Surface tension plays a core role in dominating various surface and interface phenomena. For liquid metals with high melting temperature, a profound understanding of the behaviors of surface tension is crucial in industrial processes such as casting, welding, and solidification, etc. Recently, the room temperature liquid metal (RTLM) mainly composed of gallium-based alloys has caused widespread concerns due to its increasingly realized unique virtues. The surface properties of such materials are rather vital in nearly all applications involved from chip cooling, thermal energy harvesting, hydrogen generation, shape changeable soft machines, printed electronics to 3D fabrication, etc. owing to its pretty large surface tension of approximately 700 mN/m. In order to promote the research of surface tension of RTLM, this paper is dedicated to present an overview on the roles and mechanisms of surface tension of liquid metal and summarize the latest progresses on the understanding of the basic knowledge, theories, influencing factors and experimental measurement methods clarified so far. As a practical technique to regulate the surface tension of RTLM, the fundamental principles and applications of electrowetting are also interpreted. Moreover, the unique phenomena of RTLM surface tension issues such as surface tension driven selfactuation, modified wettability on various substrates and the functions of oxides are discussed to give an insight into the acting mechanism of surface tension. Furthermore, future directions worthy of pursuing are pointed out.

Surface tension of liquid metal: role, mechanism and application

Electronics, such as printed circuit board (PCB), transistor, radio frequency identification (RFID), organic light emitting diode (OLED), solar cells, electronic display, lab on a chip (LOC), sensor, actuator, and transducer etc. are playing increasingly important roles in people’s daily life. Conventional fabrication strategy towards integrated circuit (IC), requesting at least six working steps, generally consumes too much energy, material and water, and is not environmentally friendly. During the etching process, a large amount of raw materials have to be abandoned. Besides, lithography and microfabrication are typically carried out in “Cleanroom” which restricts the location of IC fabrication and leads to high production costs. As an alternative, the newly emerging ink-jet printing electronics are gradually shaping modern electronic industry and its related areas, owing to the invention of a series of conductive inks composed of polymer matrix, conductive fillers, solvents and additives. Nevertheless, the currently available methods also encounter some technical troubles due to the low electroconductivity, complex sythesis and sintering process of the inks. As an alternative, a fundamentally different strategy was recently proposed by the authors’ lab towards truly direct writing of electronics through introduction of a new class of conductive inks made of low melting point liquid metal or its alloy. The method has been named as direct writing of electronics based on alloy and metal (DREAM) ink. A series of functional circuits, sensors, electronic elements and devices can thus be easily written on various either soft or rigid substrates in a moment. With more and more technical progresses and fundamental discoveries being kept made along this category, it was found that a new area enabled by the DREAM ink electronics is emerging, which would have tremendous impacts on future energy and environmental sciences. In order to promote the research and development along this direction, the present paper is dedicated to draft a comprehensive picture on the DREAM ink technology by summarizing its most basic features and principles. Some important low melting point metal ink candidates, especially the room temperature liquid metals such as gallium and its alloy, were collected, listed and analyzed. The merits and demerits between conventional printed electronics and the new direct writing methods were comparatively evaluated. Important scientific issues and technical strategies to modify the DREAM ink were suggested and potential application areas were proposed. Further, digestions on the impacts of the new technology among energy, health, and environmental sciences were presented. Meanwhile, some practical challenges, such as security, environment-friendly feature, steady usability, package, etc. were summarized. It is expected that the DREAM ink technology will initiate a series of unconventional applications in modern society, and even enter into peoples’ daily life in the near future.

Direct writing of electronics based on alloy and metal (DREAM) ink: A newly emerging area and its impact on energy, environment and health sciences

The implementation and exploration of liquid metals for soft electronics, especially electronic skins (e-skins), are fast increasing. The growing field has received special attention since research regarding gallium-based alloys has intensified as these alloys are much safer in comparison to their more hazardous counterpart, mercury. Liquid metal alloys of gallium provide unique physical and chemical properties for e-skin. These properties originate from their high thermal and electrical conductivities and the fact that the liquid metal is an electronic melt in contrast to the ionic liquid. The formation of 2-D oxides on the surface of liquid gallium alloys, with large van der Waals forces, also adds to their uniqueness. Liquid metals, whether in bulk form or particulated morphologies, provide stretchabilities that surpass any other systems, allowing for the formation of super malleable e-skins. As such, they present certain opportunities for developing elements with extraordinary softness, malleability, and skin compatibility: stretchable wires and electrodes, memories, electronic components such as resistors, coils, diodes, and transistors, soft sensors, energy harvesting/storage elements, and self-healing systems. Presence of the 2-D metal compound skin also helps in accessing the non-Newtonian characteristics of gallium-based alloys that permit the formation of microparticles/nanoparticles and specific fluidics and grant printability in three dimensions. Liquid alloys of gallium, their properties, and applications for e-skins are discussed in this review, and the wealth of opportunities for future applications within soft and stretchable electronics is explored.

