A Gallium-Based Magnetocaloric Liquid Metal Ferrofluid.

We investigated frequency tunability of a microfluidic-based antenna using on-demand manipulation of a gallium-based liquid metal alloy. The fluidic antenna was fabricated by polydimethylsiloxane (PDMS) and filled with the gallium-based liquid metal alloy (Galinstan ® ). It is composed of a digital number “7”-shaped feedline, and a square-shaped and a digital number “6”-shaped patterns, which are all implemented with the liquid metal. The gallium-based liquid metal was adhered to the channel surface due to its viscous oxide layer originating from the gallium oxide forming when it exposed to the air environment. We treated the liquid metal with hydrochloric acid solution to remove the oxide layer on the surface resulting in easy movement of the liquid metal in the channel, as the liquid metal surface has been transformed to be non-wettable. We controlled the physical length of the liquid metal slug filled in feedline with an applied air pressure, resulting in tuning the resonant frequency ranging from 2.2 GHz to 9.3 GHz. The fluidic antenna properties using the liquid metal’s electrical conductivity and mobility were characterized by measuring the return loss (S11), and also simulated with CST Microwave Studio.

The focus of this chapter is to review the literature on flexible and stretchable electronics fabricated using gallium-based liquid metal alloys. Over the past few decades, gallium-based liquid metal alloys have gathered growing interest due to their application in various soft electronic applications as conductive yet fluidic metals that can be patterned at room temperature. Gallium-based liquid metal alloys have a melting point at or near room temperature and offer a low toxicity as opposed to the conventional liquid metal – mercury, in addition to having negligible vapor pressure and low viscosity. The surface of these metals reacts rapidly with oxygen in air to form a thin surface oxide ‘skin’ that allows these liquids to be deposited in several useful stable shapes despite the destabilizing effects of a large surface tension and gravity. There are several methods to pattern these liquid metals – traditionally using lithography-based subtractive approaches, and unconventionally using 3D printing and other additive techniques. Gallium-based liquid metal alloys have been used for fabricating various electronic structures such as ultrastretchable and self-healing wires, interconnects, and stretchable antennas. Integrating these components, many electronic components have been built such as optical and soft memory devices, reconfigurable circuits, soft diodes, microfluidic pumps, soft strain sensors, capacitors, and circuit boards.

We report clog-free and oxide-free metal inkjet printing applicable to flexible electronics using gallium-based liquid metal alloy. Inkjet printing has been developed and expanded to make a pattern of either non-conductive or conductive materials. In order to print typical conductive material, it utilizes metal nanoparticle dispersed in solvent or melts the metal. However, those methods often encounters clogging and oxidation problem. We fabricated a simple polydimethylsiloxane (PDMS) based inkjet printer incorporated with hydrochloric acid (HCl)-impregnated paper as orifice material. A constant stream of gallium-based liquid metal alloy droplet was demonstrated using the inkjet printer. Depending on the applied flow rate, pinch off and Rayleigh instability phenomena were observed. We printed beads-on-string shape gallium-based liquid metal alloy line on various flexible substrates such as Si wafer, PDMS, and a paper. Finally, it was demonstrated that the inkjet-printed gallium-based liquid metal can maintain its line shape without disconnection even with the significant deformation of a flexible paper.

Several gallium-based liquid metal alloys are liquid at room temperature. As 'liquid', such alloys have a low viscosity and a high surface tension while as 'metal', they have high thermal and electrical conductivities, similar to mercury. However, unlike mercury, these liquid metal alloys have low toxicity and a negligible vapor pressure, rendering them much safer. In comparison to mercury, the distinguishing feature of these alloys is the rapid formation of a self-limiting atomically thin layer of gallium oxide over their surface when exposed to oxygen. This oxide layer changes many physical and chemical properties of gallium alloys, including their interfacial and rheological properties, which can be employed and modulated for various applications in microfluidics. Injecting liquid metal into microfluidic structures has been extensively used to pattern and encapsulate highly deformable and reconfigurable electronic devices including electrodes, sensors, antennas, and interconnects. Likewise, the unique features of liquid metals have been employed for fabricating miniaturized microfluidic components including pumps, valves, heaters, and electrodes. In this review, we discuss liquid metal enabled microfluidic components, and highlight their desirable attributes including simple fabrication, facile integration, stretchability, reconfigurability, and low power consumption, with promising applications for highly integrated microfluidic systems.

