Liquid Metal Droplets Research Articles

Precuring Matrix Viscosity Controls Thermal Conductivity of Elastomeric Composites with Compression‐Activated Liquid and Solid Metallic Filler Networks

AbstractPolymer matrix composites with liquid metal droplet and solid particle fillers are promising candidates for thermal interface materials (TIMs) used in electronics thermal management. To achieve good thermal transport, the particle and droplet fillers must be interconnected to form thermally conductive percolation pathways in the polymer matrix. This in turn requires displacement of the polymer between fillers as well as rupture of the oxide shell on the liquid metal droplets. This study demonstrates a multipronged strategy to achieve extensive filler bridging and a high thermal conductivity polymer TIM pad. The strategy synergistically employs reactive solid and liquid microscale fillers, a polymer matrix with tuned precure viscosity, and mechanical compression during thermal curing of the composite. The data demonstrate that the viscosity of the precursor polymer solution prior to curing plays a major role in the resulting thermal conductivity. More specifically, samples made with low viscosity ≈100 cSt solutions achieve a high thermal conductivity of ≈15 W m−1 K−1 at a curing pressure of 2 MPa. This thermal conductivity is double that achieved with high viscosity ≈2300 cSt solutions. Since many polymer systems employed in industry and research have a high precure viscosity, this insight has important implications for next‐generation TIMs.

Surface-modified liquid metal nanocapsules derived multiple triboelectric composites for efficient energy harvesting and wearable self-powered sensing

Liquid metal (LM) has been widely used as flexible electrodes for triboelectric nanogenerators (TENGs). However, the development of high-performance triboelectric composites based on LM droplets is still challenging due to the mismatched surface tension. Here, copolymer surface-modified LM nanocapsules (CPLM NCs) with core–shell structure are successfully synthesized via two-step method containing chelation reaction and in situ free-radical polymerization. The as-synthesized NCs can be incorporated into various polymers to fabricate diverse flexible polymer/NCs nanocomposites. Based on surface engineering, CPLM will not leak and drop off from nanocomposites when they suffer severe deformations, exhibiting good interfacial compatibility and robust mechanical properties. The introduction of NCs can improve the charge storage capability and electric outputs of various nanocomposites owing to more interfacial polarization sites, showing a versatile strategy for manufacturing high-powered triboelectric composites. The optimized polydimethylsiloxane/NCs-based TENG (PDMS/NCs-TENG) delivers a significant enhancement of power density by 28.3-fold and can availably power wearable electronics. PDMS/NCs-TENG can be also developed as a wearable self-powered sensor to detect physiological signals and joint-related motions in a real-time, rapid, and noninvasive way. This work not only renders a versatile strategy for the development of high-performance triboelectric materials but also provides a sustainable energy solution for wearable electronics.

Joint‐Inspired Liquid and Thermal Conductive Interface for Designing Thermal Interface Materials with High Solid Filling yet Excellent Thixotropy

AbstractFor advanced thermal interface materials (TIMs), massive inorganic addition for high isotropic thermal conductivities conflicts with suitable rheological viscosity for low contact thermal resistance. Traditional strategies rarely resolve such a contradiction, and it remains an academic and industrial challenge. Herein, inspired by the structure and function of the bone joint, a best‐of‐both‐worlds approach is reported that endows a standard polydimethylsiloxane/alumina (PDMS/Al2O3) TIM with simultaneously enhanced rheological mobility and thermal conductivity. It is conducted by employing morphology‐controllable gallium‐based liquid metal (LM) to the surface of Al2O3by a scalable mechanochemical process. At the typical polymer‐LM‐Al2O3interface, LM droplets with low cohesive energy can release the freedom for macromolecular chain relaxation and reduce the viscosity, successfully allowing the high‐loading TIMs (79 vol.%) to keep the thixotropic state and effectively reducing its contact thermal resistance with a copper substrate by 65%. At the same time, adjacent LMs merge to thermally bridge separate Al2O3particles, which facilitates the interfacial thermal conduction and enhances the thermal conductivity from 5.9 to 6.7 W m−1 K−1. Along with additional electrical insulation, this filler modification strategy is believed to inspire others to develop high‐performance polymer‐based TIMs for future advanced electronics.

