Fluid instabilities produce complex patterns across natural and industrial systems, yet their precise control remains a fundamental challenge. Here, we demonstrate that hydrodynamic metamaterials, consisting of substrates patterned with Cartesian topographies, enable programmable fingering instabilities in liquid metals. Electrochemical oxidation dramatically lowers the metal’s effective interfacial tension, yet a basic NaOH(aq) electrolyte dissolves the oxidized species, creating a dynamic balance that drives spreading and fractal-like growth. Remarkably, substrates with rectangular arrays of posts or wells dictate the type of highly radial fluidic metal patterns, even though the topographical features are significantly smaller than the instability scale. These results suggest that engineered topographies can steer the coupled electrochemical and hydrodynamic processes, offering a general strategy for controlling the fluid instabilities by harnessing interfacial tension distribution.

Nanozymes are nanomaterials with enzyme-like characteristics that are found in the fields of catalysis, biomedicine, and environmental science. In this work, we present a core-shell liquid metal nanozyme (MnOx@EGaIn) that shows pH-regulated multi-enzyme mimicking capabilities. By harnessing the amphoteric nature of liquid metal surface oxides, these liquid metal nanozymes demonstrate tunable reaction possibilities under various pH conditions (4.0-9.5). This property enables highly efficient enzyme-mimicking activities, including oxidase (OXD, specific activity, SA of 539 U/g), catalase (CAT, SA of 2621 U/g), and superoxide dismutase (SOD, SA of 2391 U/g). Moreover, these liquid metal nanozymes showed notable regenerability, allowing them to be recycled and re-synthesized from their raw material forms. This discovery not only broadens the range of materials and applications for nanozymes but also equips them with the ability to perform multiple enzyme functions while remaining regenerative, providing valuable insights for the design of next-generation enzyme-mimicking materials.

Variable stiffness electronics represent the forefront of adaptive technology, integrating rigid and soft electronics in a single system through dynamic mechanical modulation. While gallium’s high modulus tuning ratio and rapid phase transitions make it ideal for transformative electronic systems (TES), its liquid-state instability, high surface tension, and unintended phase transitions during processing pose substantial challenges. Here, we introduce STiffness-Adjustable temperature-Responsive ink (STAR ink), a chemically sinterable gallium composite electronic ink designed to overcome these obstacles. STAR ink enables high-resolution (~50 micrometers) circuit patterning, large-scale batch fabrication, and three-dimensional structure coating at room temperature. Through pH-controlled chemical sintering, STAR ink–based TES exhibits exceptional mechanical tunability (tuning ratio: 1465) and electrical conductivity (2.27 × 10 6 siemens per meter). Demonstrated applications—from multilayered variable stiffness printed circuit boards (PCBs) matching standard PCBs’ complexity to body-temperature responsive neural probe—underscore STAR ink’s potential for reconfigurable electronics across consumer electronics and biomedical devices.

A plasmonic printing technology is developed to enable rapid, room-temperature, scalable fabrication of all-metal oxide thin-film transistors and circuits.

Ga‐based liquid metals (LMs) are conductive liquids in room conditions, making them attractive for printing conductive patterns. Yet, LM extrudes as droplets because of its high effective interfacial tension and low viscosity. In contrast, liquid metal foams (LMFs) exhibit yield stress and shear‐thinning behavior, which are necessary for extruding filaments for direct ink writing (DIW). LMFs are made by stirring LM in air, thereby entraining oxide‐lined air capsules. Unlike LM mixtures containing metallic particles, LMFs do not embrittle from intermetallic phase formation. Here, DIW of LMF and the challenge of separation of dissimilar phases during extrusion are explored. Incorporating additives, glycerol and tannic acid (TA), into LMF improves the printability of the mixture. The new formulations, LMFG (LMF + glycerol) and LMFGT (LMF + TA‐glycerol solution), extrude more uniformly and produce better print quality than LMF. Interestingly, the viscosity and flow stress of LMF is between that of LMFGT and LMFG. This highlights the limitation of relying on rheological properties alone to predict printability in the case of LMF‐based mixtures. With LM as the continuous phase, these LMF‐based inks exhibit high electrical conductivity. DIW of LMF‐based inks can be done in room conditions without additional pre/ post‐processing, among other advantages.