We report a “magnetic liquid metal marble” (MLMM) obtained by coating a liquid metal’s surface with micro/nano-sized ferromagnetic iron (Fe) particles, which enables on-demand, magnetic manipulation of a liquid metal droplet for switching applications. Among liquid metals, gallium-based liquid metal alloys have been developed for a variety of applications. However, most developed applications using the gallium-based liquid metal alloy only work on deformability because of its easy-wetting property stemming from surface oxidation. By coating the oxidized surface with the 45 $\mu \text{m}$ or 45 nm diameter Fe particles, the MLMM exhibits non-wetting property investigated by evaluating apparent contact angles and sliding angles against various surfaces. On the Teflon-coated glass, the largest contact angle was measured to be ~169.0°, and the lowest sliding angle was obtained to be 17.2°, respectively. In order to move the 45- $\mu \text{m}$ diameter Fe particles-coated MLMM, we measured the minimum required magnetic flux density of 150 gauss and demonstrated the magnetic control of the liquid metal marble to turn ON light emitting diodes. In addition, we investigated that hydrochloric acid-vapor treatment on the MLMM enhanced the lyophobicity (sliding angle of 9.4°), reduced the minimum magnetic flux density (150 to 107 gauss) to actuate it, and enabled electrical switching applicability even in silicon oil with shorter delay time and high mobility of the MLMM under the applied magnetic field. [2015-0329]

Gallium-based liquid metals (GLMs) are promising anode materials for lithium-ion batteries due to their low-melting point, fluidity, high-electrical and -thermal conductivity, and high-theoretical capacity for Li (769 mAh/g, Li2Ga). Notably, GLMs undergo a reversible liquid-to-solid-to-liquid transition during lithiation and delithiation, enabling a self-healing mechanism that mitigates surface defects. This mechanism extends the cycle life of anodes utilizing GLMs either as active materials( 1–6 ) or in combination with high-volume expansion materials such as silicon (~400%)( 7–9 ).Fundamental studies on the self-healing mechanism of GLMs are limited. Previous investigations have primarily relied on macroscopic characterizations such as X-ray diffraction (XRD) in bulk electrodes( 3,10 ) or in situ transmission electron microscopy (TEM) on individual GLM nanoparticles (NPs) in solid-state cells open to the environment( 6,11 ). In this work, we adopt a multi-scale approach to gain insights into the (de)lithiation mechanism. We conducted: (1) in situ TEM observations of GLM NPs on a TEM grid current collector to simultaneously track lithiation/delithiation dynamics across multiple nanoparticles; (2) cryogenic TEM where GLM NPs on a TEM grid are cycled in a lithium-ion battery electrolyte, and the grid is subsequently transferred to a cryo-TEM for high-resolution interfacial characterization of the GLM NPs; and (3) cryogenic focused-ion beam tomography to analyze structural changes in a composite electrode comprised of GLM NPs, Super P carbon, and PVDF binder on a Cu current collector over continuous cycles within a coin or pouch cell. By investigating GLMs at multiple length scales, this study advances the understanding of how the lithiation/delithiation mechanism of GLM NPs influences capacity fade in lithium-ion batteries. In a broader context, this study establishes a framework for future multi-length scale investigations into battery materials.References Jin, X. et al. Transferring Liquid Metal to form a Hybrid Solid Electrolyte via a Wettability-Tuning Technology for Lithium-Metal Anodes. Adv. Mater. 34, 2200181 (2022).Zhu, M., Li, S., Li, B. & Yang, S. A liquid metal-based self-adaptive sulfur–gallium composite for long-cycling lithium–sulfur batteries. Nanoscale 11, 412–417 (2019).Guo, X. et al. A Self-Healing Room-Temperature Liquid-Metal Anode for Alkali-Ion Batteries. Adv. Funct. Mater. 28, 1804649 (2018).Pu, J. et al. Liquid Metal-Based Stable and Stretchable Zn-Ion Battery for Electronic Textiles. Adv. Mater. 36, 2305812 (2024).Liu, G. et al. Soft, Highly Elastic, and Discharge-Current-Controllable Eutectic Gallium–Indium Liquid Metal–Air Battery Operated at Room Temperature. Adv. Energy Mater. 8, 1703652 (2018).Huang, C. et al. Alkali-ion Batteries by Carbon Encapsulation of Liquid Metal Anode. Adv. Mater. n/a, 2309732.Hapuarachchi, S. N. S. et al. Utilizing Room Temperature Liquid Metals for Mechanically Robust Silicon Anodes in Lithium-Ion Batteries. Batter. Supercaps 1, 122–128 (2018).Ju, Z. et al. Assembled MXene–Liquid Metal Cages on Silicon Microparticles as Self-Healing Battery Anodes. Nano Lett. 24, 6610–6616 (2024).Zhao, Z. et al. Liquid Metal Remedies Silicon Microparticulates Toward Highly Stable and Superior Volumetric Lithium Storage. Adv. Energy Mater. 12, 2103565 (2022).Wang, K. et al. Core-shell GaSn@rGO nanoparticles as high-performance cathodes for room-temperature liquid metal batteries. Scr. Mater. 217, 114792 (2022).Liang, W. et al. Nanovoid Formation and Annihilation in Gallium Nanodroplets under Lithiation–Delithiation Cycling. Nano Lett. 13, 5212–5217 (2013).

