![]() ![]() The choice of method depends on specific applications and cost/complexity of the experiment. Various existing methods are applicable to liquid metal including manual methods ( Yu et al., 2014), flow focusing via microchannels ( Thelen et al., 2012), molding ( Mohammed et al., 2014), and inkjet printing ( Li et al., 2015, 2016). Liquid Metal Droplet GenerationÄroplet generation has been studied extensively in the field of microfluidics ( Xu and Attinger, 2009 Yu et al., 2011 Khoshmanesh et al., 2017 Zhu and Wang, 2017). Soft robots, reconfigurable electronics, and microelectromechanical systems (MEMS) are among these applications. In this approach, unique properties of liquid metals can be utilized toward applications in which flexibility and stretchability are necessary while the requirement of large pumps, wires, and direct contact with liquid metal is eliminated ( Eaker and Dickey, 2016). Besides generating liquid metal objects, manipulating the motion, and shape of these metal objects is also of importance. Generated microdroplets can also be used as micropumps, microcoolers, and micromixers ( Tang et al., 2016). The result can be utilized without any necessary further processing or can be considered as initial steps of intricate pattering methods which have been recently summarized by Majidi and Dickey (2015). The mechanically stabilizing oxide layer on liquid metal surfaces, along with the high surface tension delivers potentials for generating, and manipulating liquid metal objects and droplets. Control and Manipulation of Liquid Metals In this mini-review, we highlight the most recent progress made in manipulation and actuation of gallium-based liquid metals (both in droplet and flow scale) and summarize the latest applicable techniques in this field. One advantage provided by oxide layer is that it facilitates manipulation of gallium-based liquid metals. One of the leading research groups working on room-temperature liquid metals, discussed numerous emerging capabilities, and applications of gallium-based liquid metal devices enabled by the native oxide layer ( Dickey, 2014, 2017). That said, this oxide layer is not always problematic, since it can also help stabilize the liquid mechanically. When exposed to air, an oxide layer quickly forms on the surface of liquid metals, which is undesirable in some applications ( Morley et al., 2008), since it disrupts the wetting behavior and impedes physical and electrical contacts ( Giguere and Lamontagne, 1954). However, there are also some challenges in working with these liquid metals. Demonstrating superior performances in various aspects, gallium-based liquid metals have been explored for many novel applications, such as microfluidics devices ( Khoshmanesh et al., 2017), stretchable electronics ( Wang et al., 2015c Bartlett et al., 2016), reconfigurable devices ( Wang et al., 2015b), electronics cooling ( Ma and Liu, 2007), vacuum pumping ( Tang et al., 2015a), and painted conductive electrodes in liquid droplet actuation ( Eaker et al., 2017). EGaIn is a similar eutectic composition of 75.5%wt of gallium and 24.5%wt of indium ( Dickey et al., 2008). GaInSn or galinstan is a eutectic alloy composed of 68%wt gallium, 22%wt of indium, and 10%wt of tin ( Liu et al., 2012). ![]() Two important gallium-based alloys are GaInSn and EGaIn. They own all useful properties of other solid/molten metals such as high thermal conductivity, high electrical conductivity, inherently high density, and low vapor pressure, while being non-toxic, unlike mercury. These alloys not only can flow easily, but also can be shaped to some extent. Room-temperature gallium liquid metal alloys have drawn increasing research interests recently. In this mini-review, we summarize the most recent progresses achieved on liquid metal droplet generation and actuation of gallium-based liquid metals with/without an external force. These methods lead to numerous useful applications such as soft electronics, reconfigurable devices, and soft robots. This can occur manually or in the presence/absence of a magnetic/electric field. By removing or reconstructing the oxide skin, shape and state of liquid metal droplets and flows can be manipulated/actuated desirably. When exposed to air, a native oxide layer forms on the surface of gallium-based liquid metals which mechanically stabilizes the liquid. Gallium-based room-temperature liquid metals possess extremely valuable properties, such as low toxicity, low vapor pressure, and high thermal and electrical conductivity enabling them to become suitable substitutes for mercury and beyond in wide range of applications. Mechanical and Industrial Engineering, University of Illinois at Chicago, Chicago, IL, United States. ![]()
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