Let it Flow: Emergence of Liquid Metals

Abstract

Liquid metals—that is, metals with low melting points—have fascinating and unique properties. Gallium is one of the most popular liquid metals because it melts near room temperature, has low toxicity, effectively zero vapor pressure, and can be combined with other metals to form alloys. The term “liquid metal” can be more broadly defined to include metals that melt at temperatures that are easy to access under common laboratory conditions. Room-temperature liquid metals open unique possibilities to leverage the soft, fluidic characteristics of liquids while simultaneously taking advantage of properties like high surface energy and thin oxide layers, high electrical and thermal conductivity, and other metallic characteristics that are typically reserved for rigid, solid materials. This liquid-metallic combination has resulted in remarkable properties, unusual behaviors, and new materials and devices that cannot be created with typical fluids or common metals alone. Interest in liquid metals has grown over the last decade due to the interest in applications that take advantage of their properties. For example, liquid metals can be used as conductors and components in soft and stretchable electronics. Likewise, they can be used for soft biomedical devices, energy harvesting devices, and e-textiles. Liquid metals can help catalyze reactions in unique ways and have interfacial properties that can be exploited for a variety of applications, such as actuators, batteries, and substrates for the deposition of thin films. The versatility of liquid metals presents many opportunities for future materials, devices, and systems, and represents an exciting paradigm for both scientific exploration and the creation of novel technologies. The special issue on Liquid Metals for Functional Materials consists of 49 manuscripts with 37 research articles and 12 reviews across a wide breadth of the liquid metal field. The work focuses on the latest advancements from fundamental properties to applied concepts, as broadly classified as Energy and Electrochemistry, Biomedical and Healthcare, Catalysts and Reactions, Composites and Multiphase Materials, Electronics and Conductors, Fabrication and Patterning, and Interfacial Properties. This collection highlights the multidisciplinary nature of liquid metal research and provides a platform to showcase our current understanding and outline future directions. We sincerely thank Dr. Joseph Krumpfer and Dr. Esther Levy, whose dedication to this special issue energized this effort and led to the exciting collection of papers presented in this special issue. Below, we will briefly highlight the contents of the special issue through the lens of several of the key subtopics in the field. Liquid metals offer promising potential in future energy storage and harvesting. Their electrical conductivity, deformability, compatibility with electrochemistry, lithium dissolution capability, and the possibility of creating porous composites make them ideal materials for efficient ion and electron exchanges. The addition of small metallic components can significantly alter the electrochemical behavior of liquid metals. In a work published in this special issue, Tang and co-workers showed the modulation of electrochemical properties by studying fractals that are formed from liquid metal alloys (adfm.202301348). In another work, Li and co-workers presented mixtures of MXene with EGaIn for energy-harvesting (adfm.202307830). Additionally, Boley and co-workers created a galvanic cell by mechanically rupturing surface oxide of liquid metals to convert chemical energy to electrical energy (adfm.202309177). Quality of electrical conductivity and stretchability are important factors in textile-based electronics with embedded supercapacitors, which are carefully investigated in another work by So and co-workers (adfm.202310318). An interesting paper by Deng and co-workers presented the application of liquid metals for forming elastome