Recapillarity: Electrochemically Controlled Capillary Withdrawal of a Liquid Metal Alloy from Microchannels

This paper describes the mechanistic details of an electrochemical method to control the withdrawal of a liquid metal alloy, eutectic gallium indium (EGaIn), from microfluidic channels. EGaIn is one of several alloys of gallium that are liquid at room temperature and form a thin (nm scale) surface oxide that stabilizes the shape of the metal in microchannels. Applying a reductive potential to the metal removes the oxide in the presence of electrolyte and induces capillary behavior; we call this behavior “recapillarity” because of the importance of electrochemical reduction to the process. Recapillarity can repeatably toggle on and off capillary behavior by applying voltage, which is useful for controlling the withdrawal of metal from microchannels. This paper explores the mechanism of withdrawal and identifies the applied current as the key factor dictating the withdrawal velocity. Experimental observations suggest that this current may be necessary to reduce the oxide on the leading interface of the metal as well as the oxide sandwiched between the wall of the microchannel and the bulk liquid metal. The ability to control the shape and position of a metal using an applied voltage may prove useful for shape reconfigurable electronics, optics, transient circuits, and microfluidic components.     

A Novel Strategy for Preparing Stretchable and Reliable Biphasic Liquid Metal

Low‐melting liquid metal is a hugely promising material for flexible conductive patterns due to its excellent conductivity and supercompliance, especially low‐cost and environmental liquid processing technology. However, the ever‐present fluidity characteristic greatly limits the stable shape and reliability of prepared liquid metal conductive electronics. Herein, a novel solidification strategy of liquid GaIn alloys by Ni doping and heat treatment is first reported, which can efficiently create a solid phase in the liquid metal and provide an effective solution for practical applications. Particularly, the liquid characteristic is preserved for conveniently fabricating different flexible electronic circuits, and then the solidification is carried out on prepared conductive patterns by heat treatment. The solidification mechanism is revealed by the interface chemical reaction between Ni and GaIn, creating the solid phase of intermetallic compound (Ga₄Ni₃ and InNi₃) during heat treatment. Moreover, a biphasic GaInNi can be obtained by regulating the atomic ratio of gallium, indium, and nickel. As a result, the obtained GaInNi possesses extremely low sheet resistance (15 ± 4.5 to 135 ± 2.5 mΩ sq^?1) and the variation of ΔR/R₀ exhibits low level (0–2) when strained up to 100%, which offers a promising strategy to prepare stretchable and reliable liquid metal electronics.     

3D Printing of Liquid Metals: Recent Advancements and Challenges

The development of flexible electronics (FEs) has rapidly accelerated in numerous fields due to their exceptional deformability, bending, and stretchability. Room-temperature gallium-based liquid metals (LMs) are considered as efficient conductive materials for FEs due to their outstanding electrical conductivity and intrinsic flexibility. Recently, 3D printing has become a promising technique for fabricating FEs. However, the poor printability due to high surface tension and fluidity offers huge challenges in the 3D printing of LMs. This review summarizes the effective strategies to address these challenges. It primarily focuses on three points: 1) how to improve the printability of LM and its wettability with the substrate, 2) how to select the appropriate printing method to improve the printing speed and ensure the resolution of printing structure, and 3) how to provide perfect encapsulation for LM-based FEs with 3D printing. Following a brief introduction, the mainstream printing technologies and recent developments in the 3D printing of LMs are provided, with an emphasis on the selection of printing method, improvement of printability, encapsulation, and conductivity activation. Then, the revolutionary changes attained after 3D printing of LMs are specifically focused upon. Finally, opinions and potential directions for this thriving discipline are explored.     

Liquid Metal based Stretchable Room Temperature Soldering Sticker Patch for Stretchable Electronics Integration (Adv. Funct. Mater. 36/2023)

Researchers are eagerly developing various stretchable conductors to fabricate devices for next-generation electronics. Most of the major problems in stretchable electronics happen at the connection between rigid and soft parts and the development of reliable soldering material is a major hurdle in stretchable electronics. Though there are attempts to devise new soldering processes for integrating chips and stretchable conductors, they still possess limitations such as mechanical stability, mass production, sophisticated processes, and restricted candidates for conductors and substrates. Here, this study presents a room-temperature universal stretchable sticker-like soldering process that can stretchably solder multiple spots at once and directly fabricates a stretchable device in an in situ manner while a target conductor is installed on one's body. The solder developed in this research possesses high conductivity with a unique freestanding feature enabling the process. It can be elongated when directly positioned between a rigid chip and a rigid conductor, demonstrating its extraordinary stretchability. It is expected that this simple but unique stretchable soldering technique utilizing the invented solder will allow the integration of functional stretchable conductors with highly advanced rigid chips for next-generation stretchable electronics.     

