Liquid Metal Microdroplets Research Articles

Soft electronic vias and interconnects through rapid three-dimensional assembly of liquid metal microdroplets

Soft electronic vias and interconnects through rapid three-dimensional assembly of liquid metal microdroplets

Self-Adaptive Magnetic Liquid Metal Microrobots Capable of Crossing Biological Barriers and Wireless Neuromodulation.

Magnetic liquid-bodied microrobots (MRs) possess nearly infinite shape adaptivity. However, they currently confront the risk of structure instability/crushes during shape-morphing in tiny biological environments. This article reports that magnetic liquid metal (LM) MRs (LMMRs) show high structure stability and robust magnetic maneuverability. In this protocol, Fe nanoparticles are encapsulated inside less-than-10-μm LM microdroplets by establishing interfacial chemical potential barriers, yielding LMMRs. Their robust magnetic maneuverability originates from the magnetically controlled assembly of Fe nanoparticles inside LM and distinct liquid-solid interaction. With the self-adaptive shape-recovering capabilities even after 50% deformation, LMMRs can implement vertical climbing over walls up to 400% of its body length and traverse channels with the size of its two-thirds. The in vitro and in vivo experiments have both verified the effective magneto-mechanical stimulation of LMMRs upon neurons after their shape-adaptive crossing the blood-brain barrier under a driven magnetic field. Our work provides a promising strategy for wireless therapies with MRs by safely and effectively overcoming biological barriers.

Liquid metal hybrid antibacterial hydrogel scaffolds from 3D printing for wound healing

Hydrogels patches hold significant promise for wound healing. Endeavors have been dedicated to the improvement of hydrogel patch functionalities, with the aim of achieving systematic wound management. Herein, we present liquid metal (LM) hybrid antibacterial hydrogel scaffolds for wound healing via the 3D printing strategy. Gelatin/gellan gum-based hydrogel ink is adopted for printing the scaffolds, with LM microdroplets integrated within. Due to the excellent photothermal responsiveness of LM microdroplets, the LM hybrid scaffolds can convert the received near infrared light energy into heat energy, demonstrating controllable antibacterial ability. In addition, vascular endothelial growth factor is introduced into the scaffolds, which can be sustainably released and play an angiogenesis role. In vivo animal experiments demonstrate that this hydrogel scaffolds can effectively assit wound closure by eradicating bacterial infection and promoting angiogenesis. Thus, we believe that the proposed LM hybrid hydrogel scaffolds show promising prospects in wound treatment.

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.

Printing of liquid metal by electrically modulating of interface tension in liquid environment

In recent years, gallium-based liquid metal has emerged as a prominent research material in the realm of flexible electronics, owing to its exceptional electrical conductivity and deformability. The crux of developing flexible electronic devices using liquid metal as a conductive material lies in the patterning of the liquid metal. Inkjet printing technology possesses the capability to efficiently craft functional patterns by printing any inkable material. However, due to the facile oxidation of the gallium-based liquid metal surface in ambient air, leading to the formation of oxide films and the presence of high surface tension, conventional inkjet printing techniques are incapable of directly rendering liquid metal prints. This manuscript introduces a refined and remarkably efficient approach to inkjet printing utilizing the liquid metal. By deftly adjusting the interfacial tension of the liquid metal at the nozzle with ultra-low potentials, falling within the 2-V range, the achievement of on-demand, micron-scale liquid metal microdroplet ejection printing surpassing 3 kHz is brought to fruition. This article delves into unraveling the mechanisms of potential modulation on the interfacial tension of the liquid metal at the nozzle, as well as the fundamental theories pertaining to the formation and migration of liquid metal microdroplets. These insights establish a theoretical and technical foundation for the application of liquid metals in the field of electronic printing.

