There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

Post-transition elements, together with zinc-group metals and their alloys belong to an emerging class of materials with fascinating characteristics originating from their simultaneous metallic and liquid natures. These metals and alloys are characterised by having low melting points (i.e. between room temperature and 300 °C), making their liquid state accessible to practical applications in various fields of physical chemistry and synthesis. These materials can offer extraordinary capabilities in the synthesis of new materials, catalysis and can also enable novel applications including microfluidics, flexible electronics and drug delivery. However, surprisingly liquid metals have been somewhat neglected by the wider research community. In this review, we provide a comprehensive overview of the fundamentals underlying liquid metal research, including liquid metal synthesis, surface functionalisation and liquid metal enabled chemistry. Furthermore, we discuss phenomena that warrant further investigations in relevant fields and outline how liquid metals can contribute to exciting future applications.

Liquid metals: fundamentals and applications in chemistry

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Philosophical Transactions of the Royal Society of London. for the Year Mdcccxxvii. Part I. Volume Cxvii:355–388, 1827

Two-dimensional (2D) oxides have a wide variety of applications in electronics and other technologies. However, many oxides are not easy to synthesize as 2D materials through conventional methods. We used nontoxic eutectic gallium-based alloys as a reaction solvent and co-alloyed desired metals into the melt. On the basis of thermodynamic considerations, we predicted the composition of the self-limiting interfacial oxide. We isolated the surface oxide as a 2D layer, either on substrates or in suspension. This enabled us to produce extremely thin subnanometer layers of HfO2, Al2O3, and Gd2O3. The liquid metal–based reaction route can be used to create 2D materials that were previously inaccessible with preexisting methods. The work introduces room-temperature liquid metals as a reaction environment for the synthesis of oxide nanomaterials with low dimensionality.

A liquid metal reaction environment for the room-temperature synthesis of atomically thin metal oxides.

Soft electronics are intensively studied as the integration of electronics with dynamic nonplanar surfaces has become necessary. Here, a discussion of the strategies in materials innovation and structural design to build soft electronic devices and systems is provided. For each strategy, the presentation focuses on the fundamental materials science and mechanics, and example device applications are highlighted where possible. Finally, perspectives on the key challenges and future directions of this field are presented.

Materials and Structures toward Soft Electronics.

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.

Liquid metal enabled microfluidics

The continued miniaturization of electronic components demands integrated liquid cooling systems with minimized external connections and fabrication costs that can be implanted very close to localized hot spots. This might be challenging for existing liquid cooling systems because most of them rely on external pumps, connecting tubes, and microfabricated heat sinks. Here, we demonstrate an integrated liquid cooling system by utilizing a small droplet of liquid metal Galinstan, which is placed over the hot spot. Energizing the liquid metal droplet with a square wave signal creates a surface tension gradient across the droplet, which induces Marangoni flow over the surface of droplet. This produces a high flow rate of coolant medium through the cooling channel, enabling a “soft” pump. At the same time, the high thermal conductivity of liquid metal extends the heat transfer surface and facilitates the dissipation of heat, enabling a “soft” heat sink. This facilitates the rapid cooling of localized hot spots,...

An Integrated Liquid Cooling System Based on Galinstan Liquid Metal Droplets.

Patterning customized arrays of microscale Galinstan or EGaIn liquid metals enables the creation of a variety of microfabricated systems. Current techniques for creating microsized 3D structures of liquid metals are limited by the large dimension or low aspect ratio of such structures, and time-consuming processes. Here, a novel technique for creating 3D microstructures of Galinstan using dielectrophoresis is introduced. The presented technique enables the rapid creation of Galinstan microstructures with various dimensions and aspect ratios. Two series of proof-of-concept experiments are conducted to demonstrate the capabilities of this technique. First, the 3D Galinstan microstructures are utilized as 3D microelectrodes to enhance the trapping of tungsten trioxide (WO 3 ) nanoparticles flowing through a microfluidic channel. Second, the patterned Galinstan microstructures are utilized as microfins to improve the dissipation of heat within a microfluidic channel that is located onto a hot spot. The presented technique can be readily used for creating customized arrays of 3D Galinstan microstructures for a wide range of applications. This work introduces a novel technique for creating 3D microstructures of Galinstan using dielectrophoresis. It enables the rapid formation of multiple microstructures with controllable diameters and aspect ratios. Proof-of-concept experiments are conducted by utilizing the patterned microstructures as 3D microelectrodes for enhancing the trapping of suspended nanoparticles, and as microfins to improve the convective heat transfer within a microfluidic channel.

Creation of Liquid Metal 3D Microstructures Using Dielectrophoresis

Here, we demonstrate a self-sufficient, inexpensive and disposable pressure pump using commercially available latex balloons. The versatility of the pump is demonstrated against various microfluidic structures, liquid viscosities, and ambient temperatures. The flow rate of the pump can be controlled by varying the size and thickness of the balloon. Importantly, the soft structure of the balloon allows for almost instantaneous change of the flow rate upon manual squeezing of the balloon. This feature has been used for dynamically changing the flow ratio of parallel streams in a T-shaped channel or varying the size of droplets in a droplet generation system. The self-sufficiency, simplicity of fabrication and operation, along with the low-cost of the balloon pump facilitate the widespread application of microfluidic technologies for various research, education, and in situ monitoring purposes.

A self-sufficient pressure pump using latex balloons for microfluidic applications.

Here, we demonstrate a modular, reconfigurable, and self-sufficient convective heat exchanger for regulation of temperature in microfluidic systems. The heat exchanger consists of polymer tubes wrapped around a plastic pole and fully embedded in an elastomer block, which can be easily mounted onto the microfluidic structure. It is compatible with various microfluidic geometries and materials. Miniaturized, battery-powered piezoelectric pumps are utilized to drive the heat carrying liquid through the heat exchanger at desired flow rates and temperatures. Customized temperature profiles can be generated by changing the configuration of the heat exchanger with respect to the microfluidic structure. Tailored dynamic temperature profiles can be generated by changing the temperature of the heat carrying liquid in successive cycles. This feature is used to study the calcium signaling of endothelial cells under successive temperature cycles of 24 to 37 °C. The versatility, simplicity, and self-sufficiency of the heat exchanger makes it suitable for various microfluidic based cellular assays.

Jiu Yang Zhu

Papers

Liquid metal enabled microfluidics

An Integrated Liquid Cooling System Based on Galinstan Liquid Metal Droplets.

Creation of Liquid Metal 3D Microstructures Using Dielectrophoresis

A self-sufficient pressure pump using latex balloons for microfluidic applications.

Reconfigurable, Self-Sufficient Convective Heat Exchanger for Temperature Control of Microfluidic Systems.