Autonomous convergence and divergence of the self-powered soft liquid metal vehicles

Abstract Conventional rigid machines, even biologicalsystems in nature, generally do not own the capabilities likeautonomous convergence or divergence. Here, such ex-traordinary behavior was demonstrated for the first timewith the liquid metal vehicle. This synthetic soft machinefueled with an aluminum flake could initiate its autonomouslocomotion in an open-top circular channel containingNaOH solution, like a running vehicle. If cutting a largemachine into several smaller separately running vehicles,each of them still resumes its traveling state along the ori-ginal track and chases each other. If the volumes of suchdispersive vehicles were close to each other and they wereall squeezed in the channel, the vehicles would move syn-chronously with oscillation. Otherwise, such self-motionwould become desynchronized with interval between theinequable vehicles decreased gradually. If their volumeswere significantly different, and the smaller vehicles werenot squeezed in the channel, the faster vehicle wouldovertake the slower ones, until they finally coalescedseamlessly. The assembled vehicle could deform itselfalong with change of its velocity. This finding may shedlight onfuture researches on smart material, fluid mechanicsand soft matter to self-fueled machine and biomimics. Itwould also offer opportunities for constructing self-recon-figurable soft robots.Keywords Liquid metal Self-fueled machine Soft robot Autonomous convergenceand divergenceSoft matter1 IntroductionRecently, the self-propelled nano-/microsystems have at-tracted intensive attention owing to their outstanding per-formance in a wide variety of application areas like drugdelivery [1, 2] or smart sensors [3, 4]. Meanwhile, the self-propulsion of macroscopic machine has also been enor-mously tried, which, however, remains a big challenge upto now. Such machines are perceived as pretty useful toolsfor fabricating actuators and sensors. So far, there are al-ready many investigations conducted to achieve such goal[5–9]. The actuation mechanisms are generally categorizedas bubble propulsion [10], electroosmosis [11]orMarangoni effect [8]. Among them, Marangoni effect,which induces an imbalance of surface tension to push theobject forward [8], is especially effective to generate suf-ficient impetus for the autonomous motion of macroscopicobjects. Considering that the macroscopic machines madeby soft matter appear more competent for many specialtasks, several liquid materials have been concentrated on toachieve the autonomous motion [5, 7, 8]. Particularly, theroom-temperature liquid metal EGaIn (an eutectic alloy of75 % gallium and 25 % indium by mass) owns manymerits, such as large surface tension, desirable flexibility,