Liquid-metal-microdroplets-incorporated ultrasoft dielectric gel toward stretchable and healable waste-energy-harvesting devices

In this work, an ultrasoft, healable, and portable device is designed, which is capable of harnessing lost energy from AC-powered gadgets. This soft device consisting of an ultrasoft dielectric silicone gel as a matrix and liquid metal alloy (EGaIn, eutectic gallium indium) microdroplet fillers can reduce the dielectric loss of the targeted system through the polarization of the liquid metal droplets, which allows the transfer of electricity to an attached electrode electrostatically. Although this concept has been previously introduced; the device developed in this work is remarkably capable of harnessing energy after multiple catastrophic failures owing to its healability enabled by the strong cohesive property of the gel matrix. In addition, the polymeric gel is initially too soft and moist for utilization; however, it becomes easier to handle after adding liquid metal fillers that increase its modulus of elasticity owing to the oxide layer of the liquid metal, resulting in creating portable and adhesive soft devices. The practical utility of this harnessed energy is ultimately demonstrated by powering small electrical appliances. Therefore, our research sheds light on the potential of liquid metal composites in harnessing free energy from dielectric loss of electronically powered gadgets, which can prove to be a powerful energy-harvesting system in the future.

Highly sensitive and self-healing conductive hydrogels fabricated from cationic cellulose nanofiber-dispersed liquid metal for strain sensors

The emergence of liquid metal (LM) emulsion as a soft multifunctional filler brings a new opportunity for fabricating hydrogel-based strain sensors with multifunctional properties. However, the extremely large surface tension and high density of LMs inhibit emulsification. Herein, we demonstrated a strategy for stabilizing LM emulsions using cationic cellulose nanofibers (CCNFs) to encapsulate LM droplets through strong electrostatic attraction with LM. By inducing acrylic acid (AA) polymerization in the presence of a CCNF-stabilized LM emulsion, a conductive hydrogel was prepared with the formation of reversible hydrogen bonds, ionic coordination, and electrostatic interactions among CCNFs, LM droplets, and poly(acrylic acid) (PAA). The hydrogel obtained, named the CCNF-LM-PAA hydrogel, shows good conductivity (1.54 S m−1), remarkable tensile strength and elongation at break, self-adhesiveness, and quick self-healing capability. As a strain-sensing material, the CCNF-LM-PAA hydrogel exhibits a very high sensing sensitivity (gauge factor = 16.2), a low strain detection limit (less than 1%), a short response/recovery time (107/91 ms), and good durability (300 cycles). These results enable the CCNF-LM-PAA hydrogel-based strain sensor to be an excellent wearable device for monitoring various human activities. Therefore, introducing additional electrostatic interactions by using CCNFs to stabilize LM emulsions provides a practical way to enhance the strain-sensing performance of LM emulsion-based hydrogels for assembling self-attached wearable devices.

3D actuation of foam-core liquid metal droplets.

Precise manipulation of liquid metal (LM) droplets possesses the potential to enable a wide range of applications in reconfigurable electronics, robotics, and microelectromechanical systems. Although a variety of methods have been explored to actuate LM droplets on a 2D plane, versatile 3D manipulation remains a challenge due to the difficulty in overcoming their heavy weight. Here, foam-core liquid metal (FCLM) droplets that can maintain the surface properties of LM while significantly reducing the density are developed, enabling 3D manipulation in an electrolyte. The FCLM droplet is fabricated by coating LM on the surface of a copper-grafted foam sphere. The actuation of the FCLM droplet is realized by electrically inducing Marangoni flow on the LM surface. Two motion modes of the FCLM droplet are observed and studied and the actuation performance is characterized. Multiple FCLM droplets can be readily controlled to form 3D structures, demonstrating their potential to be further developed to form collaborative robots for enabling wider applications.

