Eutectic Gallium-indium Alloy Research Articles

Bubble Printing of Liquid Metal Colloidal Particles for Conductive Patterns.

Bubble printing is a patterning method in which particles are accumulated by the convection of bubbles generated by laser focusing. It is attracting attention as a method that enables the high-speed, high-precision patterning of various micro/nanoparticles. Although the bubble printing method is used for metallic particles and organic particles, most reports have focused on the patterning of solid particles and not on the patterning of liquid particles. In this study, liquid metal wiring patterns were fabricated using a bubble printing method in which eutectic gallium‒indium alloy (EGaIn) colloidal particles (≈diameter 0.7 µm) were fixed on a glass substrate by generating microbubbles through heat generation by focusing a femtosecond laser beam on the EGaIn colloidal particles. The wiring was then made conductive by replacing gallium oxide, which served as a resistance layer on the surface of the EGaIn colloidal particles, with silver via galvanic replacement. Fine continuous lines of liquid metal colloids with a line width of 3.4 µm were drawn by reducing the laser power. Liquid metal wiring with a conductivity of ≈1.5 × 105 S/m was formed on a glass substrate. It was confirmed that the conductivity remained consistent even when the glass substrate was bent to a curvature of 0.02 m-1.

Deterministic Fabrication of Liquid Metal Nanopatterns for Nanophotonics Applications

AbstractGallium‐based liquid metals (LMs) are widely used for stretchable and reconfigurable electronics thanks to their fluidic nature and excellent conductivity. These LMs possess attractive optical properties for photonics applications as well. However, due to the high surface tension of the LMs, it is challenging to form LM nanostructures with arbitrary shapes using conventional nanofabrication techniques. As a result, LM‐based nanophotonics has not been extensively explored. Here, a simple yet effective technique is demonstrated to deterministically fabricate LM nanopatterns with high yield over a large area. This technique demonstrates for the first time the capability to fabricate LM nanophotonic structures of various precisely defined shapes and sizes using two different LMs, that is, liquid gallium and liquid eutectic gallium–indium alloy. High‐density arrays of LM nanopatterns with critical feature sizes down to ≈100 nm and inter‐pattern spacings down to ≈100 nm are achieved, corresponding to the highest resolution of any LM fabrication technique developed to date. Additionally, the LM nanopatterns demonstrate excellent long‐term stability under ambient conditions. This work paves the way toward further development of a wide range of LM nanophotonics technologies and applications.

Dynamic Electric Discharge Paths in Liquid Metal Marble Arrays.

Electric discharge occurs ubiquitously in both natural and engineered systems, where the discharge paths provide critical information. However, control and visualization of discharge patterns is a challenging task. Here arrays of liquid metal marbles, droplets of a gallium-indium eutectic alloy with a copper-doped ZnS luminescent coating, are designed for pixelated visualization of electric discharge paths at optical imaging length-scales. The ZnS particles embed themselves into the surface of liquid metal droplets and are anchored by a self-limiting gallium oxide layer. The operation is achieved by generating spark discharges at inter-marble air gaps and reduced voltage drop across highly conducting liquid metal droplets. By taking advantage of the malleability of soft liquid metal marbles, the dynamic visualization platforms allow the manipulation of discharge path selections in configurable marble arrays and the embedding of artificial defect features. The systems are further integrated for characterizing dynamic changes in granular and soft systems, and for enabling logic computing and information encoded display. This demonstration holds promises for creating new-generation electric discharge-based optoelectronics.

The coalescence and oscillation of eutectic gallium indium alloy droplets

Gallium-based liquid metal droplets have excellent deformation performance, and their oscillation has huge potential in oscillator and organ-on-chip research. At present, the oscillation of gallium-based liquid metal droplets controlled by external energy (such as the application of electric fields and chemical reactions) has been thoroughly studied, while there are vacancies in the study of the oscillation due to coalescence of droplets without external energy input. A numerical simulation study on the oscillation process after the coalescence of two and three eutectic gallium indium alloy droplets is conducted. The degree of deformation of the droplet is described by the dimensionless length change in the symmetry axis direction of the line connecting the centers of the initial droplets in the top view, that is, in the y direction. The oscillation curve of the droplet is obtained by the fitting method. The results show that the oscillation of coalesced droplets has obvious periodicity and damping attenuation characteristics, and increasing the number of droplets or reducing the droplet radius will increase the damping coefficient. Furthermore, the ratio of droplet sizes and their initial distribution have an impact on the variations in kinetic energy of the three droplets in the y direction, thereby influencing the deformation of the droplets. This research enriches the theory of gallium-based liquid metal droplet deformation and plays a significant role in guiding the application of droplet oscillation in the fields of micromixers and measuring the physical parameters of solutions.