Gallium-Based Liquid–Solid Biphasic Conductors for Soft Electronics

Soft and stretchable electronics have diverse applications in the fields of compliant bioelectronics, textile-integrated wearables, novel forms of mechanical sensors, electronics skins, and soft robotics. In recent years, multiple material architectures have been proposed for highly deformable circuits that can undergo large tensile strains without losing electronic functionality. Among them, gallium-based liquid metals benefit from fluidic deformability, high electrical conductivity, and self-healing property. However, their deposition and patterning is challenging. Biphasic material architectures are recently proposed as a method to address this problem, by combining advantages of solid-phase materials and composites, with liquid deformability and self-healing of liquid phase conductors, thus moving toward scalable fabrication of reliable stretchable circuits. This article reviews recent biphasic conductor architectures that combine gallium-based liquid-phase conductors, with solid-phase particles and polymers, and their application in fabrication of soft electronic systems. In particular, various material combinations for the solid and liquid phases in the biphasic conductor, as well as methods used to print and pattern biphasic conductive compounds, are discussed. Finally, some applications that benefit from biphasic architectures are reviewed.     

Liquid‐Metal Electrode for High‐Performance Triboelectric Nanogenerator at an Instantaneous Energy Conversion Efficiency of 70.6%

Harvesting ambient mechanical energy is a key technology for realizing self‐powered electronics, which has tremendous applications in wireless sensing networks, implantable devices, portable electronics, etc. The currently reported triboelectric nanogenerator (TENG) mainly uses solid materials, so that the contact between the two layers cannot be 100% with considering the roughness of the surfaces, which greatly reduces the total charge density that can be transferred and thus the total energy conversion efficiency. In this work, a liquid‐metal‐based triboelectric nanogenerator (LM‐TENG) is developed for high power generation through conversion of mechanical energy, which allows a total contact between the metal and the dielectric. Due to that the liquid–solid contact induces large contacting surface and its shape adaptive with the polymer thin films, the LM‐TENG exhibits a high output charge density of 430 μC m^?2, which is four to five times of that using a solid thin film electrode. And its power density reaches 6.7 W m^?2 and 133 kW m^?3. More importantly, the instantaneous energy conversion efficiency is demonstrated to be as high as 70.6%. This provides a new approach for improving the performance of the TENG for special applications. Furthermore, the liquid easily fluctuates, which makes the LM‐TENG inherently suitable for vibration energy harvesting.     

Sonication‐Assisted Synthesis of Gallium Oxide Suspensions Featuring Trap State Absorption: Test of Photochemistry

Gallium is a near room temperature liquid metal with extraordinary properties that partly originate from the self‐limiting oxide layer formed on its surface. Taking advantage of the surface gallium oxide (Ga₂O₃), this work introduces a novel technique to synthesize gallium oxide nanoflakes at high yield by harvesting the self‐limiting native surface oxide of gallium. The synthesis process follows a facile two‐step method comprising liquid gallium metal sonication in DI water and subsequent annealing. In order to explore the functionalities of the product, the obtained hexagonal α‐Ga₂O₃ nanoflakes are used as a photocatalytic material to decompose organic model dyes. Excellent photocatalytic activity is observed under solar light irradiation. To elucidate the origin of these enhanced catalytic properties, the electronic band structure of the synthesized α‐Ga₂O₃ is carefully assessed. Consequently, this excellent photocatalytic performance is associated with an energy bandgap reduction, due to the presence of trap states, which are located at ≈1.65 eV under the conduction band minimum. This work presents a novel route for synthesizing oxide nanostructures that can be extended to other low melting temperature metals and their alloys, with great prospects for scaling up and high yield synthesis.     

