Liquid metal droplets fuse themselves into stretchable circuits at room temperature, driven only by surface tension gradients during solvent evaporation.
(Nanowerk Spotlight) Gallium-indium alloys are liquid at room temperature and conduct electricity almost as well as solid copper wire. That combination makes them attractive for stretchable electronics, where rigid metals crack and fail. Simply layering liquid metal onto a rubber-like substrate does not work well, though: the metal pools, loses contact, and cannot stretch with the polymer.
A more effective strategy is to disperse the liquid metal as millions of micro- or nano-sized droplets embedded throughout a flexible polymer matrix, so the entire composite deforms as a unit. The problem is that each droplet instantly grows a thin oxide shell that blocks electrical contact with its neighbors, leaving the composite an insulator.
Standard approaches use mechanical pressing, laser ablation, or ultrasonic energy to rupture the oxide and force droplets to fuse into conductive networks, a process known as sintering. Each method demands additional equipment and energy, complicating fabrication and limiting scalability.
A study published in Advanced Functional Materials (“Marangoni‐Driven Sintering of Liquid Metal Droplets for Strain‐Insensitive Flexible Electronics”) now shows that liquid metal droplets can sinter themselves during ordinary solvent evaporation at room temperature, with no applied force or post-processing. Researchers at Qingdao University and the China University of Petroleum achieved this by adding ethanol to a toluene-based polymer solution, creating a surface tension gradient that drives droplet migration and packing through the Marangoni effect.
Design and fabrication of Janus liquid metal (LM) film. (a) Schematic illustration of sonicating LM in SEBS-toluene-ethanol mixed solution. (b) Homogeneous solution after sonication. (c) Optical image of Janus film with gray (top layer) and shiny surface (bottom layer). (d) Surface tension of toluene and ethanol. (e) Explanation of the causes of Marangoni phenomenon. (f) The schematic diagram of the preparation of Janus film induced by the Marangoni effect. (Image: Reproduced with permission from Wiley-VCH Verlag) (click on image to enlarge)
The Marangoni effect is the flow of liquid caused by spatial differences in surface tension. In this system, ethanol evaporates considerably faster than toluene because of its lower boiling point. As ethanol leaves the surface of the drying film, the top layer becomes ethanol-depleted and its surface tension rises relative to the ethanol-rich interior below. This vertical gradient pulls ethanol upward and pushes the liquid metal droplets, encased in collapsing chains of the rubber-like polymer SEBS (styrene-ethylene-butylene-styrene), downward toward the substrate.
As droplets concentrate at the bottom and solvent volume contracts, the spacing between them shrinks to the nanoscale. Liquid bridges form between neighboring droplets and generate capillary forces that grow exponentially as the gaps narrow. When droplets finally make contact, the local pressure spikes to the gigapascal range, enough to rupture the oxide shells and merge the liquid metal cores. The entire process occurs during ambient drying at 25 °C with no heating, pressing, or other post-treatment.
The result is a “Janus” film with two distinct faces: a liquid-metal-rich conductive layer on the bottom and a polymer-rich insulating layer on top. At a liquid metal loading of just 17 vol%, the conductive side reached 1.1 × 10⁵ S/m. The same formulation without ethanol was roughly ten million times less conductive, underscoring how critical the Marangoni-driven assembly is.
The gradient structure also produced unusual mechanical behavior. Stretchable conductors typically suffer sharp resistance increases during elongation as conductive pathways fracture. This Janus film maintained a resistance ratio (R/R₀) of just 1.5 even at 1250% strain. The researchers attribute this to a complementary process: during initial stretching, droplets in the upper, less-dense region of the film aggregate and sinter into new pathways, actually lowering resistance.
At higher strains, liquid metal flows into microcracks and maintains continuous electrical connectivity through the network. Only at extreme elongation do breaks appear, yet the metal’s fluidity keeps resistance close to its starting value. The film held stable conductivity through hundreds of stretch-release cycles and repeated bending, twisting, and peeling.
Ethanol concentration turned out to be a sensitive control parameter. Sintering occurred only within a narrow window. Too little ethanol produced Marangoni flow too weak to drive sufficient droplet migration. Too much triggered phase separation in the polymer, trapping droplets in isolated pockets and preventing them from forming a connected network. Within the effective range, the minimum liquid metal content needed for electrical connectivity, known as the percolation threshold, dropped to less than half the value required without ethanol, a direct result of tighter droplet packing.
The approach also worked with other polymer matrices, including polyurethane, styrene-butadiene-styrene, and acrylonitrile-butadiene-styrene, provided the relative evaporation rates of solvent and non-solvent remained favorable.
The team tested several practical applications. A sandwich-structured capacitive strain sensor, with two Janus films as electrodes flanking a pure SEBS dielectric layer, responded quickly enough to track finger bending, wrist flexion, muscle contraction, and swallowing with repeatable signals. The Janus film also served as a Joule heater, reaching 100 °C at just 1.0 V, and as an electromagnetic interference shield that exceeded commercial performance standards. Because SEBS is a thermoplastic, spent films could be shredded, redissolved, and recast, preserving recyclability.
What separates this strategy from prior self-sintering methods is its minimal infrastructure. Earlier approaches required elevated temperatures, specialized inks, or additives like graphene. Here, the driving force is a difference in boiling points between two common laboratory solvents. Controlling that difference within the effective ethanol window may prove challenging at industrial scale, but the underlying mechanism is simple: engineer the right evaporation mismatch, and capillary physics does the rest.
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