Liquid-metal-based three-dimensional microelectrode arrays integrated with implantable ultrathin retinal prosthesis for vision restoration

Electronic retinal prostheses for stimulating retinal neurons are promising for vision restoration. However, the rigid electrodes of conventional retinal implants can inflict damage on the soft retina tissue. They also have limited selectivity due to their poor proximity to target cells in the degenerative retina. Here we present a soft artificial retina (thickness, 10 μm) where flexible ultrathin photosensitive transistors are integrated with three-dimensional stimulation electrodes of eutectic gallium–indium alloy. Platinum nanoclusters locally coated only on the tip of these three-dimensional liquid-metal electrodes show advantages in reducing the impedance of the stimulation electrodes. These microelectrodes can enhance the proximity to the target retinal ganglion cells and provide effective charge injections (72.84 mC cm−2) to elicit neural responses in the retina. Their low Young’s modulus (234 kPa), owing to their liquid form, can minimize damage to the retina. Furthermore, we used an unsupervised machine learning approach to effectively identify the evoked spikes to grade neural activities within the retinal ganglion cells. Results from in vivo experiments on a retinal degeneration mouse model reveal that the spatiotemporal distribution of neural responses on their retina can be mapped under selective localized illumination areas of light, suggesting the restoration of their vision.

The Dielectrophoretic Alignment of Biphasic Metal Fillers for Thermal Interface Materials.

Pad-type thermal interface materials (TIMs) with composite structures are required to exhibit high thermal conductivity while maintaining conformal contact with the heat sink, which is strongly influenced by the type and content of the thermally conductive filler. This study presents that biphasic metal particles can be effectively aligned using the dielectrophoretic chaining (DEP-C) mechanism, thereby enhancing the thermal conductivity of a pad-type TIM. A eutectic gallium-indium (EGaIn) alloy liquid metal and solid copper were used as the filler materials with two different phases. The biphasic metal particle mixture of EGaIn and Cu (EGaIn-Cu) were better aligned by DEP-C than when they presented individually because fusion between the two particles increased the effective size. As expected, the thermal conductivity of the TIM composites increased when DEP-C aligned the filler. Notably, TIMs with both EGaIn-Cu fillers showed the largest increase in thermal conductivity, of up to 64.6%, and the highest thermal conductivity values after DEP-C application compared to TIMs with only the EGaIn or Cu filler. Finally, the heat dissipation performance of the TIM composite on a lit light-emitting diode is shown, where the TIM with DEP-C-aligned fillers exhibits improved performance.

Liquid Metal‐Based Perovskite Solar Cells: In Situ Formed Gallium Oxide Interlayer Improves Stability and Efficiency

AbstractIn this study, eutectic gallium–indium alloy (EGaIn) liquid metal is used as the rear electrode for perovskite solar cells (PSCs), where the interfacial properties of the device, particularly the beneficial roles of the surface oxide of the liquid metal, are explored. The findings demonstrate that the native oxide of the EGaIn electrode significantly affects the stability of photovoltaic performance and impedance characteristics including series and shunt resistances. Based on the results, the following hypothesis is formulated: the oxide interlayer serves two crucial functions of a barrier against metal diffusion and a tunnel for enhancing charge extraction and transfer. The results of elemental mapping and trap density calculation support the former function of the hypothesis that the oxide film can effectively prevent metal penetration into the perovskite layer. Furthermore, measurements involving capacitance−voltage and time‐resolved photoluminescence confirm that the oxide film on the liquid metal eliminates the interfacial Schottky barrier, promoting efficient charge extraction and transfer processes. Finally, the investigation is extended to develop flexible PSCs using the EGaIn electrode, which consistently exhibits stable performance during repeated bending cycles. Notably, the EGaIn rear electrode can be readily removed and collected through a straightforward acid treatment, offering a promising avenue for efficient cell recycling.