Dermal interstitial fluid (ISF) is a promising source of biomarkers for point-of-care (PoC) diagnostics, yet noninvasive and reliable extraction remains a significant challenge. In this study, we present a fully passive microneedle (MN) platform that integrates hydrogel-forming MNs, a hydrogel-based osmotic pump, and paper microfluidics to enable zero-power ISF extraction and analyte transport from skin models. The system's performance was evaluated using paper microfluidic designs optimized for both bulk fluid uptake and lateral flow-based detection. Osmotic pumping with glycerol and glucose showed comparable extraction efficiencies. Cortisol, a representative stress biomarker, was successfully recovered following 15-minute, 45-minute, and 24-hour sampling durations, demonstrating the platform's suitability for both short-term and extended ISF monitoring. These results highlight the potential of this integrated MN system as a simple, cost-effective, and minimally invasive solution for passive ISF sampling and subsequent biochemical analysis.

Soft strain sensors are crucial for enabling humanoid robots to perform industrial, medical, and other human-related tasks. However, the limited internal space of humanoid robots exposes soft strain sensors to interference from line resistance, contact resistance, and alternating magnetic fields generated by motor actuator systems and power conversion circuits. To address this issue, inspired by biological neural signal systems, this work proposes an impedance-modulated soft strain sensor with high stability. The sensor combines a liquid metal (LM) resistor, a capacitor, and an inductor to form a passive band-stop filter, which is encapsulated in a soft elastomer. Tensile strain increases the resistance of the LM resistor, reducing the impedance of the sensor at resonance and converting the resistance signal into an impedance-modulated signal. Based on filter theory, the circuit structure of the sensor and the selection of the resistance, capacitance, and inductance components are analyzed in terms of stability, sensitivity, and measurement feasibility. By employing a series connection of resistance and inductance, the sensor achieves high impedance at resonance, effectively suppressing interference from line and contact resistance. Additionally, the frequency of the impedance-modulated sensor does not overlap with the frequency of electromagnetic interference (EMI) from the humanoid robot motor drivers, achieving immunity to EMI. Furthermore, a Field Programmable Gate Array-based signal acquisition system is constructed to measure the impedance-modulated signal. Finally, an application of this impedance-modulated sensor in humanoid robots and wearable devices is demonstrated.

Shape-memory polymers (SMPs) are of tremendous fundamental and practical interest as stimuli-responsive soft materials since they can spontaneously transform from one strain state to another upon environmental stimulation. Most SMPs are dense films possessing a single thermal trigger that responds to a change in temperature. In this work, we examine novel shape-memory nonwovens (SMNs) comprised of electrospun microfibers randomly arranged into mats and possessing a trigger that can be precisely varied. These SMNs derive from physicallycrosslinked triblock copolymers blended with one or more midblock-selective crystallizable hydrocarbons (HCs) with 18-40 carbon units (HC18-HC40). The dependence of the trigger (melting) temperature on blend composition is ascertained by calorimetry for systems composed of a single copolymer and one or more hydrocarbons. Judicious selection of the blends produced here yields designer materials that can be triggered near/at body temperature. Examples of blends satisfying this requirement include a styrenic thermoplastic elastomer (TPE) modified with similar to 40-95 wt% HC20 or 80-95 wt% HC18/HC20 (equimass). While the trigger temperature of the latter ternary blend is independent of TPE molecular weight, SMNs composed of a high-molecular-weight TPE possess remarkable mechanical properties, such as elongations exceeding 3000 % strain. We envisage use of these SMNs in applications such as self-fitting personal protection equipment.

