Liquid Metal Droplets Research Articles

Replacing the Gallium Oxide Shell with Conductive Ag: Toward a Printable and Recyclable Composite for Highly Stretchable Electronics, Electromagnetic Shielding, and Thermal Interfaces.

Liquid metal (LM)-based composites hold promise for soft electronics due to their high conductivity and fluidic nature. However, the presence of α-Ga2O3 and GaOOH layers around LM droplets impairs conductivity and performance. We tackle this issue by replacing the oxide layer with conductive silver (Ag) using an ultrasonic-assisted galvanic replacement reaction. The Ag-coated nanoparticles form aggregated, porous microparticles that are mixed with styrene-isoprene-styrene (SIS) polymers, resulting in a digitally printable composite with superior electrical conductivity and electromechanical properties compared to conventional fillers. Adding more LM enhances these properties further. The composite achieves EMI shielding effectiveness (SE) exceeding 75 dB in the X-band frequency range, even at 200% strain, meeting stringent military and medical standards. It is applicable in wireless communications and Bluetooth signal blocking and as a thermal interface material (TIM). Additionally, we highlight its recyclability using a biodegradable solvent, underscoring its eco-friendly potential. This composite represents a significant advancement in stretchable electronics and EMI shielding, with implications for wearable and bioelectronic applications.

Temperature-triggered liquid metal actuators for fluid manipulation by leveraging phase transition control

ABSTRACT Small-scale pumps for controlling microfluidics have promising applications in drug delivery and chemical assays. Liquid metal (LM) demonstrates excellent flow pumping performance due to its simple structure and the electrocapillary effect under an electric field. However, LM droplets risk escaping from constrained structures, which can lead to pump failure. Temperature regulation is also a critical parameter in optimizing chemical reactions in fluidic systems, however, integrating it into a compact system remains challenging. Here, we develop a temperature-triggered gallium-based actuator (TTGA) by introducing a gallium (Ga) droplet wetted on a copper (Cu) plate as the core element for flow actuation. The Cu plate prevents the Ga droplet from escaping the chamber and significantly increases the flow rate. By leveraging the electrochemical method to inhibit the supercooling effect of Ga, the TTGA enables activation/deactivation for flow actuation at different temperatures. We investigate the impact of electrode position, solution concentration, and applied voltage on TTGA’s pumping efficiency. By dynamically tuning the Ga droplet’s temperature to control phase transition, TTGA allows for accurate flow actuation control. Furthermore, placing Ga and eutectic Ga-indium (EGaIn) droplets in different channels enables the expected flow divergence for fluids with different temperatures. The development of TTGA presents new opportunities in microfluidics and biomedical treatment.

In Situ Alloying Enabled by Active Liquid Metal Filler for Self‐Healing Composite Polymer Electrolytes

AbstractSolid inorganics, known for kinetically inhibiting polymer crystallization and enhancing ionic conductivity, have attracted significant attention in solid polymer electrolytes. However, current composite polymer electrolytes (CPEs) are still facing challenges in Li metal batteries, falling short of inhibiting severe dendritic growth and resulting in very limited cycling life. This study introduces Ga62.5In21.5Sn16 (Galinstan) liquid metal (LM) as an active liquid alternative to conventional passive solid fillers, aiming at realizing self‐healing protection against dendrite problems. Compared to solid inorganics, for example silica, LM droplets could more significantly reduce polymer crystallinity and enhance Li‐ion conductivity due to their liquid nature, especially at temperatures below the polymer melting point. More importantly, LMs are unraveled as dynamic chemical traps, which are capable of blocking and consuming lithium dendrites upon contact via in situ alloying during battery operation and further inhibiting dendritic growth due to the lower deposition energy barrier of the formed Li‐LM alloy. As a proof of concept, by strategically designing an asymmetric CPE with the active LM filling, a solid‐state Li/LiFePO4 battery achieves promising full‐cell functionality with notable rate performance and stable cycle life. This active filler‐mediated self‐healing approach could bring new insights into the battery design in versatile solid‐state systems.

