A liquid metal droplet that splits and merges to pump fluid offers a simple, low-voltage engine for powering soft robots, wearables, and microfluidic systems.
(Nanowerk Spotlight) Soft machines are designed to move like living tissue rather than like engines. They bend, twist, and compress with quiet precision, yet most still depend on rigid pumps or valves hidden somewhere outside the flexible body. That reliance limits how small or independent they can become.
A soft robot may appear alive in motion, but it often breathes through a hard compressor. A wearable device may fit like a second skin, yet its power source remains an inflexible battery pack connected by tubes or wires. Engineers continue to search for a way to give these systems a heartbeat of their own, a compact and silent mechanism that moves liquid through soft channels as smoothly as muscle moves blood.
The problem is not a lack of imagination but the behavior of fluids at small scales. When liquids travel through narrow spaces, they do not flow like water in pipes. They cling to surfaces and resist sudden pressure changes. Valves stick. Pumps lose strength. Electrical methods that can move liquids, such as electroosmotic or ionic flow, often require high voltages or unusual materials. Magnetic and acoustic systems can work but only under limited conditions. The result is a paradox. The softer and smaller a device becomes, the harder it is to make it move.
A promising direction is emerging from the study of instability itself. Instead of preventing droplets from breaking apart, scientists are learning to use that process in a controlled way. Liquid metals such as gallium–indium alloys conduct electricity like copper but move like a liquid. They respond to electric and magnetic fields, change shape under low voltage, and merge again after separating. These qualities suggest a new kind of soft engine that operates without pistons or valves, one that uses the behavior of a droplet to generate motion.
A paper published in Advanced Materials (“Shapeshifting Liquid Metal Droplets for Soft Fluidic Machines”) turns this idea into a working technology. It describes how a small droplet of liquid metal, placed inside a soft channel and exposed to a magnetic field, repeatedly stretches, divides, and merges to push fluid forward. What once counted as an unstable accident becomes the driving principle of a stable device.
This research redefines what motion can mean in soft machines, showing that even a single droplet can act as an engine when guided by the right forces.
The study introduces a method called liquid metal shapeshifting, or LMSS, which uses a droplet of eutectic gallium indium alloy (E-GaIn) as both conductor and pump. Confined within a flexible channel and powered by a low-voltage current, the droplet cycles through deformation, breakup, and reunion to move fluid without solid moving parts. The device operates from a single AAA battery and produces repeatable fluid motion that can be scaled or combined in networks of small pumps.
The concept of liquid metal shapeshifting. A) Conceptual figure of the liquid metal shapeshifting droplet in unconstrained (explosive), semi-constrained (unstable) and constrained (stable, cyclic) conditions, interplaying intrinsic surface tension and the Lorentz force. B) Schematic diagram of the LMSS pump. Current, magnetic field and Lorentz force are shown as blue, red and purple arrows. C) Demonstrating the deformability and small size of the LMSS pump (scale bar 5 mm). D) (top) 4 stages of the liquid metal shapeshifting in the LMSS pump: 1-deforming, 2-pinch-off, 3-rupture, 4-coalescence and return to stage 1. (bottom) Simulation of the current density of the liquid metal at each stage. E) Comparison of specific pressure per Watt versus operating voltage of LMSS pump and other soft and commercial pumps. The detailed data are presented in Table S (Supporting Information). F) Demonstrating bidirectionality of LMSS pump driven directly from an AAA battery. Scale bar is 20 mm. (Image: Reprinted from DOI:10.1002/adma.202420265, CC BY) (click on image to enlarge)
The setup is conceptually simple. A permanent magnet sits under a soft channel molded from polydimethylsiloxane, a silicone rubber commonly used in microfluidic systems. Two copper electrodes are embedded opposite each other, bridged by one droplet of E-GaIn. The surrounding liquid is sodium hydroxide solution, which keeps the metal surface clean and maintains its high surface tension. When current flows through the droplet, the moving charges experience a sideways magnetic push known as the Lorentz force. That push deforms the droplet and stretches it along the channel.
Surface tension resists this deformation until the droplet narrows and splits into two parts. The break opens the circuit and stops the current, which removes the magnetic push. Surface tension then pulls the separate pieces back toward the electrodes. When they touch, they fuse into a single droplet again, closing the circuit and restarting the cycle. The continuous sequence of stretch, split, recoil, and merge drives the surrounding liquid forward. The motion repeats without external control or mechanical valves.