Using chitosan nanofibers to synergistically construct a highly stretchable multi-functional liquid mental-based hydrogel for assembling strain sensor with high sensitivity and broad working range

Liquid metal (LM) microdroplets have garnered significant interest as conductive materials for initiating free radical polymerization in the development of conductive hydrogels suited for strain sensors. However, crafting multi-functional conductive hydrogels that boast both high stretchability and superior sensing capabilities remains as a challenge. In this study, we have successfully synthesized LM-based conductive hydrogels characterized by remarkable stretchability and sensing performance employing acrylic acid (AA) to evenly distribute chitosan nanofibers (CSFs) and to subsequently catalyze the free radical polymerization of AA. The resultant polymer network was crosslinked within situ polyacrylic acid (PAA), facilitated by Ga3+ in conjunction with guar gum (GG)-stabilized Ga droplets. The strategic interplay between the rigid, and protonated CSFs and the pliable PAA matrix, coupled with the ionic crosslinking of Ga3+, endows the resulting GG-Ga-CSF-PAA hydrogel with high stretchability (3700 %), ultrafast self-healing, robust moldability, and strong adhesiveness. When deployed as a strain sensing material, this hydrogel exhibits a high gauge factor (38.8), a minimal detection threshold, enduring durability, and a broad operational range. This versatility enables the hydrogel-based strain sensor to monitor a wide spectrum of human motions. Remarkably, the hydrogel maintains its stretchability and sensing efficacy under extreme temperatures after a simple glycerol solution treatment.

Printing of liquid metal by laser-induced thermal bubble at the liquid–liquid interface

The dynamics of bubbles near a liquid–liquid interface represent a complex multiphase problem with numerous potential applications. This paper utilizes pulsed laser-induced microbubbles at the liquid–liquid interface to achieve the multi-orifice ejection of liquid metal microdroplets that are challenging to achieve with traditional inkjet printing. The study combines the expansion and collapse processes of thermal bubbles at the two-phase liquid interface with the ejection process of liquid metal droplets, unveiling the underlying mechanisms of liquid metal droplet ejection. In this work, the influence of laser parameters on the behavior of thermal bubbles at the liquid–liquid interface and the double-peak pressure effect was investigated. The impact of laser parameters on the ejection behavior of liquid metal droplets was also examined, which provides theoretical support for the wide applications of bubble dynamics at the liquid–liquid interface in the field of liquid metal inkjet printing.

Controlled generation of high-frequency liquid metal microdroplets

Liquid metal (LM) microdroplets with high conductivity and fluidity have drawn extensive attention in the field of electronics, biomedicine, and catalysis. The properties of LM droplets, e.g., melting points, electrical actuation, and stability, are affected by their sizes. Continuous generation of LM microdroplets with precisely controlled size will benefit their applications, which however, remains an outstanding challenge. In this study, we have designed a scalable high frequency droplet generator for the liquid metal (EGaIn) that works on the principle of manipulating the electrohydrodynamic behaviour of EGaIn. By applying electric potentials < − 0.75 V vs Ag/ AgCl, EGaIn droplets with uniform sizes can be rapidly and continuously generated. The electric potential effectively tunes the thickness of the oxide layer on the EGaIn surface which in turn modulates the surface tension of the EGaIn and induces the generation of consistent sized EGaIn droplets. The size of the EGaIn microdroplets can be varied between 40 and 200 µm, depending on the electric potential applied and the diameter of the exit orifice. Droplet generation using this method requires relatively low energies, i.e., 2.77 × 10-7 kWh, and is coupled with high frequency droplet generation ∼ 60 droplet/min. The scalability of this droplet generation methodology can be further increased via design of an array of exit orifice. With the presence of surfactants, these EGaIn droplets can remain well-dispersed without coalescing. This considerably widens their capability for further applications.

