Exploding Water Droplets Put to Work
Researchers in Japan have discovered that evaporating water droplets on slippery surfaces can repeatedly eject charged microdroplets, a phenomenon that could aid nano-manufacturing and 3D printing.
TECH TUESDAY || 2026.05.12
Water droplets may seem lovely to behold—shapely, delicate, ephemeral. But as they evaporate, a peculiar violence creeps in: they can become electrically unstable and suddenly erupt, blasting out microscopic jets of water in bursts lasting millionths of a second.
Now, researchers led by scientists at Okinawa Institute of Science and Technology (OIST) in Japan have discovered a new form of this explosive behavior in ordinary water droplets resting on slippery surfaces.
The phenomenon expands upon a 144-year-old theory of charged droplets first proposed by Lord Rayleigh, a pioneering British physicist and fluid dynamicist, and reveals that evaporating droplets can repeatedly eject sprays of tiny charged microdroplets without the need for an external high-voltage source. This could open new possibilities for nanoscale manufacturing, spray coating, inkjet printing, and chemical analysis.
The work appears in the Proceedings of the National Academy of Sciences.
Raindrops Keep Falling, and Exploding
Water droplets are normally held together by surface tension, the force that gives them their familiar rounded shape. But electrical charge pushes in the opposite direction. Charges of the same sign repel one another, creating an outward force across the droplet’s surface.
In 1882, Lord Rayleigh calculated that beyond a certain threshold of charge, now known as the Rayleigh limit, electrostatic repulsion overcomes surface tension and the droplet becomes unstable. The droplet deforms and eventually breaks apart in a process called Coulomb fission.
For decades, this behavior has been studied primarily in suspended droplets levitated in air or electric fields. But ordinary droplets resting on surfaces were thought to behave differently. Contact with a surface suppresses the shape changes needed for the instability to develop, preventing the dramatic breakup seen in free-floating droplets.
To produce Coulomb fission on a surface, the researchers, led by Dan Daniel, head of the OIST Droplet and Soft Matter Unit, deposited tiny water droplets onto plastic dishes coated with a thin layer of silicone oil. As the droplets evaporated, electrical charges naturally concentrated on their surfaces until electrostatic forces overcame surface tension.
The droplets then elongated into tear-like shapes before releasing fine jets of microdroplets and relaxing back to their original form. The cycle repeated over and over, in some cases more than 60 times across 30 minutes.
The silicone oil proved essential because it eliminated friction between the droplet and the surface below. Without that friction, the droplets were free to stretch, contract, and deform as electrical forces intensified during evaporation.
The effect unfolded in stages. As evaporation shrank the droplet, its electrical charge became increasingly concentrated. At a first threshold, the droplet elongated, with one side sharpening into a cone-like point where charges accumulated. At a second threshold, a jet of water burst outward from that tip, ejecting dozens of highly charged microdroplets within microseconds before the parent droplet relaxed and the process began anew.
The researchers discovered that droplets resting on surfaces behave differently from levitated droplets studied in earlier experiments. Instead of a single instability threshold predicted by the classical Rayleigh limit, the team identified two distinct thresholds: one governing elongation and another triggering the explosive jetting itself.
Primed for 3D Printing and Chemical Analysis
The separation between those thresholds created a measurable delay between deformation and eruption, in some cases lasting seconds or even minutes.
And that delay may prove technologically useful.
By changing the viscosity of the silicone oil layer, the researchers were able to tune the size of the emitted microdroplets. Thicker oils produced larger droplets, while thinner oils generated finer sprays. Such control could be valuable for technologies that rely on precisely generated aerosols or charged particles.
In some forms of high-resolution 3D printing, for example, tiny droplets are deposited layer by layer to build microscopic structures. Being able to tune droplet size with greater precision could improve the resolution and uniformity of those fabricated materials.
More broadly, the approach may offer a new way to manufacture nanoscale coatings or particles whose properties depend sensitively on size and shape.
The researchers said the findings could also point toward more energy-efficient laboratory techniques. One possibility is electrospray ionization, a common method used in mass spectrometry that normally relies on high-voltage electric fields to generate sprays of charged droplets. As those droplets shrink, they release ions that can then be identified and measured, allowing scientists to determine the chemical composition of a sample.
The researchers next hope to better understand the jetting process itself, including how to more precisely control the size, direction, and timing of the emitted microdroplets.
More Information:
Journal Article (open-access): Lin, Marcus, et al. "Spontaneous Coulomb Fissions of Drops on Lubricated Surfaces." Proceedings of the National Academy of Sciences, vol. 123, no. 18, 30 Apr. 2026, p. e2538161123, https://doi.org/10.1073/pnas.2538161123
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