In Situ Alloying Enabled by Active Liquid Metal Filler for Self-Healing Composite Polymer Electrolytes.

Solid inorganics, known for kinetically inhibiting polymer crystallization and enhancing ionic conductivity, have attracted significant attention in solid polymer electrolytes. However, current composite polymer electrolytes (CPEs) are still facing challenges in Li metal batteries, falling short of inhibiting severe dendritic growth and resulting in very limited cycling life. This study introduces Ga62.5In21.5Sn16 (Galinstan) liquid metal (LM) as an active liquid alternative to conventional passive solid fillers, aiming at realizing self-healing protection against dendrite problems. Compared to solid inorganics, for example silica, LM droplets could more significantly reduce polymer crystallinity and enhance Li-ion conductivity due to their liquid nature, especially at temperatures below the polymer melting point. More importantly, LMs are unraveled as dynamic chemical traps, which are capable of blocking and consuming lithium dendrites upon contact via in situ alloying during battery operation and further inhibiting dendritic growth due to the lower deposition energy barrier of the formed Li-LM alloy. As a proof of concept, by strategically designing an asymmetric CPE with the active LM filling, a solid-state Li/LiFePO4 battery achieves promising full-cell functionality with notable rate performance and stable cycle life. This active filler-mediated self-healing approach could bring new insights into the battery design in versatile solid-state systems.

Cellulose nanofibers/liquid metal hydrogels with high tensile strength, environmental adaptability and electromagnetic shielding for temperature monitoring and strain sensors

Hydrogel sensors are widely recognized in the fields of flexible electronics and human motion monitoring due to their multiple properties and potential applications. However, how to prepare hydrogels with multiple excellent properties simultaneously and how to improve the compatibility of conductive fillers with hydrogel matrices remain a major challenge. Therefore, in this work, liquid metal (LM) droplets stabilized by cellulose nanofibers (CNFs) were utilized to initiate the polymerization of acrylamide monomer (Am), which was used as a conductive filler. Meanwhile, reduced graphene oxide (rGO) was introduced to bridge the LM droplets. The hydrogels were then further crosslinked in glycerol. The constructed CNF@LM/polyacrylamide/rGO/gelatin/glycerol hydrogel possesses high tensile properties (>1317 %), high environmental adaptability (−80 to 80 °C), and adhesion properties for multifunctional sensing. What's more, it offers the high sensitivity of both a strain sensor and a temperature sensor for accurate monitoring of human movement at room temperature and even in extreme environments. In addition, this hydrogel has excellent electromagnetic shielding properties and antimicrobial properties. This research opens up a new direction for the preparation of multifunctional hydrogel sensors, expanding their applications in cutting-edge fields such as temperature monitoring, wearable smart devices, e-skin and intelligent robotics.

Lightweight, Highly Extensible, and Anti‐Leak Liquid Metal Elastomer Composites with an Interpenetrating Architecture

In this study, a reformed sugar template‐based method is employed to prepare gallium‐based liquid‐metal‐filled elastomer composites (LMECs), which allows the uniform dispersion of LM droplets within the elastomer matrix in the form of an interpenetrating morphology. In the current preparation, a portion of the liquid metal (LM) is extracted from the elastomer foam following a complete vacuum infusion, resulting in a 10% volume fraction of LM droplets remaining. These droplets exhibit strong adhesion to the inner surfaces of the elastomer foams through the Ga2O3 layer. Nevertheless, the electrical and thermal conductivities are found to be 0.72 × 103 S m−1 and 0.48 W m−1 K−1, respectively. The mechanical performance, piezo‐resistive effect, and thermal conductivity are measured to establish the structure–property relationships. Furthermore, the intrinsic microphysical mechanisms are identified from their distinctive architectures. The reformation of the conventional sugar template approach results in the low concentration and homogeneous dispersion of LM droplets, endowing the resulting LMECs with lightweight and versatile properties, and also mitigating LM leakage to some extent. Herein, this work may prove instrumental in the development of stretchable electronics, soft robots, and multifunctional materials with interpenetrating networks.