Enabling High-Performance Pressure and Proximity Dual-Mode Sensing with EGaIn/Ag/ZnO Egg-Shell Ternary Composite Particles.

Eutectic gallium-indium alloy (EGaIn) is a biocompatible liquid metal, promising for wearable electronics. Through functionalization and formation of composites, EGaIn-based materials have shown potential in multifunctional sensing devices. Here, egg-shell EGaIn/Ag/ZnO ternary composite particles were prepared through an ultrasound-assisted displacement reaction combined with room-temperature hydrolysis. The composite was further constructed as a wearable sensor capable of both pressure and proximity detection. For pressure sensing, due to the decrease in the Young's modulus of the egg-shell structure and the presence of the electrical double layers between Ag and ZnO, which enriched surface charges, the sensor showed excellent sensitivity at low pressures (2.17 KPa-1, <0.4 KPa) and thus the ability to sense body movements. For proximity sensing, the composite sensor was able to detect approaching objects that were up to 20 cm away. By combining and fitting the sensing curves for both the touchless and touching modes, the extracted parameters were used to create fingerprints for different objects, demonstrating the great potential of our sensor in the differentiation and identification of unknown objects for future robotics and artificial intelligence.

Functionalized EGaIn Electrodes with Tunable Reduced‐Graphene‐Oxide Assembled EGaIn Core–Shell Particles for Soft and Deformable Electrochemical Biosensors

AbstractGallium‐based liquid metal, eutectic gallium–indium alloy (EGaIn), is becoming a leading innovation in soft electronic devices due to its exceptional electronic conductivity and deformability. However, the formation of insulating oxide films on EGaIn surfaces poses a significant impediment to translating electrochemical reactions, thereby limiting its use as an electrochemical biosensor. Here, functionalized EGaIn electrodes with reduced‐graphene‐oxide assembled EGaIn core–shell particles (REGs) for soft and deformable electrochemical biosensors are presented. Exploiting EGaIn's capability as a reducing agent, REGs can undergo further modification with metal nanoparticles through additional galvanic replacement reactions. The REGs and metal‐coated REGs (M‐REGs) provide strong interfacial adhesion with the EGaIn current collector, exhibiting no exfoliation during and after the mechanical deformation. Moreover, RIDE and metal‐decorated RIDE (M‐RIDE) show excellent electrochemical sensing performances by taking advantage of the facile surface tunability of REGs, allowing simultaneous detection of ascorbic acid, dopamine, and uric acid, and enzymatic detection of glucose. The developed RIDE/M‐RIDE paves the way for utilizing EGaIn in the development of soft and deformable electrochemical biosensors.

Simulations of optical properties of gallium alloy nanorods based on aspect ratio and shell thickness

Liquid plasmonics in contemporary research heavily relies on gallium-based alloy nanoparticles (NPs). The focus of this study is to explore the plasmonic properties of rod-shaped gallium alloy nanoparticles. The primary aim is to examine how variations in Ag shell thickness and aspect ratio affect the optical properties based on localized surface plasmon resonance (LSPR). The outcomes of this study will provide invaluable insights into improving the performance of plasmonic nanostructures. Notably, the focus is solely on analyzing the impact of size and material on the plasmon resonance of gallium-based alloy (GaAg), distinguishing it from the more widely known eutectic gallium-indium (EGaIn) alloy and Galinstan. To investigate the influence of varying aspect ratios and noble metal shell thickness under three different light polarization conditions, the boundary element method (BEM) was utilized. MATLAB-based simulations were employed to generate near-field and far-field plots of the extinction profiles of these nanorods. A large amount of tuning of the extinction spectra observed in the visible domain (380–780 nm) of the electromagnetic spectrum can be achieved with the size variation parameters. The gallium alloy (GaAg) examined in this study exhibits superior plasmonic properties and can be effectively utilized in soft electronic components, plasmonic energy storage devices, and as an alternative to the widely promoted GaIn alloy.

