A Bright Future for Liquid Functional Materials?

Abstract

Liquid metals were merely sci-fi fantasy a few decades ago, playing a prominent role in the Terminator film franchise. Current research is leading to an array of emerging functional liquids. Here, we discuss the exciting possibilities of such fluid materials. Liquid metals were merely sci-fi fantasy a few decades ago, playing a prominent role in the Terminator film franchise. Current research is leading to an array of emerging functional liquids. Here, we discuss the exciting possibilities of such fluid materials. While immersed in science fiction movies and novels, we marvel at the wild imagination. Sometimes this requires a suspension of disbelief, but there frequently exist scientific principles behind the artistic conceptions, which makes them closer to current life and society than classics such as The Iliad and Arabian Nights. In the movie Terminator 2: Judgment Day (1991) (Figure 1), the liquid metal T-1000, nemesis to Arnold Schwarzenegger’s obsolete T-800 model, is almost indestructible. The metal-based soft matter can change its shape and mimic the color of the environment. (In the film, the liquid metal is referred to as “mimetic polyalloy”.) The concept is so ingenious, it is currently featured in the latest installment of the franchise, Terminator: Dark Fate (2019) (Figure 1). How close are we to liquid metal devices in reality? Rather than a dark fate, the future of liquid metals seems rather bright. Taking advantage of unique properties of liquid metals at room temperature, morphologic changes—for example, movement—can be realized through the application of various energy fields such as electric, magnetic, or concentration gradients. With dynamic (seemingly autonomous) movement, it is sometimes easy for us to slip into the feeling that they can be new lifeforms. We delight in adding romantic anthropomorphism to these wonderful liquids. We should open new horizons and admit the fact that beyond liquid metals, other emerging functional liquids are playing more and more important and interesting roles in various fields. Functional liquids provide environments different from those of molecular solvents (such as aqueous and organic solvents), which can trigger new synthesis mechanisms of functional materials. They can be patterned with high resolution by direct writing and microinjection due to the excellent fluidity. Like in the Terminator movies, they can take any shape and are capable of spontaneous self-healing after suffering damage, thus excelling in flexible devices and robotics. At the same time, they are easy to disperse and gather, which can be useful in biomedical fields such as drug delivery. The solid-liquid coexistence of many liquid materials makes it possible to realize the energy storage via the phase transition that is not possible with rigid materials. It can be expected that the emerging functional liquids will be one of the key materials trending in intelligent society or even exist as new life forms in the future. Herein, we will touch on the world of emerging functional liquids and introduce the main ones, including liquid metals, ionic liquids, and liquid crystals. A new class of materials composed of post-transition metal elements and zinc group metals and their alloys are now labeled liquid metal due to their ultra-low melting point. They are in a fully liquid state and exhibit excellent fluidity at relatively low temperatures. Take the example of gallium (Ga); it performs like a liquid at room temperature owing to the intrinsic effect of supercool. It even can remain liquid at −80°C when encapsulated in carbon tubes to suppress its expansion. This material can be used as a miniature thermometer within a very wide temperature range. When several kinds of liquid metals are put together to form alloys with the designed proportion, the charge density and lattice order change, thus weakening the interatomic force in the mixed metal and resulting in a further decrease of the melting point, even below 0°C. Other alloys can be formulated with unique phase-separation behaviors, exploiting nominal variations in melting temperatures.1Tang S.-Y. Mitchell D.R.G. Zhao Q. Yuan D. Yun G. Zhang Y. Qiao R. Lin Y. Dickey M.D. Li W. Phase Separation in Liquid Metal Nanoparticles.Matter. 2019; 1: 192-204Abstract Full Text Full Text PDF Scopus (44) Google Scholar Due to their wide melting range, reducing atmosphere, and electron-rich environment, liquid metals can also serve as excellent reactive solvents, in which almost all the metal elements can be dissolved, providing an access to prepare functional liquid materials. For example, the saturated liquid metal mixture formed by dissolving excessive gadolinium (Gd) in eutectic galinstan (Ga‒In‒Sn) presents both spontaneous magnetization and magnetothermal effects. As the combination of liquid (with deformable and injectable characteristics) and metal (with electrically and thermally conductive properties), liquid metal is a favorable candidate for future flexible electronics. Ga-based liquid metals, with their low toxicity and biocompatibility, naturally stand out as being one of the most popular materials for wearable electronic devices. To potentially create Terminator-like cyborgs (hopefully not for killing), the behavior of real skin must be accurately mimicked, including perceiving/sensing and interacting with the environment. As such, artificial electronic skins need to be able to generate neural-like informational synapses to respond to external stimuli and receive excitation signals. Functional materials based on liquid metals are progressively approaching this goal.2Karbalaei Akbari M. Zhuiykov S. A bioinspired optoelectronically engineered artificial neurorobotics device with sensorimotor functionalities.Nat. Commun. 