Recent progress in liquid metal printing and its applications.

This paper focuses on the latest research printing technology and broad application for flexible liquid metal (LM) materials. Through the newest template printing method, centrifugal force assisted method, pen lithography technology, and laser method, the precision of liquid metal printing on the devices was improved to 10 nm. The development of novel liquid metal inks, such as PVA-LM ink and ethanol/PDMS/LM double emulsion ink, have further enhanced the recovery, rapid printing, high conductivity, and strain resistance. At the same time, liquid metals also show promise in the application of biochemical sensors, photocatalysts, composite materials, driving machines, and electrode materials. Liquid metals have been applied to biomedical, pressure/gas, and electrochemical sensors. The sensitivity, biostability, and electrochemical performance of these LM sensors were improved rapidly. They could continue to be used in healthy respiratory, heartbeat monitoring, and dopamine detection. Meanwhile, the applications of liquid metal droplets in catalytic-assisted MoS2 deposition, catalytic growth of two-dimensional (2D) lamellar, catalytic free radical polymerization, catalytic hydrogen absorption/dehydrogenation, photo/electrocatalysis, and other fields were also summarized. Through improving liquid metal composites, magnetic, thermal, electrical, and tensile enhancement alloys, and shape memory alloys with excellent properties could also be prepared. Finally, the applications of liquid metal in micro-motors, intelligent robot feet, nanorobots, self-actuation, and electrode materials were also summarized. This paper comprehensively summarizes the practical application of liquid metals in different fields, which helps understand LMs development trends, and lays a foundation for subsequent research.

On the question of the fine fragmentation of liquid-metal droplets during steam explosions

On the question of the fine fragmentation of liquid-metal droplets during steam explosions

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.

Magnetomechanical Transport of Liquid Metal Droplets via Asymmetric Microwall Arrays

AbstractEutectic gallium indium liquid metal (LM) is a promising conductive liquid for various electronic applications. In particular, the directional transport of LM droplets has potential applications in soft electronics to control electrical conductivity. Existing methods transport LM droplets by applying an electric field to generate an interfacial tension difference within the LM droplet due to nonuniform ionic distribution of the electrical double layer. However, these methods require confined channels and tethered systems to apply the electric field. In this study, channel‐free wireless transport of LM droplets is demonstrated via on‐demand magnetomechanical actuation of asymmetric microwall arrays comprising vertically aligned ferromagnetic iron particles embedded in polydimethylsiloxane. Asymmetric microwall of two different widths is designed to generate an asymmetric bending stiffness at a given magnetic field. The asymmetric microwall bends gradually in response to a linear external magnetic field perpendicular to the alignment axis of the iron particles. Therefore, nonuniform magnetomechanical bending induces a local height gradient along the microwall, causing gravitational‐force‐driven roll‐off motion of the LM droplet. Transport direction of LM droplet is modulated by varying the geometric parameters. Finally, the opposite‐directional transport of two LM droplets is demonstrated under a linear magnetic field by pre‐programming the asymmetric bending direction of each microwall array.

Assessment of the Cytotoxicity of Nano Gallium Liquid Metal Droplets for Biomedical Applications

Gallium-based liquid metals (LMs) have emerged as novel materials for biomedical applications. Gallium (Ga) is considered significantly less toxic than other LMs, such as mercury. However, Ga droplets have not been thoroughly assessed for their cytotoxicity. Here, we report the concentration-dependent cytotoxic effects of Ga nano- and microdroplets on HaCaT cells, a human keratinocyte cell line. We found that Ga droplets demonstrated substantial cytotoxic effects at 1000 μg/mL after 24 h exposure, while lower concentrations such as 500 μg/mL indicated only a slight reduction in cell viability at longer exposure times. At higher concentrations, it was observed that the cells were unable to form confluent layers and the cytoplasmic membrane became damaged. The formation of a protein corona on the Ga droplets in cell media is also described, which will be an important consideration for future research on Ga droplets for bio-applications. Importantly, this work provides an indication of the concentration at which Ga droplets become damaging to human cells, which will be useful information for a wide range of researchers and regulatory bodies.

