Responsive Liquid Metal Droplets: From Bulk to Nano

Abstract

Droplets exist widely in nature and play an extremely important role in a broad variety of industrial processes. Typical droplets, including water and oil droplets, have received extensive attention and research, however their single properties still cannot meet diverse needs. Fortunately, liquid metal droplets emerging in recent years possess outstanding properties, including large surface tension, excellent electrical and thermal conductivity, convenient chemical processing, easy transition between liquid and solid phase state, and large-scale deformability, etc. More interestingly, liquid metal droplets with unique features can respond to external factors, including the electronic field, magnetic field, acoustic field, chemical field, temperature, and light, exhibiting extraordinary intelligent response characteristics. Their development over the past decade has brought substantial breakthroughs and progress. To better promote the advancement of this field, the present article is devoted to systematically summarizing and analyzing the recent fundamental progress of responsive liquid metal droplets, not only involving droplet characteristics and preparation methods, but also focusing on their diverse response behaviors and mechanisms. On this basis, the challenges and prospects related to the following development of liquid metal droplets are also proposed. In the future, responsive liquid metal droplets with a rapid development trend are expected to play a key role in soft robots, biomedicine, smart matter, and a variety of other fields.

Liquid metal droplet dynamics

“…Droplets, small drops of liquid, contain a rich and mysterious wealth in related research fields including physics, chemistry, engineering, materials science, bionics, biology, and others. 1 A great deal of all-sided and detailed works has been conducted to profile the unique features of individual droplet or multiple droplets and explore their potential in various fields. 2 The formation of droplets is the result of surface tension, which is defined as the energy source required to change per unit surface area of a material.…”

“…9 Meanwhile, the boiling points of Ga-based LMs are extremely high (over 2000°C) which permit a wide liquid temperature range for safe handling to meet various application needs. 1 In addition, this class of special functional materials combines the high electrical and thermal conductivity virtues of metal, as well as the fluidic and deformable properties of fluids. 5,10 Owing to these outstanding physical merits, Ga-based LMs are believed to be alternative materials to realize numerous unconventional functions and have quickly raised widespread concerns of multidisciplinary scientists.…”

Directional Motion of Gallium-Based Liquid Metal Induced by Asymmetric Chemical Surrounding

“…This means these metals can serve as safer demonstrations. − In a previous demonstration, gallium and its alloys formed a beating heart that occurred in a uniform solution, where oscillations were induced by the periodic change of surface tension. Self-propulsion or deformation of the liquid metals resulted from a surface tension gradient , that was derived from ionic imbalance, redox reaction, − or electric field. , …”

“…19−22 In a previous demonstration, gallium and its alloys formed a beating heart that occurred in a uniform solution, where oscillations were induced by the periodic change of surface tension. Self-propulsion or deformation of the liquid metals resulted from a surface tension gradient 23,24 that was derived from ionic imbalance, 25 redox reaction, 26−28 or electric field. 29,30 In the previous gallium-based liquid metal beating heart demonstrations, the position of the iron nails or the electrodes in contact with the liquid metal droplets required careful adjustment to trigger a "heartbeat" or oscillation.…”

Toxicity and Biocompatibility of Liquid Metals

“…Compared to conventional metal-based biomaterials, liquid metals offer many extraordinary opportunities owing to their excellent properties including high electron mobility, [4,5] thermal conductivity, [6][7][8] and deformability. [9][10][11] Notably, liquid metal-based biomaterials have shown wide variety of applications, [12][13][14][15][16][17][18] such as biomedical imaging, [19,20] photothermal therapy, [21][22][23][24][25][26][27] injectable electronics, [28,29] bone repair, [30][31][32] and nerve electrodes or signal transmission. [33,34] (Figure 1).…”