High-Efficiency and Flexible Inkjet-Printed Quasi Metasurface Harvester for Microwave Power Transfer Application

This paper demonstrates an inkjet-printed quasi metasurface(QMS) flexible harvester for microwave power transfer(MPT). The substrate and radiator are separately based on Polyimide and EGaIn(Eutectic gallium indium alloy, in which the mass fraction of Ga and In are 75.5% and 24.5%, respectively). Due to the high electromagnetic energy loss of flexible materials, the electric field distribution is improved by etching on the transmission structure, consequently, the energy loss of the material is reduced. The simulated results show that under vertical incidence at 5.8 GHz by trench etching operation, the absorption efficiency(AE) and harvesting efficiency(HE) are both increased from 93% and 65% to 99% and 82%, respectively. The measured results show that the maximum HE is up to 78%, more than 60% HE can be maintained under multi-angle incident waves and more than 70% HE can be maintained in different conformal cases.

Specific adsorption induced electric double layer on gallium-based liquid metal electrode

Gallium-based liquid metal (GBLM) has been identified as a promising electrode material in flexible aqueous batteries, which are widely employed in wearable electronic devices. The electrode–electrolyte interface has significant effects on battery performance, but its molecular-level structure remains elusive. This work, for the first time, attempts to unravel electric double layer (EDL) at the interface between GBLM electrode and aqueous electrolyte through density functional theory (DFT) calculation and ab initio molecular dynamic (AIMD) simulation. It is found that such EDL structure origins from the specific adsorption of gallates (e.g. Ga(OH)4− and H2GaO3−) on GBLM (e.g. eutectic gallium-indium alloy), and these gallates build up the inner Helmholtz layer (IHL), attracting a diffusion layer with opposite charges. Further, the adsorption properties of gallates and the capacitive behavior of EDL are analyzed, in which the charging of GBLM electrode is systematically considered. The results manifest that the excess negative charge on GBLM surface interferes with the adsorbed gallates and amplifies the interface potential change across EDL. The proposed EDL structure contributes to deep understandings of the electrochemical processes occurring at electrode–electrolyte interface in GBLM aqueous batteries.

Pressure-independent through-plane electrical conductivity measurements of highly filled conductive polymer composites

Highly filled conductive polymer composites (CPCs) are widely used in applications such as bipolar plate materials for polymer electrolyte membrane fuel cells and redox flow batteries, electromagnetic interference shielding and sensors due to their useful electrical properties. A common method for determining through-plane electrical conductivities $\left(\sigma_{\rm tp} \right)$ of such highly filled CPCs applies a conductive carbon paper between electrodes and sample with application of external pressure to improve electrical contact. We show the pressure-dependence of the measured $\sigma_{\rm tp}$ can be eliminated by using a liquid metal such as the gallium-indium eutectic alloy (EGaIn) as contact material. Results indicate that EGaIn reduces contact resistances and cause three to five times larger $\sigma_{\rm tp}$ compared to measurements with carbon paper contacts and pressures up to 20 bar.

Stretchable Strain Sensor with Small but Sufficient Adhesion to Skin.

Stretchable strain sensors that use a liquid metal (eutectic gallium-indium alloy; E-GaIn) and flexible silicone rubber (Ecoflex) as the support and adhesive layers, respectively, are demonstrated. The flexibility of Ecoflex and the deformability of E-GaIn enable the sensors to be stretched by 100%. Ecoflex gel has sufficiently large adhesion force to skin, even though the adhesion force is smaller than that for commercially available adhesives. This enables the sensor to be used for non-invasive monitoring of human motion. The mechanical and electrical properties of the sensor are experimentally evaluated. The effectiveness of the proposed sensors is demonstrated by monitoring joint movements, facial expressions, and respiration.