A swarm of liquid metal microdroplets for recyclable catalysis with high spatial-temporal precision

Nanocatalysts have aroused broad interest due to their long-lasting water-remediation capability. However, conventional nanocatalysts based on noble-metal nanoparticles rely on passive solute diffusion. Therefore, demands for improvements in yields of desired products, reductions in the use of noble metals, and improvements in nanocatalyst recyclability by magnetic separation pose significant challenges to the passive nanocatalysts. Herein, we developed a general method for liquid metal microdroplets modification with noble metal nanoparticles (e.g., Au and Ag), resulting in microdroplets with exceptional catalytic properties to decompose organic pollutants. The LM-Fe@PDA@Au and LM-Fe@PDA@Ag catalysts were dynamically well assembled, and they moved according to predesigned trajectories under magnetic-field control. The liquid metal microdroplets, on the one hand, are microscale matrices for embedding the magnetic particles; on the other hand, are microscale carriers for the fixation of noble metal nanoparticles, to maintain the uniform segregation of the nanoparticles. Due to their excellent mobility and three-dimensional convection, the catalysts could conduct an on-the-fly catalysis with high spatial-temporal precision at preassigned locations. Compared to conventional passive nanocatalysts based on noble-metal nanoparticles, the liquid metal microdroplets catalysts, which were based on the active colloidal swarms, showed temporally and spatially enhanced catalytic activity and improved catalytic rate constants. The catalysts could be easily collected using a magnet, and their inner liquid metal microdroplets could be recycled by treating the catalysts with a base. The work is of potential industrial significance to enhance the catalytic efficiency and shorten the catalytic time during water remediation, with the precious inner components recyclable.

High-Thermal-Conductivity and High-Fluidity Heat Transfer Emulsion with 89 wt % Suspended Liquid Metal Microdroplets.

Colloidal suspensions of thermally conductive particles in a carrier fluid are considered promising heat transfer fluids for various thermal energy transfer applications, such as transportation, plants, electronics, and renewable energy systems. The thermal conductivity (k) of the particle-suspended fluids can be improved substantially by increasing the concentration of conductive particles above a "thermal percolation threshold," which is limited because of the vitrification of the resulting fluid at the high particle loadings. In this study, eutectic Ga-In liquid metal (LM) was employed as a soft high-k filler dispersed as microdroplets at high loadings in paraffin oil (as a carrier fluid) to produce an emulsion-type heat transfer fluid with the combined advantages of high thermal conductivity and high fluidity. Two types of the LM-in-oil emulsions, which were produced via the probe-sonication and rotor-stator homogenization (RSH) methods, demonstrated significant improvements in k, i.e., Δk ∼409 and ∼261%, respectively, at the maximum investigated LM loading of 50 vol % (∼89 wt %), attributed to the enhanced heat transport via high-k LM fillers above the percolation threshold. Despite the high filler loading, the RSH-produced emulsion retained remarkably high fluidity, with a relatively low viscosity increase and no yield stress, demonstrating its potential as a circulatable heat transfer fluid.

A Liquid Metal Microdroplets Initialized Hemicellulose Composite for 3D Printing Anode Host in Zn-Ion Battery.

Maintaining a steady affinity between gallium-based liquid metals (LM) and polymer binders, particularly under continuous mechanical deformation, such as extrusion-based 3D printing or plating/stripping of Zinc ion (Zn2+ ), is very challenging. Here, an LM-initialized polyacrylamide-hemicellulose/EGaIn microdroplets hydrogel is used as a multifunctional ink to 3D-print self-standing scaffolds and anode hosts for Zn-ion batteries. The LM microdroplets initiate acrylamide polymerization without additional initiators and cross-linkers, forming a double-covalent hydrogen-bonded network. The hydrogel acts as a framework for stress dissipation, enabling recovery from structural damage due to the cyclic plating/stripping of Zn2+ . The LM-microdroplet-initialized polymerization with hemicellulosescan facilitate the production of 3D printable inks for energy storage devices.

GaOOH Crystallite Growth on Liquid Metal Microdroplets in Water: Influence of the Local Environment.

