Let it Flow: Emergence of Liquid Metals

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 elastomer seals in stretchable lithium-ion storage units (adfm.202309861). Finally, the special issue includes a review by Yan and co-workers on advances in energy storage based on liquid metal systems (adfm.202309706). The low toxicity of gallium-based liquid metals makes them suitable for medical applications. The use of liquid metals in these areas has been growing in popularity, and this issue features several papers that review recent biomedicine-related liquid metal works. For example, one paper by Seo and co-workers summarized the use of low-melting-point metals for biomedical applications (adfm.202307708). One paper by Park and co-workers reviewed the usage of gallium-based liquid metals for bioelectronics (adfm.202307990) while another paper by Lim and co-workers reviewed healthcare-related applications of liquid metals in wearables and soft robotics (adfm.202309989). Additionally, Wei and co-workers reviewed liquid metal-based biosensors (adfm.202308173). In more specific applications, Truong and co-workers created silver-gallium amalgamated particles that show antibacterial properties and show promise as a spray-coating on implantable devices (adfm.202310539). In another work by Markvicka and co-workers, composites of elastomers and liquid metals were developed with acoustic properties that improve the image quality of wearable ultrasound devices for long-term patient monitoring (adfm.202308954). Additionally, liquid-metal electrodes and particles were used by Kim and co-workers to create soft biosensors capable of detecting ascorbic and uric acid, dopamine, and glucose (adfm.202311696). Currently, a significant portion of global greenhouse emissions and energy consumption stems from industrial-scale chemical reactions used in the production of ammonia, fuel, hydrogen, polymers, and other chemicals. Progress in enhancing catalysis and reaction rates using solid materials has been limited. Exploring the untapped properties of liquid metals holds great promise for introducing new paradigms in these chemical processes. Researchers offered several interesting works in this special issue on catalysts and reactions using liquid metals. Here, Daeneke and co-workers showed a delicate liquid metal system with incorporated copper for electrocatalytic oxidation of ethanol (adfm.202304248). Kalantar-Zadeh and co-workers demonstrated that liquid metal can be used as the reservoir for zinc metal for sourcing it into the synthesis metal–organic frameworks (adfm.202300969). While another study put forward by Yarema and co-workers presented an approach to use liquid metal to create Pd-Zn nanocrystals (adfm.202309018). In another seminal manuscript, Sitti and co-workers utilized liquid metal initiated polymerization to create hydrogel composites (adfm.202308238). A liquid metal reaction media was also used by Zavabeti and co-workers for the creation of atomically thin bismuth oxides that enabled strong piezoelectric systems (adfm.202307348). O'Mullane and co-workers showed that liquid metals have a potential use in plasma-assisted carbon dioxide reduction by incorporating liquid metal droplets (adfm.202307846). The capabilities of liquid metals are not just limited to inorganic systems. Miyako and co-workers showed that liquid metal catalysts can also be effectively used in immunostimulants (adfm.202305886). Liquid metal can be combined with diverse components such as polymers, metals, carbon-based materials, or other organic or inorganic materials to create composites and multiphase systems. This can create novel composites with enhanced functional or mechanical properties relative to solid-based inclusions or other phases can be added into liquid metal to provide new properties not native to liquid metal. These concepts were well captured in the special issue. One review by Kramer-Bottiglio and co-workers focused on multiphase composites containing liquid metal and other (x) fillers for unique combinations of properties (adfm.202309529) while another review by Lee and co-workers showed how adding particles into liquid metal can create magnetic liquid metal (adfm.202311153). A third review by Tee and co-workers discussed liquid metal composites for wearable electronics (adfm.202400284). Additionally, a set of papers presented liquid-metal polymer composites with different functionalities. One work by Bartlett and co-workers showed how liquid-metal droplets in elastomers could create electrically conductive reversible adhesives for soft electronics (adfm.202304101), while another by Li and co-workers showed hydrogel composites with high toughness and conductivity (adfm.202308113). This polymer composite architecture also enabled thermally stable soft materials for high-temperature applications as presented by Majidi and co-workers (adfm.202309725) while Zhou and co-workers used liquid metal droplets as cross-links to enable recyclable conductive composites (adfm.202308032). The high deformability of liquid metal wires was also used in conjunction with a magnetic soft composite by Park and co-workers to create artificial muscles (adfm.202302895). Like solid metals, liquid metals have a high electrical conductivity, making them suitable as conductors in electronics, but with the added properties of liquids, such as discretization into droplets. One review by Hussain and co-workers examined liquid-metal droplets and their use for electronics such as sensors, switches, transistors, actuators, and more (adfm.202308116). Liquid metals are also especially suitable for soft, flexible, and stretchable electronics. Toward this, Zhang and co-workers created a soft, lightweight composite with networks of liquid-metal fibers (adfm.202308128) while Zhang and co-workers developed liquid-metal composite materials that show increased conductivity with applied strain (adfm.202310225). In another contribution, Zheng and co-workers demonstrated a permeable and stretchable liquid-metal fibers for sensors and other electronics (adfm.202308120). Bartlett and co-workers presented a liquid metal-based conductive adhesive for integration of soft electronics and rigid devices (adfm.202313567) while Hjort and