Liquid Metal‐Based Organohydrogels for Wearable Flexible Electronics

AbstractCurrently, developing organohydrogel‐based wearable flexible electronics (WFEs) has aroused intensive research interest, since they possess superior environmental stabilities than hydrogel‐based WFEs, but the development of organohydrogels is still in the infant stage. Herein, an efficient strategy is developed to achieve the fast preparation of organohydrogels by exploring the high reactivity of eutectic gallium–indium alloy in inducing radical polymerization, and constructing the glycerol/water binary solvent system. The obtained organohydrogels, named liquid metal‐based ion‐conducting organohydrogels (LMIOs), possess excellent environmental adaptability over a wide temperature range (−20 to 100 °C) and high transparency. With outstanding mechanical properties and self‐healing abilities, LMIOs can act as strain sensors, showing high sensing sensitivities in broad strain range (0.1–1000%) and fast response speed (&lt;100 ms). Therefore, LMIOs perform well in sensing strains from human motions. Besides, LMIOs can be constructed on latex gloves and prosthetic fingers, forming an LMIO skin on their surfaces. The latex gloves equipped with the LMIO skin can accurately recognize the gesture changes of users. The LMIO skin on the prosthetic finger behaves like human skin, possessing the capability to detect the finger's movements and self‐healing ability. Hence, LMIOs present great prospects in skin‐attachable devices, robotics, and prosthetics.

Ultrasoft and Ultrastretchable Wearable Strain Sensors with Anisotropic Conductivity Enabled by Liquid Metal Fillers.

Herein, ultrasoft and ultrastretchable wearable strain sensors enabled by liquid metal fillers in an elastic polymer are described. The wearable strain sensors that can change the effective resistance upon strains are prepared by mixing silicone elastomer with liquid metal (EGaIn, Eutectic gallium-indium alloy) fillers. While the silicone is mixed with the liquid metal by shear mixing, the liquid metal is rendered into small droplets stabilized by an oxide, resulting in a non-conductive liquid metal elastomer. To attain electrical conductivity, localized mechanical pressure is applied using a stylus onto the thermally cured elastomer, resulting in the formation of a handwritten conductive trace by rupturing the oxide layer of the liquid metal droplets and subsequent percolation. Although this approach has been introduced previously, the liquid metal dispersed elastomers developed here are compelling because of their ultra-stretchable (elongation at break of 4000%) and ultrasoft (Young’s modulus of <0.1 MPa) mechanical properties. The handwritten conductive trace in the elastomers can maintain metallic conductivity when strained; however, remarkably, we observed that the electrical conductivity is anisotropic upon parallel and perpendicular strains to the conductive trace. This anisotropic conductivity of the liquid metal elastomer film can manipulate the locomotion of a robot by routing the power signals between the battery and the driving motor of a robot upon parallel and perpendicular strains to the hand-written circuit. In addition, the liquid metal dispersed elastomers have a high degree of deformation and adhesion; thus, they are suitable for use as a wearable sensor for monitoring various body motions.

Liesegang Phenomenon of Liquid Metals on Au Film.

Room temperature liquid metals (LM) such as gallium (Ga) own the potential to react with specific materials which would incubate new application categories. Here, diverse self-organized ring patterns due to nonequilibrium reaction-diffusion and spreading-limitation of Ga-based LM clusters on gold (Au) film are reported, among which diffusion is the controlling step and the self-limiting oxide layer plays the role of kinetic barrier. Such phenomena, classically known as the Liesegang rings, mainly occur in electrolyte media. Unlike existing systems, the present periodic crystallization mechanism enables highly symmetric spatiotemporal periodic Liesegang rings on a smaller scale under ambient conditions. Typically, the Ga-Au and eutectic gallium-indium alloy (EGaIn)-Au reaction-diffusion-spreading systems are constructed, obtaining the revert type and hybrid type concentric Liesegang patterns, respectively. The competitive patterning behavior of the intermediate phase products AuGa2 and AuIn2 in hybrid Liesegang patterns is further analyzed by altering the initial Ga/In mass ratio, first-principles calculations, and molecular dynamic simulations. When the mass ratio of In in GaIn alloy exceeds 15%, it will preferentially react with Au. The discovery of LM Liesegang phenomenon is expected to be a flashpoint for self-organized reaction-diffusion systems and offers promising rules for diverse areas such as materials synthesis and the jewelry design industry.

A novel coating to avoid corrosion effect between eutectic gallium-indium alloy and heat sink metal for X-ray optics cooling.

