Liquid-metal Micro Research Articles

Liquid Metal Micro- and Nanodroplets: Characteristics, Fabrication Techniques, and Applications.

Gallium-based liquid metal micro- and nanodroplets are being extensively explored in innumerable emerging technologies. Although many of these systems involve the interfaces of liquid metal with a continuous phase liquid (e.g., microfluidic channels and emulsions), the static or dynamic phenomena at the interface have been scarcely discussed. In this study, we begin by introducing the interfacial phenomena and characteristics observed at the interface between a liquid metal and continuous-phase liquids. Based on these results, we can employ various methods to fabricate liquid metal droplets with tunable surface properties. Finally, we discuss how these techniques can be directly applied to a wide range of state-of-the-art technologies including microfluidics, soft electronics, catalysts, and biomedicines.

Nanoarchitectonics and Applications of Gallium‐Based Liquid Metal Micro‐ and Nanoparticles

AbstractIn recent years, gallium‐based liquid metal (GLM) is an attractive material because of its excellent characteristics such as ultra‐low melting point, high fluidity, high conductivity, self‐healing and so on. GLMs have unique properties at micro and nano scales, such as core‐shell structure, sinterability, biocompatibility and photothermal conversion ability and so on, which promote the development of relevant application directions. Therefore, various preparation methods of GLM particles and conductive connection of GLM nanoparticles through various sintering methods are reviewed in this paper. In addition, GLM particles also show great prospects in flexible circuits, sensor detection, medical treatment, energy production and other fields

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).

FABRICATION OF UNDERCOOLED BISMUTH TIN LIQUID METAL PARTICLES WITH HIGH YIELD

Despite increasing attention to the liquid metals, most of the studies in this field have focused on the gallium-based alloys due to their low melting points. The examples of metastable undercooled liquid metal particles are rare due to the thermodynamic challenges in achieving significant level of undercooling. In this study, the fabrication of undercooled bismuth-tin (BiSn) liquid metal micro- /nano-particles at eutectic composition was studied. The droplet emulsion technique was used for particle formation in broad size range. The effects of the particle size and the shell formation reactions on the yield of undercooled particles were investigated. The fabricated particles were characterized using back- scattered scanning electron microscopy (BSE-SEM) and differential scanning calorimetry (DSC). The particle size distribution and the ratio of undercooled particles were statistically analyzed. Optimization of the processing conditions and the successful selection of oxidants enabled undercooling of BiSn liquid metal particles. In doing so, both micro- and nano-size particles could be fabricated with high yield (≥ 97%). The crystallization temperature was measured to be 0.37 Tm and the particles could preserve their liquid state at room temperatures for months.

Liquid Metal Droplet and Graphene Co-Fillers for Electrically Conductive Flexible Composites.

Colloidal liquid metal alloys of gallium, with melting points below room temperature, are potential candidates for creating electrically conductive and flexible composites. However, inclusion of liquid metal micro- and nanodroplets into soft polymeric matrices requires a harsh auxiliary mechanical pressing to rupture the droplets to establish continuous pathways for high electrical conductivity. However, such a destructive strategy reduces the integrity of the composites. Here, this problem is solved by incorporating small loading of nonfunctionalized graphene flakes into the composites. The flakes introduce cavities that are filled with liquid metal after only relatively mild press-rolling (<0.1 MPa) to form electrically conductive continuous pathways within the polymeric matrix, while maintaining the integrity and flexibility of the composites. The composites are characterized to show that even very low graphene loadings (≈0.6 wt%) can achieve high electrical conductivity. The electrical conductance remains nearly constant, with changes less than 0.5%, even under a relatively high applied pressure of >30 kPa. The composites are used for forming flexible electrically-conductive tracks in electronic circuits with a self-healing property. The demonstrated application of co-fillers, together with liquid metal droplets, can be used for establishing electrically-conductive printable-composite tracks for future large-area flexible electronics.