Breakup dynamics and scaling laws of liquid metal droplets formed in a cross junction

The formation of liquid metal droplets in a microfluidic cross junction with different viscosities of the continuous phase is experimentally and theoretically investigated. The flow pattern is distinguished into squeezing and dripping by the presence of gaps. The differences in driving forces between the two patterns are analyzed theoretically and confirmed experimentally by Micro-PIV results. Roles of the squeezing force and the shear force in droplet formation are varying due to the dynamic evolutions of the gap width which reduces with time. Therefore, critical values of the initial gap width are obtained which are the outcome of the competition between neck thinning and tip growing. Compared to water–oil systems, gaps are more difficult to be formed due to the extremely high interfacial tension. Moreover, universal flow pattern maps are constructed using dimensionless parameters of Wed, Cac, and Rec, proving the important roles of both the squeezing force and the shear force. A scaling law of the droplet volume considering the influence of the viscosity ratio is proposed and a very good agreement between experimental data and theoretical predictions is obtained for different liquid systems.

Graphene and Liquid Metal Integrated Multifunctional Wearable Platform for Monitoring Motion and Human-Machine Interfacing.

Motion sensors are an essential component of many electronic systems. However, the development of inertial motion sensors based on fatigue-free soft proof mass has not been explored extensively in the field of soft electronics. Nontoxic gallium-based liquid metals are an emerging class of material that exhibit attractive electromechanical properties, making them excellent proof mass materials for inertial sensors. Here, we propose and demonstrate a fully soft laser-induced graphene (LIG) and liquid metal-based inertial sensor integrated with temperature, humidity, and breathing sensors. The inertial sensor design confines a graphene-coated liquid metal droplet inside a fluidic channel, rolling over LIG resistive electrode. The proposed sensor architecture and material realize a highly mobile proof mass and a vibrational space for its oscillation. The inertial sensor exhibits a high sensitivity of 6.52% m-1 s2 and excellent repeatability (over 12 500 cycles). The platform is fabricated using a scalable, rapid laser writing technique and integrated with a programmable system on a chip (PSoC) to function as a stand-alone system for real-time wireless monitoring of movement patterns and the control of a robotic arm. The developed printed inertial platform is an excellent candidate for the next-generation of wearables motion tracking platforms and soft human-machine interfaces.

A fully soft, self-powered vibration sensor by laser direct writing

Soft electronics are comprised of stretchable and user-friendly components that mechanically conform to complex surfaces and epidermis, extending their applicability over commercial electronics. Vibration sensors that detect operation conditions of common motions generally rely on rigid materials or tedious fabrications. Here, a fully soft self-powered vibration sensor (SSVS) engineered by laser-assisted fabrication is proposed. Different from its rigid counterparts, the device is constructed with entirely ductile materials including a liquid metal kernel and an elastic shell. Furthermore, laser direct writing elaborates intricate patterns and functional surfaces of SSVS in a rapid and mask-free manner. A ubiquitous IR laser combined with an origami-inspired transfer creates arbitrary out-of-plane electrodes on soft media, while a pulsed UV laser judiciously tunes nonstick microstructures to the liquid metal droplet. To demonstrate the capability on curved surfaces, the SSVS is interfaced with an exhaust silencer of an air compressor and detects failures of screw connection. Additionally, a cap reinforced by SSVS with wireless transmission is devised for motion monitoring, which effectively distinguishes slip from walking, jumping and running. This work affords promising routes toward monitoring complex machinery and remote guardianship for the vulnerable populations.

Tough Bonding of Liquid Metal-Elastomer Composites for Multifunctional Adhesives.