Measurements in the study show that the droplet becomes unstable and begins to split when its perimeter grows to about 1.8 times the original. At that point, the Weber number—a dimensionless measure of how strong the driving forces are compared to surface tension—is near 1, meaning the two forces balance. The threshold current for breakup is about 3 amperes. Below that, the droplet only elongates without oscillation.
The physical design is minimal and inexpensive. The soft body measures 15 millimeters long, 2 millimeters thick, and 7 millimeters wide, with a microchannel 15 millimeters long, 5 millimeters wide, and 0.1 millimeter deep. It weighs 0.45 grams and costs less than half a British penny to make. Despite this simplicity, it produces stable cycles at voltages as low as 0.01 volts and power levels between 0.006 and 0.4 watts.
The performance depends on geometry and electrical input. A single LMSS unit can resist a back pressure of 3 kilopascals when idle because surface tension seals the droplet. Flow direction reverses when the current direction changes. Series connections of pumps raise pressure, while parallel connections raise flow. Four pumps in series generate 24 kilopascals. Four in parallel yield a flow rate of about 3.7 milliliters per minute. Pairs of pumps often synchronize their cycles automatically, simplifying coordination in arrays.
Geometry strongly affects behavior. Narrower channels increase pressure, while wider ones favor flow. Asymmetric electrode placement helps produce higher pressure because it creates uneven stretching. Symmetric placement gives steadier flow. If channels are too tall, small satellite droplets may detach during operation, though this can be reduced by optimizing dimensions.
The device performs best in sodium hydroxide solution, which prevents oxide buildup on the metal surface. It also functions for short periods in air or salt solution but becomes less stable. In long tests, a single droplet pump ran for more than 7,000 cycles without losing function. Thermal images at 6 and 8 amperes show only minor heating, confirming low energy loss and chemical stability.
The study moves beyond laboratory tests to show practical uses. One demonstration connects the pump to a soft bending actuator that moves more than 90 degrees in less than 12 seconds at 8 amperes. Another pairs two actuators so they move in opposite directions when pressure alternates, forming a controllable motion similar to a simple robotic limb. A third application channels colored liquids through a soft surface to create patterns and color mixes. Each pump controls one color, and combinations of three pumps can mix red, green, and blue to produce a full range of hues.
A wearable prototype illustrates the broader potential. The pump circulates a suspension of titanium dioxide particles in a thin soft skin that blocks ultraviolet radiation. When active, the circulating suspension cuts UV-A intensity by about 40 percent at moderate concentration. The entire system, powered by a small lithium-polymer battery or a single AAA cell, runs continuously for more than eight minutes. Measured operating voltage remains between 0.2 and 0.35 volts, with current near 6 amperes. The result is a self-contained soft device that operates safely at very low voltage.
Efficiency can rise further through stronger magnetic fields. Simulations show that adding a return path for the magnetic flux could increase field strength eightfold for the same magnet. Because pumping power depends on both current and field, that adjustment could reduce electrical energy use by about 64 times to roughly 0.006 watts while maintaining similar performance. When compared with commercial micro-pumps and previous soft designs, the LMSS system delivers higher flow and pressure per watt at far lower voltage.
Several physical ideas underlie the mechanism. Surface tension is the contractile force that pulls a liquid surface inward. Coalescence is the merging of two droplets into one. The Rayleigh–Plateau instability explains why long fluid columns naturally break into droplets. The Weber number compares how strongly a force pushes a liquid to how strongly surface tension pulls it back. By adjusting geometry and current so that these forces alternate in balance, the LMSS pump turns instability into function.
Safety and practicality strengthen its appeal. Voltages are far below those considered risky for human contact. The soft silicone structure and small mass reduce mechanical hazard. Operation is silent and vibration-free. Fabrication uses standard microfluidic methods and low-cost materials. Together these features suggest scalable applications in soft robotics, microfluidics, and wearable systems that move or circulate liquids without rigid machinery.
The central advance is conceptual as much as technical. The LMSS design transforms a droplet’s natural instability into an energy cycle. Rather than suppressing breakup, it relies on it. The repeated act of stretching and merging becomes a reliable engine that turns electrical input into hydraulic power. This approach replaces bulky compressors and pistons with an autonomous droplet that pumps itself.
This work defines a clear pathway for creating pumps and actuators that are soft, safe, and efficient. The study demonstrates stable operation, tunable pressure and flow, and practical applications ranging from actuation to color change to ultraviolet protection.
Future work on improving magnetic field design and managing oxidation will likely enhance performance further. The essential idea is simple yet powerful. A droplet of liquid metal can supply motion and pressure inside a soft machine, providing a foundation for devices that are both flexible and independent.
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