Research on Characteristics of Mercury Droplet Flow in Micro Acceleration Switch

Unlike the solid-solid contact of traditional micro acceleration switch, liquid metal can be utilized to solve the disadvantage of solid-solid contact, such as contact wear and signal bounce. Liquid-metal micro switches have demonstrated no contact bounce, low switch-on time, low contact resistance, long life, and the capability to handle large currents. So the liquid-metal micro acceleration switches have attracted researcher’s attentions increasingly. To investigate the flow characteristics of mercury droplet in micro acceleration switch, this paper establishes numerical models of micro acceleration switch by using FLUENT. The effect of contact angle, droplet volume and temperature are discussed in this paper. Based on the VOF-CSF model of FLUENT, a serial of simulations were performed. The simulation results show that the influence of contact angle and droplet volume on flow characteristics is serious, and the influence of temperature is small. As compared with theoretical analysis, simulations are in good agreement with theoretical analysis.

Enhanced Liquid Metal Micro Droplet Generation by Pneumatic Actuation Based on the StarJet Method

We present a novel pneumatic actuation system for generation of liquid metal droplets according to the so-called StarJet method. In contrast to our previous work, the performance of the device has been significantly improved: the maximum droplet generation frequency in continuous mode has been increased to fmax = 11 kHz (formerly fmax = 4 kHz). In addition, the droplet diameter has been reduced to 60 μm. Therefore, a new fabrication process for the silicon nozzle chips has been developed enabling the production of smaller nozzle chips with higher surface quality. The size of the metal reservoir has been increased to hold up to 22 mL liquid metal and the performance and durability of the actuator has been improved by using stainless steel and a second pneumatic connection to control the sheath flow. Experimental results are presented regarding the characterization of the droplet generation, as well as printed metal structures.

Pneumatic dispensing of nano- to picoliter droplets of liquid metal with the StarJet method for rapid prototyping of metal microstructures

This study presents a new, simple and robust, pneumatically actuated method for the generation of liquid metal micro droplets in the nano- to picoliter range. The so-called StarJet dispenser utilizes a star-shaped nozzle geometry that stabilizes liquid plugs in its center by means of capillary forces. Single droplets of the liquid metal can be pneumatically generated by the interaction of the sheathing gas flow in the outer grooves of the nozzle and the liquid metal. For experimental validation, a print head was build consisting of silicon chips with a star-shaped nozzle geometry and a heated actuator (up to 280°C). The silicon chips are fabricated by Deep Reactive Ion Etching (DRIE). Chip designs with different star-shaped geometries were able to generate droplets with diameters in the range of the corresponding nozzle diameters. The StarJet can be operated in two modes: Either continuous droplet dispensing mode or drop on demand (DoD) mode. The continuous droplet generation mode for a nozzle with 183 μm diameter shows tear-off frequencies between 25 and 120 Hz, while droplet diameters remain constant at 210 μm for each pressure level. Metal columns were printed with a thickness of 0.5–1.0 mm and 30 mm height (aspect ratio >30), to demonstrate the directional stability of droplet ejection and its potential as a suitable tool for direct prototyping of the metal microstructures.

High-reliability, high-performance RF micromachined switch using liquid metal

Existing mechanical relays have reliability-related issues including unstable contact resistance and limited cycle life. These problems are caused by mechanical fatigue and wearing of the contact surfaces. In the liquid metal micro switch (LiMMS), we employ a liquid-liquid contact to address these issues. The switching operation is achieved by forming a gap in the liquid metal using gas expansion. Our prototype has a microchannel on a glass substrate to hold a tiny amount of mercury, 0.15 mm in width and 0.1 mm in height, and TaN thin-film heaters on a ceramic substrate to expand the operating gas by heating. The substrates are bonded together and the total size of the device is 5/spl times/5/spl times/1.4 mm. We successfully demonstrated 70-m/spl Omega/ on-resistance, 1 ms-switching speed, the ability to handle up to 1 A of dc current and over 1/spl times/10/sup 8/ cycle operation. LiMMS also has good RF performance, better than 1 dB insertion loss, and better than 20 dB isolation up to 18 GHz.