Liquid metal (LM) composites, which consist of LM droplets dispersed in highly deformable elastomers, have recently gained interest as a multifunctional material for soft robotics and electronics. The incorporation of LM into elastic solids allows for unique combinations of material properties such as high stretchability with thermal and electrical conductivity comparable to metals. However, it is currently a challenge to incorporate LM composites into integrated systems consisting of diverse materials and components due to a lack of adhesion control. Here, a chemical anchoring methodology to increase adhesion of LM composites to diverse substrates is presented. The fracture energy increases up to 100× relative to untreated surfaces, with values reaching up to 7800 J m-2 . Furthermore, the fracture energy, tensile modulus, and thermal conductivity can be tuned together by controlling the microstructure of LM composites. Finally, the bonding technique is used to integrate LM composites with functional electronic components without encapsulation or clamping, allowing for extreme deformations while maintaining exceptional thermal and electrical conductivity. These findings can accelerate the adoption of LM composites into complex soft robotic and electronic systems where strong, reliable bonding between diverse materials and components isrequired.

MXene-Reinforced Liquid Metal/Polymer Fibers via Interface Engineering for Wearable Multifunctional Textiles.

Stretchable conductive fibers are an important component of wearable electronic textiles, but often suffer from a decrease in conductivity upon stretching. The use of liquid metal (LM) droplets as conductive fillers in elastic fibers is a promising solution. However, there is an urgent need to develop effective strategies to achieve high adhesion of LM droplets to substrates and establish efficient electron transport paths between droplets. Here, we use large-sized MXene two-dimensional conductive materials to modify magnetic LM droplets and prepare MXene/magnetic LM/poly(styrene-butadiene-styrene) composite fibers (MLMS fibers). The MXene sheets decorated on the surface of magnetic LM droplets not only enhance the droplet adhesion to substrate but also bridge adjacent droplets to establish efficient conductive paths. MLMS fibers show several-fold improvements in tensile strength and elongation and a 30-fold increase in conductivity compared with pure LM-filled fibers. These conductive fibers can be easily woven into multifunctional textiles, which exhibit strong electromagnetic interference shielding and stable Joule heating performances even under large tensile deformation. In addition, other advantages of MLMS textiles, such as high gas/liquid permeability, strong chemical resistance (acid and alkaline conditions), high/low-temperature tolerance (-40-150 °C) and water washability, make them particularly suitable for wearable applications.

AFM Manipulation of EGaIn Microdroplets to Generate Controlled, On-Demand Contacts on Molecular Self-Assembled Monolayers.

Liquid metal droplets, such as eutectic gallium-indium (EGaIn), are important in many research areas, such as soft electronics, catalysis, and energy storage. Droplet contact on solid surfaces is typically achieved without control over the applied force and without optimizing the wetting properties in different environments (e.g., in air or liquid), resulting in poorly defined contact areas. In this work, we demonstrate the direct manipulation of EGaIn microdroplets using an atomic force microscope (AFM) to generate repeated, on-demand making and breaking of contact on self-assembled monolayers (SAMs) of alkanethiols. The nanoscale positional control and feedback loop in an AFM allow us to control the contact force at the nanonewton level and, consequently, tune the droplet contact areas at the micrometer length scale in both air and ethanol. When submerged in ethanol, the droplets are highly nonwetting, resulting in hysteresis-free contact forces and minimal adhesion; as a result, we are able to create reproducible geometric contact areas of 0.8-4.5 μm2 with the alkanethiolate SAMs in ethanol. In contrast, there is a larger hysteresis in the contact forces and larger adhesion for the same EGaIn droplet in air, which reduced the control over the contact area (4-12 μm2). We demonstrate the usefulness of the technique and of the gained insights in EGaIn contact mechanics by making well-defined molecular tunneling junctions based on alkanethiolate SAMs with small geometric contact areas of between 4 and 12 μm2 in air, 1 to 2 orders of magnitude smaller than previously achieved.