co-workers presented a laser engraving methodology for creating liquid metal-based interconnects (adfm.202309707). Other papers in this issue describe the use of liquid metals in specific electronic devices. One review presented by Park and co-workers examined liquid-metal systems that respond to a variety of stimuli, and that can be used for electronics such as wearable sensors (adfm.202308703). The compliant nature of liquid metal was used by Lacour and co-workers to make sensors that can measure softness (adfm.202308698). A liquid-metal inductive sensor was created by Jeong and co-workers that is capable of distinguishing between various stimuli on a finger (adfm.202305776), and a wearable liquid-metal antenna was directly printed by Hashimoto and co-workers (adfm.202311219). Liquid metals can be patterned into useful shapes such as circuits, antennas, and wires to create soft and stretchable analogs to existing electronic devices. Relative to conventional metals, such as copper or aluminum, the fluidic nature of liquid metal allows it to be patterned in unique ways, including injection and 3D printing. In many cases, patterning is facilitated by the thin, solid oxide layer that forms rapidly on the surface of liquid metals in the presence of oxygen. This solid oxide "skin" allows the liquid metal to retain shapes that would normally be impossible with liquids due to surface tension. This special issue offers several interesting works on the fabrication and patterning of liquid metals. Liquid metals were 3D printed in ceramics by Yang and co-workers for use in microwave absorption (adfm.202307499). In another example, Syed and co-workers separated the oxide layer from the liquid metal to enable the fabrication of gas sensors (adfm.202309342). Park and co-workers also took advantage of the ability to use liquid metal particles to pattern stretchable electronics (adfm.202309660). The particles can be used to make thermally conductive composites as demonstrated by Lee and co-workers (adfm.202306698) or be cast as a film and subsequently merged together to form conductive traces as presented by Dickey and co-workers (adfm.202308574). Liquid metals have remarkable interfacial properties. For example, they have the largest interfacial tension of any liquid at room temperature, with values nearly an order of magnitude larger than that of water. Yet, the surface tension effectively can be lowered to near zero by using electrochemical oxidation. This phenomenon, as well as several others, can be used to control the flow and shape of liquid metal, as reviewed by Wang and co-workers in this special issue (adfm.202309614). In addition, studies by Daniels and co-workers provide new insights into this electrochemical phenomenon by carefully measuring the tension as a function of electrical potential (adfm.202311501) or by confining the metal to porous tubes as demonstrated by Khan and co-workers (adfm.202307919). Another interesting property of liquid metals is that they can react to form surface oxides on their surface. Herein, Tabor and co-workers enhance the mechanical strength of the skin by depositing thin silica layers on the oxide (adfm.202308167). Further, Elbourne and co-workers evaluated the structure of the oxide on liquid metal droplets (adfm.202310147) while Koo and co-workers utilized the oxide-coated liquid metal to form more stable solar cells (adfm.202311597). We are grateful for all the authors, reviewers, and editors who made this special issue possible. We hope this special issue will help highlight the challenges and exciting opportunities for the development and utilization of liquid metal in diverse applications. M.D.B., M.D.D., A.T.O., and K.K-Z. contributed equally to this work. The authors declare no conflict of interest Michael D. Bartlett is an associate professor and John R. Jones III Faculty Fellow of Mechanical Engineering at Virginia Tech. He received his B.S.E. from the University of Michigan, Ph.D. from the University of Massachusetts Amherst, and was a postdoctoral fellow at Carnegie Mellon University. Michael leads the Soft Materials and Structures Lab, which investigates multifunctional soft materials and composites with highly controllable mechanical and functional properties for the creation of soft electronics and robotics based on liquid metal, switchable and intelligent adhesives, and adaptive materials. Michael Dickey is the Camille and Henry Dreyfus Professor in the Department of Chemical & Biomolecular Engineering at NC State University. He received a B.S. in chemical engineering from Georgia Institute of Technology (1999) and a Ph.D. from the University of Texas (2006) under the guidance of Professor Grant Willson. From 2006 to 2008, he was a post-doctoral fellow in the lab of Professor George Whitesides at Harvard University. He completed a sabbatical at Microsoft in 2016 and EPFL in 2023. Michael's research interests include soft matter (liquid metals, gels, polymers) for soft and stretchable devices (electronics, energy harvesters, and soft robotics). Aaron Ohta is a professor in the Department of Electrical and Computer Engineering at the University of Hawaii at Manoa. He received his B.S. degree from the University of Hawaii at Manoa, his M.S. degree from the University of California, Los Angeles, and his Ph.D. degree from the University of California, Berkeley, all in electrical engineering. Aaron's research interests include reconfigurable circuits and systems using liquid metals and other materials, microfluidics, and microelectromechanical systems (MEMS). Kourosh Kalantar-Zadeh is a professor and Head of School of Chemical and Biomolecular Engineering at the University of Sydney. He is involved in research in the fields of analytical chemistry, materials sciences, gastroenterology, electronics, and sensors. Professor Kalantar-Zadeh is best known for his works on ingestible sensors, liquid metals, and 2D semiconductors. He led his group to the invention of an ingestible chemical sensor: human gas sensing capsule, one of the breakthroughs in the field of medical devices. He has received several international awards for his scientific contributions including the 2017 IEEE Sensor Council Achievement, and 2020 Robert Boyle Prize of RSC.