence of drops in yield stress materials is not entirely understood. In this work, we investigate the effects of the surrounding material plasticity on the rise and interfacial coalescence initiation of Newtonian drops using direct numerical simulations. Plastic effects contribute to the formation of smaller and spherical films, which facilitate the film drainage process on the one hand, but also to an increase in the resistance of the film to flow, which makes the drainage process more difficult on the other hand. The fluids’ interfaces are less deformable for high surface tension regimes, and the flow resistance effects become more significant than the film shape change effects. As a result, the drainage time tends to increase with the level of plasticity. For low surface tension regimes, the fluid interface is more deformable, and film drainage time tends to decrease with an increase in the level of plasticity.

Development and Evaluation of Natural Anti-dandruff Shampoo

The purpose of the study was to develop a pure herbal anti-dandruff shampoo and assess its efficacy and safety. The herbal shampoo was prepared by mixing various amounts of the following extracts into a 10% aqueous solution: Neem (Azadirachta indica), Lemongrass (Cymbopogon citrates), Reetha (Sapindus mukorossi), Sheekakai (Sapindus indica), Amla (Indian gooseberries), Bhringaraj (Eclipta alba), and Brahmi (Bacopa monnieri). The pH was changed, and methylparaben was added as a preservative in very minute amounts. To ascertain the physicochemical characteristics of the prepared shampoos, several tests including visual inspection, pH, wetting time, percentage of solid contents, foam volume and stability, surface tension, detergency, dirt dispersion, etc. were carried out. By giving 20 student volunteers a blind test, the conditioning performance of the herbal shampoo formulation was also assessed. The herbal shampoo was presented simply and appealingly. It demonstrated high detergency, good cleaning, and low surface tension. After 5 minutes, the shampoo’s formulation provided good foam stability. It was discovered that the scores for the conditioning performance of the hair after using a herbal shampoo ranged from 1.75 to 3.4. The results revealed that the specifically formulated shampoo had outstanding conditioning performance, comparable to a shampoo that is easily found in markets. Malassezia furfur was used as a test subject for the efficiency of the multi-herbal anti-dandruff formulations. Formulation F4 demonstrated strong antifungal activity, as measured by the zone of inhibition and lowest inhibitory concentration. However, additional research and development are required to improve its quality and safety.