DOI: 10.1002/adma. 201405438 macroscopic counterpart still remains a big mystery since it is generally difficult to obviate the requirement of external physical energy in order to generate sufficient mechanical momentum. Typically, to achieve controllable locomotion of a macroscopic object, external physical energies from the surrounding electromagnetic field,[14–16] electric field,[17, 18] or thermal gradients [19] are employed to transform them into mechanical energy. This however may somewhat restrict the independence of the motors and make their actuation problematical once the external sources are inaccessible. In principle, the driving force must be increased by several orders approximately when scaling up the operational motors from nano-/micrometer to milli-/centimeter to overcome the resistance from the aqueous environment. It is noteworthy that the velocity, which usually refers to the velocity relative to the body length of the objects, is extremely sensitive to the size of the object.[20] Thus, for a macroscopic motor, it is often necessary to expedite the absolute velocity, and correspondingly, it requires a further stronger driving force. As such, it is challenging to actuate milli-/centimeter scaled objects with a velocity on the order of centimeters per second without any exposure of external stimuli.

The macroscopic motors that are external-energy independent have been perused over decades. Such motors can be classified into solid motors [21–23] and liquid motors.[21, 24–26] Under the macroscopic scale, the flexibility or rigidity influences the application of the material significantly. The macroscopic motors based on soft materials are capable of deformation, and thus they are competent for performing special missions under tough conditions. Most of the existing self-propelled liquid motors are organic droplets like 4-octylaniline,[24] oleic anhydride,[25] and aniline oil.[26] Here, from an alternative, the room temperature liquid metals such as EGaIn (75% gallium and 25% indium) and Galinstan (68.5% gallium, 21.5% indium, and 10% tin) are introduced to fabricate radically different liquid motors. Regarding the perspective of materials, different materials own varied properties and merits, which render different performances and potential applications. The liquid metals own many favorable properties, including pretty large surface tension, desirable flexibility, high electrical conductivity and low toxicity in comparison with mercury,[27] which make them promising for designing soft robot or microfluidic systems.