A comprehensive study on active mixer performance using liquid metal droplets: an experimental approach

In this experimental study, we delve into the performance of an active mixer that utilizes liquid metal droplets. Our mixer configuration follows an innovative Y-type design, where two liquid metal-based pumping mechanisms operate within the minichannel inlet branches. By applying alternating current voltage to both sides of the liquid metal, we induce a surface tension gradient, propelling the fluid from the reservoirs toward the minichannel. Notably, the two entering liquids are distinctly colored, allowing us to quantify mixing using a Mixing Index (MI) definition and image processing techniques. Our investigation centers on four key variables governing mixer performance: applied voltage, liquid metal droplet diameter, input angle between the branches, and minichannel width. Given the experimental nature of our research, we employ a central composite design method within the framework of design of experiments (DoE). From our obtained experimental results, we establish correlations between MI and the other variables. Our examination of the four mentioned parameters revealed that liquid metal droplet diameter exerts the most significant influence on the mixing index. Following this, in descending order of impact, we find applied voltage, input angle, and channel width. In the optimal dimensions of our mixer, we achieve a remarkable maximum MI of 0.867.

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 collective transportation of many self-propelled liquid metal droplets in Tesla valve

Gallium-based liquid metal droplet is active particle that maintains a liquid phase at room temperature and perform Self-propelled motion in electrolyte solutions. Unlike passive rigid particles, the collective transportation of swarming liquid metal droplets is strongly influenced by the surface tension, the shape deformation, the collision dynamics and the coalescence process of liquid metal droplets. We drive the swarming micron-sized Galinstan droplets to pass a Tesla microfluidic valve by applying external electric field, the diodicity of liquid metal droplet flow in Tesla valve is verified experimentally, which increases with respect to an increasing voltage. Theoretical simulations of flow field within the Tesla microfluidic valve suggested different turbulent flow field with opposite input flows. The coalescence of liquid metal droplets was enhanced when the Tesla valve blocks the flow. The collective transportation of swarming self-propelled liquid metal droplets has promising applications in biomedics, flexible electronics and swarm microrobotic technology.

A study on the impact of liquid metal droplets onto metal and elastomer substrates

A study on the impact of liquid metal droplets onto metal and elastomer substrates

Electrocapillarity‐Induced Hurricane‐in‐a‐Tube Enables the Generation and Patterning of Liquid Metal Droplets

Electrocapillarity‐Induced Hurricane‐in‐a‐Tube Enables the Generation and Patterning of Liquid Metal Droplets

Liquid metal droplets impacting superhydrophobic surface in a viscous (non-oxidizing) environment

The hypothesis of the present research is the existence of distinct spatial-temporal characteristics of non-oxidized liquid metal (LM) droplets impacting a solid surface. To provide a quantitative claim to this hypothesis, we created a test matrix based on the well-known impingement regime map bounded by two dimensionless quantities—Weber number (We) and Ohnesorge number (Oh). The range of these quantities is from 10−2 to 102 (We) and 10−3 to 101 (Oh), leading to Reynolds number (Re) (≡We1/2/Oh) to vary from 10−2 to 104. The class of LMs opted for are post-transition metals—eutectic gallium alloys—due to their several desired practical features, such as low melting point, non-toxicity, and low vapor pressure. The research is conducted using numerical experiments performed using C++ OpenFOAM libraries. To ensure the reliability of the code, we tested our work with numerous impingement behaviors of fluids available in the literature. A plethora of droplet behaviors are reported, such as deposition, rebound, bubble entrapment, and splash. Several features of droplet impingement were critically examined, such as temporal spreading factor, maximum spreading factor, and contact time of droplets on the surfaces. Moreover, the conventional scaling laws regarding the impingement behavior of droplets were tested, with new ones proposed where deemed necessary. Furthermore, a distinct route for the entrapment of droplet is observed, caused by the bulging of LM droplet during the recoiling stage. Emphasis is made to form delineations for these impingement characteristics using dimensionless groups (i.e., We, Oh, and Re).