Gallium-based liquid metals form alloys with a melting point close to or below room temperature. On the surface of these liquid metals, a thin oxide skin is formed once in contact with oxygen, and this oxide skin can be leveraged to stabilize liquid metal micro- and nanodroplets in a liquid. During sonication and storage of these droplets in aqueous solution, gallium oxide hydroxide (GaOOH) forms on these droplets, and given enough time or treatment with heat, a full shape transition and dealloying are observed. In this article, we show that GaOOH can be grown at room temperature and that the growth is dependent on both the local environment and temperature. GaOOH growth on liquid metal microdroplets located at the air/water interface is considerably faster than in the bulk phase. Interestingly, hydrolysis to GaOOH is hampered and stops at 15 °C in bulk water after 6 h. In contrast, hydrolysis commences even at 15 °C for liquid metal microdroplets located at the air/water interface, and full surface coverage is obtained after around 24 h (compared to 12 h at 25 °C at the air/water interface). The X-ray photoelectron spectroscopy (XPS) measurement suggests that gallium oxide is dissolved and Ga(OH)3 is formed as a precursor that reacts in a downstream reaction toward GaOOH. This improved understanding of the GaOOH formation can be leveraged to control the liquid metal micro- and nanodroplet shape and composition (i.e., for biomedical applications).

Photothermal Thin Films with Highly Efficient NIR Conversion for Miniaturized Liquid-Crystal Elastomer Actuators.

This work presents the development of highly efficient photothermal thin films (PTFs) and the demonstration of their application on miniaturized polymer-based soft actuators. The proposed PTF, which comprises acrylic-based black paint and EGaIn liquid metal (LM) microdroplets, serves as an excellent absorber for efficiently converting near-infrared (NIR) irradiation into heat for actuating liquid-crystal elastomer (LCE) actuators. The introduction of LM microdroplets into the PTFs effectively increases the overall thermal efficiency of PTFs. Miniaturized soft crawlers monolithically integrated with the NIR-driven LCE actuators are also implemented for demonstrating the application of the proposed PTF. The crawler’s locomotion, which is inspired by the rectilinear movement of snakes, is generated with the proposed PTF for inducing the LC-to-isotropic phase transition of the LCEs. The experimental results show that introducing LM microdroplets into the PTF can effectively reduce the thermal time constants of LCE actuators by 70%. Under periodic on/off NIR illumination cycles, the locomotion of crawlers with different dimensions is also demonstrated. The measurement results indicate that the proposed PTF is not only essential for enabling photothermal LCE actuation but also quite efficient and durable for repeated operation.

On‐Demand Programming of Liquid Metal‐Composite Microstructures through Direct Ink Write 3D Printing (Adv. Mater. 20/2022)

Elastomer Composites Liquid metal elastomer composite microstructure governs material properties and performance for applications that require soft, multifunctional response. In article number 2200182, Michael D. Bartlett, Eric J. Markvicka, and co-workers utilize direct ink writing 3D printing to create elastomer composites with unique liquid metal microdroplet architectures, including smooth to discrete transitions from spherical to needle-like microstructures and connected liquid metal networks.

High-Performance Liquid Metal/Polyborosiloxane Elastomer toward Thermally Conductive Applications.

With the combination of high flexibility and thermal property, thermally conductive elastomers have played an important role in daily life. However, traditional thermally conductive elastomers display limited stretchability and toughness, seriously restricting their further development in practical applications. Herein, a high-performance composite is fabricated by dispersing room-temperature liquid metal microdroplets (LM) into a polyborosiloxane elastomer (PBSE). Due to the unique solid-liquid coupling mechanism, the LM can deform with the PBSE matrix, achieving higher fracture strain (401%) and fracture toughness (2164 J/m2). Meanwhile, the existence of LM microdroplets improves the thermal conductivity of the composite. Interestingly, the LM/PBSE also exhibits remarkable anti-impact, adhesion capacities under complex loading environments. As a novel stretchable elastomer with enhanced mechanical and thermal behavior, the LM/PBSE shows good application prospects in the fields of thermal camouflages, stretchable heat-dissipation matrixes, and multifunctional shells for electronic devices.