This review focuses on the sintering of liquid metal particles (LMPs). Here, sintering means the partial merging or connecting of particles (or droplets) to form a network of percolated and, thus, conductive electrical pathways. LMPs are attractive materials because they can be suspended in a carrier fluid to create printable inks or distributed in an elastomer to create soft, stretchable composites. However, films and traces of LMPs are not typically conductive as fabricated due to the native oxide that forms on the surface of the particles. In the case of composites, polymers can also get between particles, making sintering more challenging. Sintering can be done via a variety of ways, such as mechanical, thermal, and chemical processing. This review discusses the mechanisms to sinter these particles, patterning techniques that use sintering, unique properties of sintered LMPs, and their practical applications in fields such as stretchable electronics, soft robotics, and active materials.

The transition metal carbide (TMC) Ti3C2Tx exhibits exceptional electrical conductivity, photothermal conversion, and mechanical flexibility. The latter two properties are useful for creating coatings that convert light into heat, which can enable photothermal actuation. However, the performance of such actuators utilizing Ti3C2Tx is constrained by unfavorable surface terminations (e.g., -F) formed during traditional wet acid etching synthesis, which compromise both electrical conductivity and photothermal efficiency. This study introduces a novel plasma-enabled atomic layer etching (plasma-ALE) approach to precisely engineer the surface termination of Ti3C2Tx post-synthesis. The one-step ALE process transforms the surface chemistry from fluorine-dominated to oxygen-dominated terminations, resulting in an 80% increase in electrical conductivity and significantly enhanced photothermal conversion efficiency. By incorporating actuation-enhancing cellulose nanofibrils, ALE-treated Ti3C2Tx/cellulose actuators demonstrate dramatic increases in both bending angle and force under near-infrared light illumination, outperforming other 2D material-based actuators. Various actuator manufacturing techniques, including vacuum filtration and aerosol jet printing, have been employed, demonstrating plasma-ALE’s compatibility with multi-scale pattern designs ranging from millimeter to centimeter dimensions. Furthermore, plasma-ALE treatment facilitates actuators capable of grasping and locomotion. This work paves the way for advanced surface engineering of TMCs and their integration into multifunctional soft robotic systems.

This study investigates the self-assembly of hybrid poly(amidoamine)-peptide dendrimers (DendriPeps) into shear-responsive vesicle-like structures with nanometric thickness, called "Nanocoats", that are capable of encapsulating nano- and microscale particles. To assess the material-agnostic coating power of DendriPeps, we tested the formation of Nanocoats on a variety of synthetic and biological substrates, including polystyrene nanoparticles, poly(N-isopropylacrylamide) microgels, gallium-indium liquid metal nanodroplets, and bacteriophages and lentiviruses. Specifically, we utilized spectroscopic and microscopic techniques to monitor the reversible assembly of Nanocoats on the surface of the particles upon controlling the shear stress of the surrounding aqueous phase. Furthermore, we evaluated the use of Nanocoats as a glue mediating the formation of particle clusters, whose size, in terms of the number of particles and coating thickness, can be dynamically controlled by adjusting the shear stress. Finally, we harnessed the reconfigurability of DendriPep Nanocoats to develop vectors for the shear-controlled delivery of a bioactive payload. To that end, we achieved the controlled release of the antibacterial peptide polymyxin B from DendriPep-coated microgels by applying shear stresses of 0.5-1 Pa. These results demonstrate the potential of DendriPeps to develop reconfigurable systems for biomedical applications that leverage localized shear gradients.

Liquid metals (LM) are emerging plasmonic nanomaterials with transformable surface plasmon resonances (SPR) due to their liquid-like deformability. This study delves into the plasmonic properties of LM nanoparticles, with a focus on EGaIn (eutectic gallium-indium)-based materials. Leveraging Finite-Difference Time-Domain (FDTD) simulations, we explored the localized SPR (LSPR) effects of EGaIn nanoparticles with various shapes, including nanospheres, dimers, nanorods, nanodisks, nanoellipses, nanocubes, and nanocuboids, in the broad range of the ultraviolet (UV)-visible-near infrared (NIR) spectrum. While EGaIn is conventionally known as a UV-active metal alloy, this study reveals unique LSPR features (e.g., higher order resonances, polar and quadrupolar modes) of EGaIn nanostructures in the broader visible and NIR wavelength ranges, providing a comprehensive map of LSPR properties for different shapes of EGaIn nanoparticles. These findings offer new insights into the dependence of the optical properties of EGaIn nanoparticles on their geometries for diverse applications, ranging from biosensing, nanoelectronics, to optomechanical systems.