From Droplets to Devices: Recent Advances in Liquid Metal Droplet Enabled Electronics (Adv. Funct. Mater. 31/2024)

From Droplets to Devices: Recent Advances in Liquid Metal Droplet Enabled Electronics (Adv. Funct. Mater. 31/2024)

Liquid metal-based flexible heat sink for adaptive thermal management

The deformable heat sinks are critical for flexible electronics, wearable personal thermoregulation and energy management. However, the available soft polymer materials with poor thermal characteristics face challenges in achieving efficient heat transfer. Herein, we report an excellent flexible heat sink enabled by a gallium-based liquid metal (LM) for adaptive thermal management. The flexible heat sink is prepared by embedding LM droplets and copper particles into the elastic matrix forming solid–liquid multiphase composites, achieving a considerable thermal conductivity (3.5 W/mK@300 % stretching deformation), outstanding high-temperature resistance, and excellent conformity. The cooling and hydrodynamic characteristics of the flexible heat sink with different structures, including parallel (plane), S-type (plane), and annular (three-dimensional) channels, are investigated and optimized by the thermal experiment and developing the fluid–structure coupling numerical simulation. The results show that the maximum temperature and heat flux achieved by the S-type heat sink are 36.7 °C and 0.68 W/cm2 respectively, due to the secondary flow enhancing convection thermal transfer intensity. The annular heat sink has outstanding advantages in suppressing temperature and improving uniformity because the thermal transfer path is optimized by stretching, especially for larger deformation. Furthermore, the annular heat sink has been proven feasible for lithium battery cooling and pre-insulation.

Automobile exhaust flexible thermoelectric harvester enabled by liquid metal-based heatsink

The softening thermoelectric generator based on a deformable heatsink is expected to facilitate achieving remarkable temperature differences and thermal harvesting capabilities while alleviating exhaust back pressure and engine operating perturbations. However, the available soft polymer materials with poor thermal characteristics face challenges in realizing efficient heat transfer. Herein, we reported a highly integrated automobile exhaust flexible thermoelectric generator (IAE-FTEG) enabled by a liquid metal (LM)-based stretchable heatsink for cylindrical thermal sources to achieve efficiently harnessing waste heat energy and continuously powering multiple vehicle-mounted sensors. IAE-FTEG exhibits excellent flexibility (the bending radius at the apex of 0.57 cm) and outstanding heat transfer capability by introducing the porous sandwich-based soft electrode films and LM-based thermal interface material, achieving an output power enhancement of 25.7 %. The flexible heatsink is prepared by embedding LM droplets and copper particles into the elastic matrix forming solid–liquid multiphase composites, which obtain considerable thermal conductivity (2.40 W/mK@200 % stretching deformation) and excellent geometric conformity. We built the bench and the real vehicle test platform of the IAE-FTEG waste heat recovery system. The simulation test results demonstrate that the maximum power density achieved by the IAE-FTEG waste heat recovery system is 244 kW/m3. The real vehicle tests show that the IAE-FTEG waste heat recovery system has a relatively stable output performance with an average power density of 117 kW/m3 under urban conditions and can realize a stable power supply for back-end loads. Our IAE-FTEG can offer new opportunities in enabling efficient curved-surface waste heat recovery, flexible wearable electronics, and personalized thermal management.

3D laser activatable alloy-free solid/liquid biphasic conductive composites for additive manufacturable electronics

Recently, biphasic soft conductors have gained tremendous attention owing to their characteristic advantages by co-existence of solid and liquid metals. In this study, we developed the 3D printable composite that can generate alloy-free solid/liquid biphasic conductive features by 3D green-laser activation process. It comprises copper nanoparticles with localized surface plasmonic resonance in the visible wavelength regime, eutectic indium-gallium liquid metal droplets, and polyvinylpyrrolidone as a multifunctional polymeric additive. We clarify that, based on the photothermal simulation results, an instant thermal heating is triggered in the vicinity of copper nanoparticles upon green laser irradiation, which activates, comprehensively, the overall microstructural evolution reactions, including a rupture of the oxide shell surrounding the liquid metal droplets, a formation of networked copper particulate assemblies, and a combustion-free carbonization of polymeric substances. This allows us to generate double-layered biphasic conductive features in which the solid conductive compartments are in individual contact with the densified liquid metal bulks, enabling to achieve a high conductivity, excellent flexibility, self-healing capability, and solder processability without the critical problem of leakage of liquid metals.