Fabrication of Highly Stretchable and Wearable Elastomer Structure with Liquid Metal Encapsulated in Auxetic Channel

Recently, soft material structures, in which liquids and droplets are introduced into solids in various shapes, have been attracting attention. These structures can be used in the fields of soft electronics, soft robotics, and wearable devices, etc. In this study, we focus on the auxetic structures, which are patterned structures with negative Poisson's ratio. Herein, we fabricated an auxetic channel structure in elastomer using 3D printer and aimed to develop a novel wearable device with liquid metal inflow. This room-temperature liquid metal has been widely used in flexible and stretchable sensors, focusing on embedding liquid metal in microchannels, liquid metal microdroplets formation, captive sensors, and liquid metal nanoparticles, etc.

High-Thermal-Conductivity and High-Fluidity Heat Transfer Emulsion with 89 Wt.% Suspended Liquid Metal Microdroplets

High-Thermal-Conductivity and High-Fluidity Heat Transfer Emulsion with 89 Wt.% Suspended Liquid Metal Microdroplets

Highly stretchable and sensitive strain sensor based on liquid metal composite for wearable sign language communication device

In recent years, the demand for wearable strain sensors is growing due to their motion monitoring and clinical data collection capabilities. The sensors with positive piozeconductivity, i.e. their conductivity increases when stretched, are more favored in wearable electronics because they can maintain high conductivity under stretching. However, reported composites with positive piezoconductivity mostly show low sensitivity. Furthermore, few wearable electronic systems based on strain sensors are built completely with data processing capabilities. Herein, we present a wearable sensor based on a positive piezoconductive composite composed of liquid metal microdroplets, metal microparticles, and elastic matrix. The resistance of the composite is extremely high at the relaxed state but drops drastically under any deformation. It has high stretchability (fracture strain >200%) and sensitivity (gauge factor ∼100), which shows excellent performance in detecting human motions such as joint bending. Based on this composite, we demonstrate a wearable and portable sign language communication device with a fully programmable and configurable system to help people with language disabilities communicate effectively with others from the visual and auditory perspectives. By bending the fingers, the device can type words or sentences on screen and speak relevant letters through the audio.

Enhancing Thermal Transport in Silicone Composites via Bridging Liquid Metal Fillers with Reactive Metal Co-Fillers and Matrix Viscosity Tuning.

Polymer matrix composites containing room temperature liquid metal (LM) microdroplets offer a unique set of thermo-mechanical characteristics that makes them attractive candidates for high performance thermal interface materials. However, to achieve the desired level of the composite thermal conductivity, effective bridging of such fillers into interconnected percolation networks needs to be induced. Thermal percolation of the LM microdroplets requires two physical barriers to be overcome. First, the LM microdroplets must directly contact each other through the polymer matrix. Second, the native oxide shell on the LM microdroplet must also be ruptured. In this work, we demonstrate that both physical barriers can be penetrated to induce ample bridging of the LM microdroplets and thereby achieve higher thermal conductivity composites. We accomplish this through a synergistic combination of solid silver and LM fillers, tuning of the silicone oil "matrix" viscosity, and sample compression. We selected silver as the solid additive because it rapidly alloys with gallium to form microscale needles that could act as additional paths that aid in connecting the LM droplets. We systematically explore the impact of the composition (filler type, volume fraction, and matrix oil viscosity) and applied pressure on the thermal conductivity and multiscale structure of these composites. We reveal the microscopic mechanism underlying the macroscopic experimental trends and also identify an optimal composition of the multiphase Ag-LM-Silicone oil composite for thermal applications. The identified design knobs offer path for developing tunable LM-based polymer composites for microelectronics cooling, biomedical applications, and flexible electronics.

Heat Transfer Enhancement in an Oscillating Heat Pipe with Self-Dispersion Liquid Metal Micro-Droplets

Heat Transfer Enhancement in an Oscillating Heat Pipe with Self-Dispersion Liquid Metal Micro-Droplets