Owing to the parasitic vibration effect of the cooling medium and pipes of X-ray optics, the vibration decoupling cooling method based on eutectic gallium-indium (EGaIn) alloy has become very crucial for fourth generation synchrotron radiation advanced light sources. However, there is an issue that the corrosion of the EGaIn alloy to the heat sink metal [e.g., copper (Cu) plate] results in the solidification and the failure of eliminating the parasitic vibration effect. To deal with the problem, a novel anti-corrosion coating based on tungsten (W) is presented in this paper. It possesses better corrosion resistance performance compared with the traditional coating of nickel (Ni). The experimental investigation was carried out, in which the EGaIn alloy was exposed to several typical metal materials in conditions of various time durations and various temperatures, which were considered as controls. Furthermore, the corrosion effects are analyzed and evaluated in two aspects of micromorphology and the chemical composition by using an optical microscope and a scanning electron microscope as well as x-ray diffraction. The results show that non obvious corrosion occurred for W, 0.33mm and 48 µm thick transition micro-area, respectively, for Cu and Ni. In addition, new substances CuGa2 and Ni3Ga7 occurred, respectively, for Cu and Ni for 36 hours at 250 °C. The EGaIn alloy will freeze after corroding 18 µm substrate for Ni or 30 µm for Cu. Furthermore, the W coating that was prepared by magnetron sputtering has been implemented for feasibility validation.

Plasma-Assisted Growth of Two-Dimensional Ga&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt;/ Gas Heterophases on Liquid Alloy Substrate for Nanoelectronic Applications

Ultra-low-level incorporation of trace impurities and dopants into two-dimensional (2D) semiconductors is a challenging step towards the development of functional electronic devices based on 2D materials. Herein, the incorporation of sulfur atoms into 2D Ga2O3 surface oxide film of eutectic gallium-indium alloy (EGaIn) is achieved through plasma-enhanced metal-catalyst dissociation of H2S gas on EGaIn substrate. This process led to the growth of GaS crystalline nanodomains inside of amorphous 2D Ga2O3 sublayer films. Consequently, 2D lateral heterophases were developed between amorphous Ga2O3 and crystalline GaS nanodomains. The materials characterization revealed the alteration of Photoluminescence (PL) characteristics and change of valence band maximum (VBM) of functionalized 2D films. The comprehensive studies by conductive atomic force microscopy (c-AFM) showed considerable enhancement of conductivity of 2D Ga2O3/GaS materials (300 times improvement) compared with that of 2D Ga2O3 film. This technique has a great potential for fabrication of 2D metal oxide devices with tunable electronic characteristics similar to nanojunction memristors and transistors.

Liquid Metal-Tailored PEDOT:PSS for Noncontact Flexible Electronics with High Spatial Resolution.

Electric field-based noncontact flexible electronics (EF-NFEs) allow people to communicate with intelligent devices through noncontact human-machine interactions, but current EF-NFEs with limited detections (usually <20 cm) distance often lack a high spatial resolution. Here, we report a versatile material for preparing EF-NFE devices with a high spatial resolution to realize everyday human activity detection. Eutectic gallium-indium alloy (EGaIn) was introduced into poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) chains to fabricate this material, named Ga-PP. The introduction of EGaIn successfully regulates the intra- and interchain interactions of PEDOT chains and thus increases the π-electron accumulation on Ga-PP chains, which facilitates improvement of the electron storage of Ga-PP and its noncontact sensing ability. The water solubility of the obtained Ga-PP can reach approximately 15 mg/mL, comparable to that of commercial PEDOT:PSS, thus making Ga-PP suitable for various design strategies to prepare EF-NFE devices. We demonstrate that a conductive textile with a noncontact sensing ability can be achieved by immersing a commercial silk fabric into a Ga-PP solution for 5 min. With a detection distance exceeding 1 m, the prepared Ga-PP-based conductive textile (Ga-PP-CT) possesses outstanding noncontact sensing sensitivity, showing advantages in tracing the locations of signal sources and distinguishing motion states. Surprisingly, even when placed in water, Ga-PP-CT can be used to monitor the movement signals of athletes in different sporting events and output specific noncontact response signals for different sports. Intriguingly, the Ga-PP solution itself can be used to construct noncontact sensing conductive circuits, displaying the potential to be incorporated into smart electronics.