Interfacing of surfaces with gallium-based liquid metals – approaches for mitigation and augmentation of liquid metal adhesion on surfaces

Gallium-based liquid metals (GLMs) exist as atypical liquid-phase metals at and near room temperature while being electrically and thermally conductive, enabling copious applications in soft electronics and thermal management systems. Yet, solid metals are affected by interfacing with GLMs, resulting in liquid metal embrittlement and device failure. To avert this issue, mechanically durable and electrically tunable diffusion barriers for long-term reliable liquid metal-solid metal interfacing based on the deposition of various diamond coatings are designed and synthesized, as they feature high chemical inertness and extraordinary mechanical resistance. The diamond coatings show superlyophobicity (GLM contact angle ≥ 155°) and are nonstick toward GLMs, thereby achieving high mobility of GLM droplets (sliding angle 8-12°). The excellent barrier and anti-adhesion performance of the diamond coatings are proven in long-term experiments (3 weeks) of coated titanium alloy (Ti) samples in contact with GLMs. The electrical performance of the conductive diamond coating deposited on Ti is reliable and stable over a period of 50 h. As proof-of-concept applications a switch and a thermal management device based on liquid metals are demonstrated, signifying that coating diamond films on metals is a potent means to achieve stable integration of solid metals with GLMs.

Gallium-based liquid metals (LMs) are materials that possess some unique properties. Just like any other metal, they have high thermal conductivity and low interfacial resistance, but they are in a liquid phase at room temperature. In contrast to mercury alloys, gallium alloys are non-toxic. They don’t evaporate and they can’t be inhaled. The viscosity of those alloys is very similar to water, but they are six times as dense as water. All those properties make Gallium-based liquid metals very good candidates for thermal interface material (TIM) in electronics applications. On the other hand, the reaction and incompatibility of those alloys with some metals is one of the challenges for this type of TIM. The other challenge is that those materials are not just thermally, but also electrically conductive and that is not a desirable property for TIM. A suitable barrier that will prevent any leakage of LMs and the best way to apply the appropriate volume of LM in high-volume production (HVP) would be one of the most important things for any application. A key challenge is applying the liquid metal consistently through a traditional dispensing method due to its property and behavior which involves high surface tension. Through advanced dispensing techniques like jetting technology liquid metal can be applied reliably on a flat, uneven surface or in arrays of minuscule confined spaces or cavities. This paper highlights the dispensing quality, weight repeatability from one substrate to another, and valve hardware stability. This paper will also address the challenges faced during dispensing of liquid metal in high volume manufacturing, and how to achieve desired bondline thickness with jet dispensing for higher throughput and process reliability.