When in a pristine state, gallium and its alloys have the largest interfacial tensions of any liquid at room temperature. Nonetheless, applying as little as 0.8 V of electric potential across eutectic gallium indium (EGaIn) placed within aqueous NaOH (or other electrolyte) solution will cause the metal to behave as if its interfacial tension is near zero. The mechanism behind this phenomenon has remained poorly understood because NaOH dissolves the oxide species, making it difficult to directly measure the concentration, thickness, or chemical composition of the film that forms at the interface. In addition, the oxide layers formed are atomically-thin. Here, we present a suite of techniques which allow us to simultaneously measure both electrical and interfacial properties as a function of applied electric potential, allowing for new insights into the mechanisms which cause the dramatic decrease in interfacial tension. A key discovery from this work is that the interfacial tension displays hysteresis while lowering the applied potential. We combine these observations with electrochemical impedance spectroscopy to evaluate how these changes in interfacial tension arise from chemical, electrical, and mechanical changes on the interface, and close with ideas for how to build a free energy model to predict these changes from first principles.

Low-melting-point metals, especially those that are liquid at room temperature, are being explored for an increasing number of applications. Like solid-phase metals, liquid metals have high electrical and thermal conductivity. However, liquid metals are also conformal, flexible, and stretchable, even when using thick films and large volumes. The shapes and structures that can be achieved with liquid metals span a variety of geometries and scales, ranging from thin films to 3D structures, and from nanoscale to macroscale feature sizes. Furthermore, liquid metals based on alloys of gallium have been shown to have low toxicity, making them suitable for biomedical devices and wearable electronics. These unique properties of low-melting-point metals make them useful materials in a variety of applications such as soft electronics, catalysis, and microfluidics. This Special Section is a collection of ten research articles and one review article, contributed by experts in applications that utilize low-melting-point metals. The articles can be broadly sorted into three themes: 1) liquid metals for stretchable electronics, 2) liquid metals for energy storage devices and recyclable devices, and 3) fabrication processes enabled by or tailored to the use of low-melting-point metals. The topics covered by this special section illustrate the wide applicability of low-melting-point metals, and the utility of this class of materials in important and trending research areas. Liquid metals are inherently suitable for stretchable electronics, as they can be used to realize deformable electrically conductive materials. In addition, repeated cycles of stretching and bending can result in fatigue of solid materials, but liquid–metal components are unaffected. Furthermore, the low toxicity of gallium-based liquid metals makes them suitable for wearable electronics. In this special section, Liu and co-workers demonstrate a wearable sensor that uses a spiral structure of liquid metal to measure a variety of human motion (article number 2300896). Du and co-workers have reviewed the broader field of stretchable and flexible sensors that employ liquid metals (article number 2300431). Lim and co-workers describe a method of fabricating electrodes that use liquid metal in a sponge-like structure, and demonstrate stretchable sensors and flexible electronic breadboards using these "sponge electrodes" (article number 2301589). Malakooti and co-workers have also created stretchable conductors but with a different approach: printing elastomers impregnated with liquid–metal microdroplets (article number 2301324). Under strain, the microdroplets become electrically connected, and remain conductive after the strain is released. Bae and co-workers also used the direct printing of conductive material, but in their work liquid metal was used as an ink to create a stretchable thermoelectric device (article number 2301171). As seen in these papers, not all of the components of stretchable electronics need to be deformable. Jang and co-workers created a stretchable display that consists of an array of light-emitting-diode pixels with liquid–metal interconnects (article number 2301413). It is beneficial to have flexible and stretchable energy sources for stretchable electronics. Low-melting-point metals are also useful in this area of research. As in other stretchable devices, liquid metals can be used for electrodes in energy storage devices. Toward this end, Tavakoli and co-workers show that graphene oxide coatings on eutectic gallium–indium liquid metal films make them more stable in acidic or alkaline solutions (article number 2301428). The coating thus makes electrodes made from these liquid metals more robust, with a higher capacitance per unit area. This is useful for energy storage in devices such as supercapacitors. In a separate article, Tavakoli and co-workers demonstrate a different type of energy storage for stretchable electronics: a strain-tolerant