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.

Chameleon-inspired tunable multi-layered infrared-modulating system via stretchable liquid metal microdroplets in elastomer film

This report presents liquid metal-based infrared-modulating materials and systems with multiple modes to regulate the infrared reflection. Inspired by the brightness adjustment in chameleon skin, shape-morphing liquid metal droplets in silicone elastomer (Ecoflex) matrix are used to resemble the dispersed “melanophores”. In the system, Ecoflex acts as hormone to drive the deformation of liquid metal droplets. Both total and specular reflectance-based infrared camouflage are achieved. Typically, the total and specular reflectances show change of ~44.8% and 61.2%, respectively, which are among the highest values reported for infrared camouflage. Programmable infrared encoding/decoding is explored by adjusting the concentration of liquid metal and applying areal strains. By introducing alloys with different melting points, temperature-dependent infrared painting/writing can be achieved. Furthermore, the multi-layered structure of infrared-modulating system is designed, where the liquid metal-based infrared modulating materials are integrated with an evaporated metallic film for enhanced performance of such system.

Jujube-cake inspired lightweight and flexible MXene/liquid metal/bacterial cellulose electromagnetic interference shielding film with excellent Joule heating and thermal conductivity

The modern 5G communication electronics and systems require lightweight and flexible films that have superior electromagnetic interference (EMI) shielding performance as well as high thermal conductivity. This work reports a facile “one-pot” synthesis strategy to create EMI shielding films with a structure inspired by “jujube cake”. The two-dimensional Ti3C2Tx MXene nanosheets were combined with one-dimensional bacterial cellulose (BC) to form a mechanically entangled supporting framework resembling the structure of a “sponge cake”, wherein zero-dimensional liquid metal (LM) droplets like “jujubes” were ingeniously introduced. A series of multifunctional Ti3C2Tx/LM/BC (TLB) EMI shielding films with highly efficient conductive networks and complete thermal conductivity pathways were prepared through a simple, eco-friendly and highly scalable fabrication process involving vacuum-assisted filtration and hot pressing. Such ultrathin (18 μm) and lightweight (0.63 g cm−3) TLB composite film demonstrates an outstanding specific SE (SSE/t) of 21695.8 dB cm2 g−1. Meanwhile, it exhibits a remarkable in-plane thermal conductivity of 10.44 W m−1 K−1 and exceptional Joule heat performance from room temperature to 95 °C at 3.0 V in seconds. These attractive properties and scalable fabrication of TLB composite film showcase its potential in the realm of flexible electronics, particularly for applications pertaining to EMI shielding protection, electromagnetic compatibility and thermal management.

In-situ microfibrilization of liquid metal droplets in polymer matrix for enhancing electromagnetic interference shielding and thermal conductivity

Herein, liquid metal microfibers (LMM) were constructed in poly (ε-caprolactone) (PCL) matrix and PCL/carbon nanotubes (CNT) composites via an in-situ microfibrilization of liquid metal (LM) droplets by a layer-by-layer stacking method. The aspect ratios of LMMs in the composites can be easily adjusted by controlling the number layers. The effect of LMM aspect ratios on electromagnetic interference (EMI) shielding effectiveness (SE) and thermal conductivity is discussed. The results show that the EMI SE value and the thermal conductivity increase with increasing aspect ratios of LMMs. In addition, the EMI shielding mechanism of PCL/LMM and PCL/LMM/CNT composites is evaluated comprehensively through the combination of electromagnetic simulation and experimental investigation. The efficiently conductive network can be formed in the composites with LMMs, which enhance EMI SE and thermal conductivity. Furthermore, the electric field distribution on the LMM surface is uneven, which enhances the polarization loss ability to electromagnetic waves.