2019; 10: 3873Crossref PubMed Scopus (28) Google Scholar In addition, the added specific responsive components can act as synapses in liquid materials, such as TiO2, which is sensitive to the light. As alternative materials for electronic devices and soft robots, liquid metal composites also have the unique advantage of self-healing. When being damaged by tearing or impaling, liquid metal may be designed to spontaneously and instantaneously flow, splice, and fuse with the surrounding liquid metal to reform a new conductive path without any external force field stimulation.3Markvicka E.J. Bartlett M.D. Huang X. Majidi C. An autonomously electrically self-healing liquid metal-elastomer composite for robust soft-matter robotics and electronics.Nat. Mater. 2018; 17: 618-624Crossref PubMed Scopus (331) Google Scholar For precisely constructed wearable electronics, higher integrated resolution is inevitably required for continued improvement of functions. Traditional pattern techniques, such as evaporation and lithography, only achieve the higher resolution on a two-dimensional (2D) scale whereas liquid metal can be used as ink for high-resolution three-dimensional (3D) printing.4Park Y.G. An H.S. Kim J.Y. Park J.U. High-resolution, reconfigurable printing of liquid metals with three-dimensional structures.Sci. Adv. 2019; 5: eaaw2844Crossref PubMed Scopus (74) Google Scholar Currently, an as-printed thin wire can only be printed 1.9 microns wide and can perfectly maintain the 3D structure of the electronic device after the spontaneous oxidation of liquid metal antenna due to its extremely high specific surface area. What’s more, it has been reported that the liquid metals can exhibit more biological-like behaviors, such as breathing, eating, self-moving, and leaping. The emerging liquid functional materials based on liquid metals are gradually being applied in various fields as a result of their excellent properties, like flexible shapes, stable interface, high integrated resolution, and self-healing ability. It should also be noted that the self-limiting oxidation layer on the surface of liquid metals is few atomic layers thick, which provides a platform to explore naturally existing 2D materials. Different from solid metals, the interfacial interactions of liquid metal can change from the metallic bond to van der Waals interaction within a few atoms thickness at their its surface. Coupling with the fluidity of liquid, it can be easily separated to obtain atomistically thin and wafer-level liquid metal oxides.5Kim Y.D. Hone J. Materials science: Screen printing of 2D semiconductors.Nature. 2017; 544: 167-168Crossref PubMed Scopus (39) Google Scholar In the chemical vapor deposition (CVD) process to grow 2D materials, the employment of liquid metals also provides new opportunities.6Zeng M. Fu L. Controllable fabrication of graphene and related two-dimensional materials on liquid metals via chemical vapor deposition.Acc. Chem. Res. 2018; 51: 2839-2847Crossref PubMed Scopus (30) Google Scholar The vacancies in the liquid metal bulk facilitate them as the container of heteroatoms. The liquid metal solvent-based strategy can be extensively employed for growing 2D materials. A smooth and isotropic liquid surface without grain boundaries and defects is conducive to reduce the nucleation density in the growth process. Together with controlling mass transfer and segregation of dissolved precursors in the liquid bulk during the phase transition of liquid metal, the self-limited growth of 2D materials with ultra-thin thickness and high quality can be achieved. It is interesting to note that the 2D material single crystals will smoothly stitch with each other to form grain boundary-free film due to the excellent rheological property of liquid metals. It is also found that 2D material single crystals grown on liquid metals can spontaneously self-assemble into ordered arrays with highly uniform sizes and directions, which lays the foundation for a 2D integrated electronic system. Besides the unique advantages of the liquid metal in the controllable growth and assembly of 2D materials, its fluidity and smooth surface also enable the grown materials to be directly slid to arbitrary substrate. This fast, nondestructive, and uncontaminated transfer method facilitates the practical application of 2D materials. Ionic liquids are a kind of multifunctional material with highly tunable and adjustable properties. Simply put, liquid ionic compounds are obtained by increasing the size and structure asymmetry of cations or anions, thus reducing the force between anions and cations and enabling their liquid state at a low temperature. Biological science is one of the emerging applications of ionic liquids. Similar to many liquid mediums, ionic liquids can be used as a reaction platform, especially in the field of biocatalysis, wherein many biological enzymes exhibit high activity. Based on the fact that almost all the enzyme reactions occur in water, researchers have found that by adding a small amount of water into ionic liquids, hydrated ionic liquids can be formed, which contributes to the enhancement of the enzyme activity for the degradation of biological materials. Moreover, since ions are involved in material transport, energy conversion, information transfer, and other processes during normal activities of a living system, ionic liquids show the great potential for both drug delivery and drug manufacturing.7Egorova K.S. Gordeev E.G. Ananikov V.P. Biological activity of ionic liquids and their application in pharmaceutics and medicine.Chem. Rev. 2017; 117: 7132-7189Crossref PubMed Scopus (671) Google Scholar Extending the concept, poly(ionic liquids) can be formed by the combination of ionic liquids with a primary chain of polymerization to obtain new structures and shapes. The complexity of the structures of poly(ionic liquids) allows multiple interactions, such as electrostatic, hydrogen bonding, and π-π stacking to be introduced to potentially adjust the conductivity, water solubility, and stability of these kinds of materials. These promote their