rechargeable battery (article number 2301189). This battery uses a liquid–metal current collector and a gallium-carbon anode, and is capable of self-healing damage to the gallium-carbon electrode. The battery can still be repaired after more extensive damage, and the metals can be recovered and recycled at the end of the battery's lifetime. Handschuh–Wang and co-workers have also developed devices that can be recycled (article number 2301483). In this case, these are transient stretchable circuits made from gelatin biogel substrates with liquid metal conductive elements. The circuits can be quickly and easily degraded, as the biogel substrate dissolves in hot water in less than a minute. The liquid metal and biogel materials can then be recovered and recycled. The articles mentioned above employ a variety of methods to fabricate the devices and circuits that use low-melting-point metals. These fabrication processes have resulted in many types of novel devices and circuits. However, this special section contains two articles that focus on fabrication methods with broader applicability. Gui and co-workers show that molds made of elastomer and polycarbonate membranes can be used to create 3D metal structures with a minimum size of 10 µm (article number 2301625). In this work, a bismuth-indium alloy with a melting point of 72 °C fills the mold in its liquid state, then is cooled to create the final metal structure, forming a variety of 2D or 3D shapes. Lazarus and co-workers describe a fabrication method that integrates the direct laser writing of microfluidic channels with larger features and substrates made by stereolithography (article number 2301980). These multi-scale structures help with the introduction of liquid metal into microchannels, and enable the fabrication of nH-range coil-type inductors. We thank all the authors in this special section for their valuable contributions to this area of applied research. We also appreciate the other experts who have volunteered their expertise and time during the peer review process. We are especially grateful to Dr. Joseph Krumpfer and Dr. Esther Levy for their efforts in making this special section possible. The papers in this special section span a variety of important and interesting topics, all made possible by the use of low-melting-point metals. We hope that this collection of articles informs and stimulates further research using this unique class of liquid–metal materials. Furthermore, because of the variety of applications, and the increasing popularity of liquid metals, this Special Section has been organized jointly with another Special Issue on liquid metals in Advanced Functional Materials. Interested readers are encouraged to also explore this accompanying special issue (see Guest Editorial for details). The authors declare no conflict of interest. Aaron Ohta is a professor in the Department of Electrical and Computer Engineering at the University of Hawaii at Manoa. He received his B.S. degree from the University of Hawaii at Manoa, his M.S. degree from the University of California, Los Angeles, and his Ph.D. degree from the University of California, Berkeley, all in electrical engineering. Aaron's research interests include reconfigurable circuits and systems using liquid metals and other materials, microfluidics, and microelectromechanical systems (MEMS). Michael D. Bartlett is an associate professor and John R. Jones III Faculty Fellow of Mechanical Engineering at Virginia Tech. He received his B.S.E. from the University of Michigan, his Ph.D. from the University of Massachusetts Amherst, and was a postdoctoral fellow at Carnegie Mellon University. Michael leads the Soft Materials and Structures Lab, which investigates multifunctional soft materials and composites with highly controllable mechanical and functional properties for the creation of soft electronics and robotics based on liquid metal, switchable and intelligent adhesives, and adaptive materials. Michael Dickey is the Camille and Henry Dreyfus professor in the Department of Chemical & Biomolecular Engineering at NC State University. He received a BS in Chemical Engineering from Georgia Institute of Technology (1999) and a PhD from the University of Texas (2006) under the guidance of Professor Grant Willson. From 2006–2008 he was a post-doctoral fellow in the lab of Professor George Whitesides at Harvard University. He completed a sabbatical at Microsoft in 2016 and EPFL in 2023. Michael's research interests include soft matter (liquid metals, gels, polymers) for soft and stretchable devices (electronics, energy harvesters, and soft robotics). Kourosh Kalantar-Zadeh is a professor and head of the School of Chemical and Biomolecular Engineering at the University of Sydney. He is involved in research in the fields of analytical chemistry, materials sciences, gastroenterology, electronics, and sensors. Professor Kalantar–Zadeh is best known for his works on ingestible sensors, liquid metals, and 2D semiconductors. He led his group to the invention of an ingestible chemical sensor: a human gas sensing capsule, one of the breakthroughs in the field of medical devices. He has received several international awards for his scientific contributions including the 2017 IEEE Sensor Council Achievement, and 2020 Robert Boyle Prize of RSC.