Lightweight Liquid Metal Entity

Liquid metals are of great importance in developing wearable devices and soft robotics owing to its high conductivity and flexibility. However, the high density of such metals turned out to be big concern for many practical situations. With generalized purpose, a new conceptual material as lightweight liquid metal entity, which can be as light as water, is proposed here. For illustration, an unconventionally ultralight material composed of eutectic galliumindium alloys (eGaIn) and glass bubbles is demonstrated, whose density can be reduced below 2.010 even 0.448 g cm^‐3, even lighter than water, but still maintains excellent conformability, electric conductivity, and stiffness variety under temperature regulation. Such material is further adopted to build various complicated structures through origami or force regulation, representing various application scenarios and can be reused for eight times without evident loss in function. Based on these tests, buoyancy component for water‐related devices is designed, which offers the functions of a switch and loading element. The lightweight liquid metal entities are promising for making diverse advanced soft robotics and underwater devices in the near future.     

Liquid-Metal-Based Dynamic Thermoregulating and Self-Powered Electronic Skin

Electronic skin (E-skin) is an emerging and promising human-machine interface. Besides skin-like functions of tactile perception and stretchability, skin-like comfortabilities, including breathability, moisture permeability, softness, and thermoregulating ability are, also crucial factors for E-skins. Thermoregulation is one of the most important roles of human skin. People can feel comfortable when their skins are regulated at a certain range of temperature. Moreover, it is a dynamic process according to the surrounding temperature. Current E-skins do not have the function of dynamically regulating their temperature. Here, a thermoregulating E-skin (TE-skin) based on liquid metal as a phase change material with its melting point in the comfortable temperature range of human skin is reported. Compared with conventional E-skins, the TE-skin can dynamically termoregulate according to the surrounding temperature through a phase change. Combining with the principle of triboelectric nanogenerator, the TE-skin is also able to act as a self-powered sensor. Based on the self-powered TE-skin, an intelligent dialing communications system is further developed, which can be used to call a cellphone on human skin. For the first time, this study introduces the dynamic thermoregulating concept to E-skins and could open up new opportunities for E-skin developments.     

Creation of Liquid Metal 3D Microstructures Using Dielectrophoresis

Patterning customized arrays of microscale Galinstan or EGaIn liquid metals enables the creation of a variety of microfabricated systems. Current techniques for creating microsized 3D structures of liquid metals are limited by the large dimension or low aspect ratio of such structures, and time‐consuming processes. Here, a novel technique for creating 3D microstructures of Galinstan using dielectrophoresis is introduced. The presented technique enables the rapid creation of Galinstan microstructures with various dimensions and aspect ratios. Two series of proof‐of‐concept experiments are conducted to demonstrate the capabilities of this technique. First, the 3D Galinstan microstructures are utilized as 3D microelectrodes to enhance the trapping of tungsten trioxide (WO₃) nanoparticles flowing through a microfluidic channel. Second, the patterned Galinstan microstructures are utilized as microfins to improve the dissipation of heat within a microfluidic channel that is located onto a hot spot. The presented technique can be readily used for creating customized arrays of 3D Galinstan microstructures for a wide range of applications.     

Liquid Metals-Assisted Synthesis of Scalable 2D Nanomaterials: Prospective Sediment Inks for Screen-Printed Energy Storage Applications

The advents in flexible and smart technology like wearable electronics have accelerated the demand for high-performance energy-storage devices. These devices could significantly reduce the size of the next-generation wearable smart electronics. A selection of suitable printing technology and its product typically offer a reasonable manufacturing pathway like high deposition rate, low materials waste, scalable fabrication, and high-performance production. Therefore, the production of novel functional inks with desirable rheological properties that authorize high-resolution printing, are some major challenges of this technology. This work has an emphasis on the recent advancements in supporting and utilizing liquid metals chemistry to synthesis high-quality and scalable 2D nanomaterials by liquid-phase free exfoliation and facile sonication-assisted methods. These are novel concepts in synthesizing 2D nanomaterials particularly for those which either have not intrinsic layered crystal structures or those with strong interaction between their crystal layers which are difficult to synthesized using conventional approaches. It also provides some potentials to make sustainable ink formulation of such 2D nanostructures for the fabrication of high-quality screen-printed patterns for sustainable energy applications. Subsequently, it deals with the possibilities and challenges of printing such 2D nanomaterials (namely, 2D metal oxides) for micro-supercapacitor and micro-battery applications on an industrially viable scale.     