We investigated characteristics of highly flexible and stretchable electrodes consisting of Galinstan (i.e., a gallium-based liquid metal alloy) under various conditions including sub-zero temperature (i.e., <0 °C) and demonstrated solar-blind photodetection via the spontaneous oxidation of Galinstan. For this work, a simple and rapid method was introduced to fabricate the Galinstan electrodes with precise patterns and to exfoliate their surface oxide layers. Thin conductive films possessing flexibility and stretchability can be easily prepared on flexible substrates with large areas through compression of a dried suspension of Galinstan microdroplets. Furthermore, a laser marking machine was employed to facilitate patterning of the Galinstan films at a high resolution of 20 μm. The patterned Galinstan films were used as flexible and stretchable electrodes. The electrical conductivity of these electrodes was measured to be ~1.3 × 106 S m−1, which were still electrically conductive even if the stretching ratio increased up to 130% below 0 °C. In addition, the surface oxide (i.e., Ga2O3) layers possessing photo-responsive properties were spontaneously formed on the Galinstan surfaces under ambient conditions, which could be solely exfoliated using elastomeric stamps. By combining Galinstan and its surface oxide layers, solar-blind photodetectors were successfully fabricated on flexible substrates, exhibiting a distinct increase of up to 14.7% in output current under deep ultraviolet irradiation (254 nm wavelength) with an extremely low light intensity of 0.1 mW cm−2, whereas no significant change was observed under visible light irradiation.

We report the fabrication and characterization of a reversible resonant frequency tunable antenna based on liquid metal actuation. The antenna is composed of a coplanar waveguide fed monopole stub printed on a copper-clad substrate, and a tunnel-shaped microfluidic channel linked to the printed metal. The gallium-based liquid metal can be injected and withdrawn from the channel in response to an applied air pressure. The gallium-based liquid metal is treated with hydrochloric acid to eliminate the oxide layer, and associated wetting/sticking problems, that arise from exposure to an ambient air environment. Elimination of the oxide layer allows for reliable actuation and repeatable and reversible tuning. By controlling the liquid metal slug on-demand with air pressure, the liquid metal can be readily controllable to connect/disconnect to the monopole antenna so that the physical length of the antenna reversibly tunes. The corresponding reversible resonant frequency changes from 4.9 GHz to 1.1 GHz. The antenna properties based on the liquid metal actuation were characterized by measuring the reflection coefficient and agreed well with simulation results. Additionally, the corresponding time-lapse images of controlling liquid metal in the channel were studied.

Gallium based eutectic alloys are one of the promising substitutes for mercury. These alloys are nontoxic and environment friendly and have similar or equivalent properties as that of mercury. These alloys exhibit good electrical and thermal properties. However, these alloys form oxides when in contact with oxygen atoms. Due to this, the material surface tension changes, which renders them unsuitable for those applications where the mobility of the fluid is a valuable factor, such as in thermometers and blood pressure monitoring devices. There are few methods reported to remove the oxide layer from the surface of the gallium-based alloys, like acid and base treatment prior to use, but all these methods are not durable and long-lasting. Here in this work, we report gallium oxide coating as a simple approach to convert mercury manometer glass tube, which has glass as a substrate to a nonwetting surface against surface-oxidized gallium-based liquid metal alloys. These alloys form an oxidized layer in ambient air (O-2 &amp;gt;1 ppm) and show stickiness to almost all surfaces that impact the residue-free movement of the liquid metal droplets. Herein, the physical vapor deposition technique was used for gallium oxide coating on substrates such as silicon wafer and glass slide. Moreover, various characterizations were carried out to support our outcomes. This method does not require any micro/nano machining or specific nanoscale surface topology. The contact angle was measured with or without coated gallium oxide film on the glass substrate, and the static contact angle (c.a.) 137.69 degrees and with bared glass 94.30 degrees.

Gallium-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 ≈100nm and inter-pattern spacings down to ≈100nm 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.

Gallium-based liquid metals (LMs) featuring both high conductivity and fluidity are ideal conductors for soft and stretchable electronics. However, their liquid nature is a double-edged sword in many key applications since LMs are inherently prone to mechanical damage. Although additional encapsulation is frequently used for the protection of delicate LM electrodes, it hinders the electrical interfacing with other objects for interconnection, sensing, and stimulation. Here, different from conventional patterning methods that deposit LM on or inside substrates, we for the first time report a simple strategy to create surface-embedded LM of eutectic gallium-indium (EGaIn) circuits with mechanical damage endurance. This was achieved by using direct magnetic printing to overcome the high surface tension of LM, allowing it to be passively filled into the laser-patterned microgrooves on soft substrates. We show that the surface-embedded LM circuits are resistant to mechanical erasure, washing, and peeling. We also show the applications of our surface-embedded LM electrodes in respiration monitoring and electrical stimulation of nerves. This work provides a simple and efficient way to create mechanically reliable LM microelectrodes, holding great promise for wearable and implantable bioelectronics.

Surface Chemistry of Gallium-Based Liquid Metals