This paper reports a simple, disposable, and wearable patch inspired by lateral flow assay (LFA) that can continuously and non-invasively sample sweat for colorimetric sensing of potassium (K+). K+ is an important biomarker for healthy cardiovascular and neuromuscular functions of the body. Current techniques for K+ sensing in sweat commonly use ion-selective electrodes that require complex fabrication steps, signal drift issues, and active perspiration for continuous operation. Here, the sensor uses passive osmotic-capillary principles to collect sweat from the skin without the need of external power for sampling. A skin-mounted hydrogel patch equilibrated with glucose and glycerin solutions can extract fluid from the skin at rest because of the high osmotic strength of the hydrogel compared to sweat. The gel delivers fluid to a commercial colorimetric assay via a paper strip. The assay contains the reagent dipicrylamine which is selective to K+. On-body human trials prove that these patches function with low sweat volumes (similar to 2-3 mu L) and measure K+ concentration in sweat extracted both during exercise and rest. The sweat potassium concentration is independent of the sweat rate and its magnitude matches that of blood. Such economical patches could open opportunities for at-home or in-field point-of-care (POC) diagnostics.

Rapid progresses concerning wearable electronics call for stretchable batteries. Owing to the merits of high energy density and wide operating voltage range, organic electrolyte-based lithium-ion batteries (LIBs) take a large proportion in the powering markets. Due to the high sensitivity to moisture and oxygen, it is difficult to obtain long-term reliable stretchable organic electrolyte-based LIBs when they are directly exposed to the air. Benefiting from the outstanding gas impermeability of liquid metal (LM) and relatively good barrier properties for reactive acid gas molecules by polyisobutylene (PIB), an LM-PIB seal is demonstrated for organic electrolyte-based LIBs. In the seal, LM can prevent the inward permeation of oxygen and water vapor, and PIB can block the outward permeation of hydrolysis acid generated from the side reactions during the operation of the organic electrolyte-based LIBs, which helps prevent the detrimental interaction between the hydrolysis acid and LM. A specific capacity of over 120 mAh g-1 is achieved and the battery shows consistent electrochemical performances under multiple deformations. This study offers a different design approach for long-term operation of stretchable organic electrolyte-based LIBs. Stable seals for stretchable organic electrolyte-based lithium-ion batteries (LIBs) are demonstrated by the integration of liquid metal (excellent hermetic properties and low Young's modulus) and polyisobutylene (good barrier properties against acid gases). The seal can block the inward permeation of the ambient moisture/oxygen and the outward permeation of reactive gases from the inner cell of organic electrolyte-based battery system. With such seal, the organic electrolyte based LIB exhibits good stretchability and operation stability.image

Abstract Four-dimensional (4D) printing is an advanced manufacturing technology that has rapidly emerged as a transformative tool with the capacity to reshape various research domains and industries. Distinguished by its integration of time as a dimension, 4D printing allows objects to dynamically respond to external stimuli, setting it apart from conventional 3D printing. This roadmap has been devised, by contributions of 44 active researchers in this field from 32 affiliations world-wide, to navigate the swiftly evolving landscape of 4D printing, consolidating recent advancements and making them accessible to experts across diverse fields, ranging from biomedicine to aerospace, textiles to electronics. The roadmap’s goal is to empower both experts and enthusiasts, facilitating the exploitation of 4D printing’s transformative potential to create intelligent, adaptive objects that are not only feasible but readily attainable. By addressing current and future challenges and proposing advancements in science and technology, it sets the stage for revolutionary progress in numerous industries, positioning 4D printing as a transformative tool for the future.