applications in various fields from biological medicine to the adsorption/dissociation and catalysis. As for the energy storage, it can be used to prepare stable and secure storage facilities like organic transistors. Electromechanical actuators, which act like a muscle in a reversible way under the voltage, have also attracted much attention. Using ionic liquids instead of aqueous electrolytes can greatly improve the working window and service life of sensitive equipment. Liquid crystals—a class of materials between a crystalline solid state and an isotropic liquid state—exhibit both crystal anisotropy and flowable characteristics of ordinary liquids. A liquid crystal may flow like a liquid, but its molecules can be oriented in a crystal-like way. This special structure makes it one of the candidates in soft materials for functional stimulus response, which necessitates quick response speed and low signal consumption under the stimulation of external fields. Liquid crystals have long been used in display devices such as television screens. Recently, the discovery of some new functional liquid crystals has injected new vitality into this field. Graphene oxide (GO), a common 2D material, spontaneously forms nematic liquid crystals upon reaching a critical solvent concentration. Since the topology of liquid crystals is fluid, GO liquid crystals can form folded structures. Based on this discovery, researchers successfully prepared GO films with patterned fold structures, which can effectively regulate the mechanical and electrical properties of GO films.8Jiang Y. Guo F. Xu Z. Gao W. Gao C. Artificial colloidal liquid metacrystals by shearing microlithography.Nat. Commun. 2019; 10: 4111Crossref PubMed Scopus (11) Google Scholar In addition to displays, optical driven materials have received recent attention. Optical driven phenomena are very common in the living world, exemplified by the heliotropism of plants and the phototaxis of animals. Optical driven phenomena can also be found in the material field, of which the most representative one is liquid crystals. Azobenzene-containing liquid crystalline polymers are capable of spontaneous movement if dealt with optical light. Both phototropy and photo-induced phase transition of liquid crystals can happen within a few hundred microseconds.9Bisoyi H.K. Li Q. Light-driven liquid crystalline materials: from photo-induced phase transitions and property modulations to applications.Chem. Rev. 2016; 116: 15089-15166Crossref PubMed Scopus (397) Google Scholar Due to this intrinsic photosensitivity, liquid crystals are often used to facilitate the formation of directional patterns that allow for rich topological transformations. Liquid crystals can also be found in living bodies, such as solutions of collagen and peptides, as well as DNA and biofilms. Liquid crystals have a lot in common with biological systems; for example, the fluidity of liquid crystals is in agreement with the motility of biological systems. Just like organisms, liquid crystals are supersensitive to external stimulations. Upon heating, lighting, or other external fields, they may undergo color changes, phase changes, or motions in response to stimulation. More importantly, the ordered structure of liquid crystal molecules is constantly broken down, and to some extent is renewable, similar to the organs of living organisms, which can maintain their vital functions. Those distinct and unique properties enable us to understand the structural changes of organisms and then study the various life activities expressed by living organisms. Like liquid metals and ionic liquids, liquid crystals have favorable biocompatibility, which lays the foundation for their applications in bionic fields, such as high-performance fibrous materials, and provides a new perspective on living systems. Liquid means fluidity, evolution, and advance. As briefly discussed, liquid functional materials offer unlimited possibilities, including preparation of new materials, biomedicine, self-healing, sensing, artificial intelligence, etc., in a variety of frontier fields (Figure 2). The interdisciplinary collision and fusion have opened new horizons to understand this fluid world. Recently, work has reported liquid magnetic materials, which not only maintain the magnetism of solid counterparts but also have the advantages of liquid flow reconstruction.10Liu X. Kent N. Ceballos A. Streubel R. Jiang Y. Chai Y. Kim P.Y. Forth J. Hellman F. Shi S. et al.Reconfigurable ferromagnetic liquid droplets.Science. 2019; 365: 264-267Crossref PubMed Scopus (59) Google Scholar At the same time, the functional liquid family is being continuously enriched. We can expect that more and more known functional materials will emerge brand new in liquid form. In current laboratory studies, it has been found liquid functional materials can spontaneously move like mollusks, exhibiting phototaxy, and are constantly destroyed and regenerated, just like living cells. While they cannot be called a living system, they have been making us think and may completely change our impression on life. Indeed, for carbon-based life—almost covering all living forms on the earth—a majority of life activities occur at the liquid-liquid, liquid-gas or liquid-solid interface. Liquid materials—with the processable appearance, exchange of matter and energy with the environment, information transition, and response to external stimuli—might one day be regarded as a new form of “life,” which can broaden and deepen our understanding of living systems. Unlike the Terminator films, there is no dark fate for liquid functional materials, only a bright future. The research was supported by the Natural Science Foundation of China (Grants 21673161 and 21905210 ), the Sino-German Center for Research Promotion (Grant 1400 ), the Science and Technology Department of Hubei Province (Grant 2017AAA114 ), and the Postdoctoral Innovation Talent Support Program of China (Grant BX20180224 ).