GEARS ABRADE, pistons crack, pumps clog. If engineers had their way, machines would have no moving parts at all. Alas, a sedentary lump of metal would be a paperweight, rather than a useful machine. So, perhaps just one moving component would be an acceptable compromise.
Such machines are now beginning to appear. The component in question is a cleverly chosen liquid, any one of a number of alloys of gallium that melt below room temperature. These have three alluring properties: the highest surface tensions of any known liquid (nearly ten times that of water), good electrical conductivity, and extreme chemical reactivity, in the form of a willingness to donate electrons—a process known as oxidation—to nearby compounds.
In a recent paper published in Matter, Kourosh Kalantar-Zadeh at the University of New South Wales used these properties to design a machine with a single moving part: a continuous-flow reactor. As the name suggests, these are devices for performing chemical reactions under sustained pumping, which offers more precision than mixing chemicals in vats. The mechanical pumps that power them, however, are frequently fouled.
Dr Kalantar-Zadeh designed and 3D-printed a circular track, 14cm in circumference, for reactants to flow around. It was interrupted by a single cavity containing a droplet of liquid metal hooked up to an electrical power source. A voltage applied to the droplet produces a gradient in its surface tension. That, in turn, leads to a pressure difference across the metal’s surface strong enough to pull reactants across, allowing electron donation to happen and the desired chemical reaction to occur. The deformation of the droplet as it tenses and relaxes within its cavity pumps the resultant chemical away, allowing fresh reactants to flow in and the process to begin again. The researchers successfully applied this model to three different reactions, including the reduction of flakes of graphene oxide, which is useful for purifying water and in energy storage.
And this is only the latest in a growing body of experiments and prototyping aimed at exploiting the unique properties of liquid metals. In 2014, Dr Kalantar-Zadeh’s group developed a pump capable of driving liquids around a circuit by similar means, but which did not exploit the metal’s reactivity. In 2016, miniature robots were fitted with liquid metal wheels that could be steered across an aqueous solution by manipulating their surface tension. In 2018, wheels containing droplets of liquid metal were developed that changed their centres of gravity in the presence of electric fields, thereby causing the wheels to rotate. In 2021, another group of researchers devised a liquid metal-powered motor that suffered far less wear and tear than those built from solid parts.
The laws of physics dictate just where such liquid-metal machines will be most effective. The forces produced by surface tension dominate at small scales. At larger scales they are eclipsed by those generated by electromagnetism, on which conventional motors rely. This means that liquid-metal engineering will be most useful for objects that are roughly centimetre-size and below. As this is the regime where maintenance and repairs are the most fiddly and costly, such a feature is good news.
Researchers are exploring their use in “labs-on-a-chip”, which are portable devices for conducting a variety of scientific tests in the field, far from conventional laboratory infrastructure. Some scientists have crafted them into modular antennas, whose resonant frequencies can be changed by adjusting their shape. Others hope to weave them into soft robots, where they could act as artificial muscles, or to use them in 3D-printed electronics.
And even though they will likely be limited to smaller devices, the appeal of liquid metals is easy to see. Blobs of liquid experience none of the friction-induced wear-and-tear that eventually causes gearboxes, valves and the like to break down. Any damage or disruption they suffer is naturally self-healing. Gallium alloys, moreover, are easy to make, harmless to the touch, and have very low rates of evaporation, meaning that users are unlikely to inhale them accidentally. Just the ticket, in other words, for creative engineers. ■
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This article appeared in the Science & technology section of the print edition under the headline “Liquid engineering”