Metallic structures with hierarchical open pores that span several orders of magnitude are ideal candidates for various catalyst applications. However, porous metal materials prepared using alloy/dealloy methods still struggle to achieve continuous pore distribution across a broad size range. Herein, we report a printable copper (Cu)/iron (Fe) composite ink that produces a hierarchical porous Cu material with pores spanning over 4 orders of magnitude. The manufacturing process involves four steps: 3D-printing, annealing, dealloying, and reannealing. Because of the unique annealing process, the resulting hierarchical pore surface becomes coated with a layer of Cu-Fe alloy. This feature imparts remarkable catalytic ability and versatile functionality within fixed bed reactors for 4-nitrophenol (4-NP) reduction and Friedlander cyclization. Specifically, for 4-NP reduction, the porous Cu catalyst demonstrates an excellent reaction rate constant (k app = 86.5 x 10-3 s-1) and a wide adaptability of the substrate (up to 1.26 mM), whilst for Friedlander cyclization, a conversion over 95% within a retention time of only 20 min can be achieved by metal-organic-framework-decorated porous Cu catalyst. The utilization of dual metallic particles as printable inks offers valuable insights for fabricating hierarchical porous metallic structures for applications, such as advanced fixed-bed catalysts.

Native oxides form on the surface of many metals. Here, using gallium-based liquid metal alloys, Johnson-Kendall-Roberts (JKR) measurements are employed to show that native oxide dramatically lower the tension of the metal interface from 724 to 10 mN m-1. Like conventional surfactants, the oxide has asymmetry between the composition of its internal and external interfaces. Yet, in comparison to conventional surfactants, oxides are an order of magnitude more effective at lowering tension and do not need to be added externally to the liquid (i.e., oxides form naturally on metals). This surfactant-like asymmetry explains the adhesion of oxide-coated metals to surfaces. The resulting low interfacial energy between the metal and the interior of the oxide helps stabilize non-spherical liquid metal structures. In addition, at small enough macroscopic contact angles, the finite tension of the liquid within the oxide can drive fluid instabilities that are useful for separating the oxide from the metal to form oxide-encased bubbles or deposit thin oxide films (1-5 nm) on surfaces. Since oxides form on many metals, this work can have implications for a wide range of metals and metal oxides in addition to explaining the physical behavior of liquid metal.

Gallium-based liquid metals (LMs) have surface tension an order of magnitude higher than water and break up into micro-droplets when mixed with other liquids. In contrast, silicone oil readily mixes into LM foams to create oil-in-LM emulsions with oil inclusions. Previously, the LM was foamed through rapid mixing in air for an extended duration (over 2 h). This process first results in the internalization of oxide flakes that form at the air-liquid interface. Once a critical fraction of these randomly shaped solid flakes is reached, air bubbles internalize into the LM to create foams that can internalize secondary liquids. Here, we introduce an alternative oil-in-LM emulsion fabrication method that relies on the prior addition of SiO2 micro-particles into the LM before mixing it with the silicone oil. This particle-assisted emulsion formation process provides a higher control over the composition of the LM-particle mixture before oil addition, which we employ to systematically study the impact of particle characteristics and content on the emulsions' composition and properties. We demonstrate that the solid particle size (0.8 mu m to 5 mu m) and volume fraction (1%-10%) have a negligible impact on the internalization of the oil inclusions. The inclusions are mostly spherical with diameters of 20-100 mu m diameter and are internalized by forming new, rather than filling old, geometrical features. We also study the impact of the particle characteristics on the two key properties related to the functional application of the LM emulsions in the thermal management of microelectronics. In particular, we measure the impact of particles and silicone oil on the emulsion's thermal conductivity and its ability to prevent deleterious gallium-induced corrosion and embrittlement of contacting metal substrates.