Electronic Skins Based on Liquid Metals

Gallium (Ga) and some of its alloys have a range of properties that make them an attractive option for microelectronic interconnects, including low melting point, non-toxicity, and the ability to wet without fluxing most materials-including oxides-found in microelectronics. Some of these properties result from their ability to form stable high melting temperature solid solutions and intermetallic compounds with other metals, such as copper, nickel, and aluminium. Ga and Ga-based alloys have already received significant attention in the scientific literature given their potential for use in the liquid state. Their potential for enabling the miniaturisation and deformability of microelectronic devices has also been demonstrated. The low process temperatures, made possible by their low melting points, produce significant energy savings. However, there are still some issues that need to be addressed before their potential can be fully realised. Characterising Ga and Ga-based alloys, and their reactions with materials commonly used in the microelectronic industry, are thus a priority for the electronics industry. This review provides a summary of research related to the applications and characterisation of Ga-based alloys. If the potential of Ga-based alloys for low temperature bonding in microelectronics manufacturing is to be realised, more work needs to be done on their interactions with the wide range of substrate materials now being used in electronic circuitry.

Ga-Based Alloys in Microelectronic Interconnects: A Review

Wearable tactile sensors can be used for haptic perception and are widely utilized in soft robotics, intelligent prosthetics, and other human interfaced/interface applications. The development of tactile sensors with for multifunctional tactile sensing capacity remains a challenge. This article presents the design and fabrication of a wearable tactile sensor based on galinstan liquid metal for the simultaneous sensing of temperature and contact force independently. The decoupling of temperature and contact force sensing, which is based on the measured output voltage signals, is achieved by a designed Wheatstone bridge circuit and by the structural design of the fingerprint-patterned microfluidic channels and the top oval-shaped protrusion. Characterization tests show that the fabricated tactile sensor has a relatively high force sensing sensitivity of 0.32 N−1 and its temperature sensing sensitivities are 0.41% °C−1 at 20~50 °C and 0.21% °C−1 at 50~80 °C. Two wearable tactile sensors are worn on the index finger and thumb of the human hand and are used to detect temperature changes and contact forces during grasping applications. The results show that the developed liquid metal-based tactile sensor has a high flexibility and durability, and can accurately measure the contact forces and temperatures simultaneously. Thus, the developed wearable tactile sensor has great potential for robotic manipulation and healthcare condition sensing in humans.

Liquid Metal-Based Wearable Tactile Sensor for Both Temperature and Contact Force Sensing

The density and surface tension of liquid high-purity gallium and indium as a function of temperature were studied. The sessile drop method was used to obtain these parameters. Experiments were carried out from the melting points to ∼1300 K in a high vacuum. The confidence error of the experiments was 0.5 and 1% for density and surface tension, respectively. The results of this research are compared with recommended reference data in the literature.

Density and surface tension of heavy liquid-metal coolants: Gallium and indium

The density and surface tension of gallium-lead melts have been determined using the sessile drop technique in the temperature range of 600–1000°C According to the form of polytherms of surface tension of a system with Ga-Pb exfoliation, a conclusion was made about the primary distribution of lead ions on the melt-vapor boundary The excess entropies of melt formation were calculated The comparison of experimental and calculated values of excess entropies showed that primary interaction between similar ions highlights the linear character of density polytherms

Density and surface tension polytherms of gallium-lead melts

Cu–W composite alloys are obtained using the liquid-phase impregnation method of noncompacted W powders and sintered porous W and W + Cu specimens. The structure of the alloys and its influence of pre-crystallization processing by low-frequency oscillation (LFO) on the “Cu melt–W powder” compositions are investigated. The technological parameters of obtaining low-porosity (1–2%) alloys are defined. It is proved that varying the thermo-time LFO exposure makes it possible to modify the W concentration in the matrix, creating the composite layers with high tungsten content (80–90%). The LFO treatment of the “copper melt + noncompacted W powders” compositions has a number of advantages compared to the routine (liquid-phase impregnation of compacted tungsten powder) industrial technology of production of Cu–W alloys. There are significant reduction of stages (up to 1–2), the possibility of replacement of working atmospheres (hydrogen, vacuum) by cheaper “Ar + CO,” and the dispersion of W phase.

V. P. Chentsov

Papers

Density and surface tension of heavy liquid-metal coolants: Gallium and indium

Density and surface tension polytherms of gallium-lead melts

Optimization of Liquid-Phase Method of Synthesis of Cu–W Alloys