Hybrid Liquid Metal Machine

Abstract

Clearly, liquid metal alone cannot solve everything. However, if combining with other materials or structures, liquid metal would aid to make complicated system, which is not possible otherwise. This chapter illustrates a typical way to fabricate a kind of oscillating system made of hybrid liquid metal and rigid needle. As is known to all, oscillation is a widely seen dynamic phenomenon (Jenkins in Phys Rep 525:167–222, 2013 [1]; Cross and Hohenberg in Rev Mod Phys 65:851–1112, 1993 [2]; PikovskyjU and Kurths in Phys Rev Lett 78:775–778, 1997 [3]) in mechanical (Hendricks in Science 3:775–776, 1884 [4]), electrical (Tesla in Proc IEEE 87(7):1282, 1999 [5]), biological (Aschoff in Science 148:1427–1432, 1965 [6]), chemical (Petrov et al. in Nature 388:655–657, 1997 [7]) systems, etc. Oscillatory chemical reactions such as Belousov–Zhabotinsky (BZ) reaction (Petrov et al. in Nature 361:240–243, 1993 [8]), mercury beating heart (Lin et al. in Proc Natl Acad Sci U S A 71:4477–4481, 1974 [9]) are classical examples of nonequilibrium thermodynamics switching between different patterns. This lab (Yuan et al. Adv Sci 3:1600212, 2016 [10]) found the first ever oscillation phenomenon of a copper wire embraced inside the liquid metal machine via chemical and mechanical coupling. Previously, it was revealed that gallium-based liquid metal owns rather important value to serve as shape transformable material (Zhang et al. in Adv Mater 27:2648–2655, 2015 [11]; Sheng et al. Adv Mater 26:5889–5889, 2014 [12]) due to its unique property of high electrical conductivity, excellent fluidity, and low melting point. In addition, feeding the liquid metal with aluminum would lead to self-powered motors which could keep long-term actuation performance in alkaline solution (Zhang et al. in Adv Mater 27:2648–2655, 2015 [11]) due to surface tension gradient and H2 propulsion mechanism. The study discovered even more unusual effects that apart from self-actuation, such liquid metal machine would trigger a copper wire to reciprocally move back and forth across the liquid metal body. When contacting a copper wire to the liquid metal motor, it will be wetted and swallowed and then oscillates horizontally like a violin bow at the frequency of about 1.2 Hz. Moreover, the oscillation could be easily regulated and speeded up by touching a steel needle on the liquid metal motor surface. This fundamental phenomenon can be explained by the wetting behavior difference due to chemical reaction. Given appropriate designing, such autonomous oscillator composed of hybrid solid and liquid metal structures can be developed as a core switch element in periodically regulating devices to realize various particular fluidic, electrical, mechanical, and optical functions. The present finding refreshes the basic understanding of the soft machines commonly conceived in textbook as well as add new knowledge to the wetting science (Gennes et al. in Phys Today 57:66–67, 2004 [13]). It also opens a basic way to fabricate self-powered wire oscillator using liquid metal as the main machine body. Further, through combining with other external material, substrate, and environmental factors, more hybrid liquid metal machines can still be made. Some of such typical strategies will be discussed thereafter.