Liquid–Metal-Superlyophilic and Conductivity–Strain-Enhancing Scaffold for Permeable Superelastic Conductors

Liquid metal (LM) has recently been used as an advanced stretchable material for constructing stretchable and wearable electronics. However, due to the poor wettability of LM and the large dimensional change during stretching, it remains very challenging to obtain a high conductivity with minimum resistance increase over large tensile strains. To address the challenge, an LM-superlyophilic and stretchable fibrous thin-film scaffold is reported, on which LM can be readily coated or printed to form permeable superelastic conductors. In contrast to conventional LM-based conductors where LM particles are filled into an elastic matrix or printed on the surface of an elastic thin film, the LM can quickly infuse into the LM-superlyophilic scaffold and form bi-continuous phases. The LM-superlyophilic scaffold shows unprecedented advantages of an extremely high uptake of the LM and a conductivity-enhancement characteristic when stretched. As a result, the LM-based conductor displays and ultrahigh conductivity of 155 900 S cm⁻¹ and a marginal resistance change by only 2.5 fold at 2 500% strain. The conductor also possesses a remarkable durability over a period of 220 000 cycles of stretching tests. The printing of LM onto the LM-superlyophilic scaffold for the fabrication of various permeable and wearable electronic devices is demonstrated.     

Inertial Microfluidics with Integrated Vortex Generators Using Liquid Metal Droplets as Fugitive Ink

This work demonstrates a simple method for fabricating nearly spherical dome structures on top of lithographically defined microfluidic channels using gallium‐based liquid metal droplets as fugitive ink. The droplets remain stable during the pouring and curing of polydimethylsiloxane and can be easily removed by applying a basic solution. This facilitates the formation of domes with diameters of a few hundred micrometers patterned on the desired locations of the channel. The expansion of the channel at the interface of the dome leads to formation of a large vortex inside the dome. Experiments using high‐speed imaging along with numerical simulations show the utility of the vortex‐induced flow rotation for orbiting of human monocytes and polystyrene microbeads inside the dome. The lateral displacement of liquids caused by the vortex is further utilized for creating controllable multiband flow/color profiles within a T‐mixer. The method enables the fabrication of customized, complex, and 3D microfluidic systems utilizing planar microfabricated structures.     

Graphene Oxide Encapsulating Liquid Metal to Toughen Hydrogel

Stretchable and tough hydrogels are highly required for various flexible devices. Liquid metal (LM) emerges as an attractive applicant in preparing functional hydrogels due to its unique features. However, the high fluidity of LM and incompatibility between LM and polymer matrix make it hard to fabricate tough hydrogels. Herein, inspired by the function of ligaments in biological structure, graphene oxide (GO) nanosheets are introduced to encapsulate LM droplets. GO nanosheets form strong interaction with both LM and polymer matrix to create a stable shell that prevents LM droplet from fracture and exudation to polymer network. The flexible LM/GO core–shell microstructure avoids phase separation and produces a tough hydrogel with stress of high up to 303 kPa at 1240% elongation. It also shows notch insensitivity and strong adhesion to various surfaces. This study opens the possibility of using LM in stretchable and tough hydrogels.     

3D Visible-Light-Driven Plasmonic Oxide Frameworks Deviated from Liquid Metal Nanodroplets

Eutectic gallium-indium (EGaIn) liquid metal droplets have been considered as a suitable platform for producing customized 3D composites with functional nanomaterials owing to their soft and highly reductive surface. Herein, the synthesis of a 3D plasmonic oxide framework (POF) is reported by incorporating the ultra-thin angstrom-scale-porous hexagonal molybdenum oxide (h-MoO₃) onto the spherical EGaIn nanodroplets through ultrasonication. Simultaneously, a large number of oxygen vacancies form in h-MoO₃, boosting its free charge carrier concentration and therefore generating a broad surface plasmon resonance across the whole visible light spectrum. The plasmonic chemical sensing properties of the POF is investigated by the surface-enhanced Raman scattering detection of rhodamine 6G (R6G) at 532 nm, in which the minimum detectable concentration is 10⁻⁸ m and the enhancement factor reached up to 6.14 × 10⁶. The extended optical absorption of the POF also allowed the efficient degradation of the R6G dye under the excitation of ultraviolet-filtered simulated solar light. Furthermore, the POF exhibits remarkable photocurrent responses towards the entire visible light region with the maximum response of ≈ 1588 A W⁻¹ at 455 nm. This work demonstrates the great potential of the liquid metal-based POFs for high-performance sensing, catalytic, and optoelectronic devices.