Jet Effusion from a Metal Droplet Irradiated by a Polarized Ultrashort Laser Pulse

Fragmentation of liquid-metal droplets irradiated by linearly and circularly polarized femtosecond laser pulses is observed in our experiment. The obtained shadowgraph snapshots demonstrate that a circularly polarized pulse may produce several randomly oriented jets effused from the expanding droplets, while a linearly polarized laser pulse generates strictly the cruciform jets. The latter orientation is tied with polarization plane, rotation of which causes rotation of the cruciform jets by the same angle. To shed light on the experimental data we perform molecular dynamics simulation of droplet expansion induced by angle-dependent heating. Our simulation shows that the jet directions are determined by an oriented angle-dependent energy distribution within a frontal hemisphere layer of droplet after absorption of linearly polarized light. As a result, the produced flow velocity field guided by surface tension forms two high-speed opposite jets oriented across the electric field vector as in our experiment. A shock-wave pulse generated in the frontal layer has angle-dependent amplitude inherited from the oriented energy deposition. The release part of shock pulse produces a cavitation zone nearby the droplet center, and thus an expanding spherical shell is formed from the droplet. The flow velocities within a rearside hemisphere of the shell, produced after reflection of the shock wave from the rear side of droplet, generate two low-speed opposite jets oriented along the electric field vector. Thus we find that the cruciform jets are originated independently from the frontal and rear sides of droplet, and a pair of frontal jets is faster than a pair of rearside jets.

Viscoelastic Metal-in-Water Emulsion Gel via Host-Guest Bridging for Printed and Strain-Activated Stretchable Electrodes.

Stretchable conductive electrodes that can be made by printing technology with high resolution is desired for preparing wearable electronics. Printable inks composed of liquid metals are ideal candidates for these applications, but their practical applications are limited by their low stability, poor printability, and low conductivity. Here, thixotropic metal-in-water (M/W) emulsion gels (MWEGs) were designed and developed by stabilizing and bridging liquid metal droplets (LMDs) via a host–guest polymer. In the MWEGs, the hydrophilic main chain of the host–guest polymers emulsified and stabilized LMDs via coordination bonds. The grafted cyclodextrin and adamantane groups formed dynamic inclusion complexes to bridge two neighboring LMDs, leading to the formation of a dynamically cross-linked network of LMDs in the aqueous phase. The MWEGs exhibited viscoelastic and shear-thinning behavior, making them ideal for direct three-dimensional (3D) and screen printing with a high resolution (∼65 μm) to assemble complex patterns consisting of ∼95 wt % liquid metal. When stretching the printed patterns, strong host–guest interactions guaranteed that the entire droplet network was cross-linked, while the brittle oxide shell of the droplets ruptured, releasing the liquid metal core and allowing it to fuse into continuous conductive pathways under an ultralow critical strain (<1.5%). This strain-activated conductivity exceeded 15800 S/cm under a large strain of 800% and exhibited long-term cyclic stability and robustness.

Magnetic Microswarm and Fluoroscopy‐Guided Platform for Biofilm Eradication in Biliary Stents

Biofilm eradication from medical implants is of fundamental importance, and the treatment of biofilm-associated pathogen infections on inaccessible biliary stents remains challenging. Magnetically driven microrobots with controlled motility, accessibility to the tiny lumen, and swarm enhancement effects can physically disrupt the deleterious biostructures while not developing drug resistance. Magnetic urchin-like capsule robots (MUCRs) loaded with magnetic liquid metal droplets (MLMDs, antibacterial agents) are designed using natural sunflower pollen, and the therapeutic effect of swarming MUCR@MLMDs is explored for eradicating complex mixtures of bacterial biofilm within biliary stents collected from patients. The external magnetic field triggers the emergence of the microswarm and induces MLMDs to transform their shape into spheroids and rods with sharp edges. The inherent natural microspikes of MUCRs and the obtained sharp edges of MLMDs actively rupture the dense biological matrix and multiple species of embedded bacterial cells by exerting mechanical force, finally achieving synergistic biofilm eradication. The microswarm is precisely and rapidly deployed into the biliary stent via endoscopy in 10 min. Notably, fluoroscopy imaging is used to track and navigate the locomotion of microswarm in biliary stents in real-time. The microswarm has great potential for treating bacterial biofilm infections associated with medicalimplants.