A water-powered nanogenerator uses droplet formation to convert flowing water into electricity, providing a clean, inexpensive energy source for small devices, sensors, and self-powered environmental systems.
(Nanowerk Spotlight) Capturing electricity from moving water remains a persistent challenge at small scales. Streams, pipes, and rainfall provide constant motion, but converting that motion into useful power efficiently and reliably has proved difficult. Large hydroelectric plants generate electricity effectively but only at substantial scales. At smaller scales, devices that rely on direct interaction between water and solid materials struggle to maintain efficiency and durability.
Triboelectric nanogenerators offer one path forward. These systems operate through the triboelectric effect, in which electrons move between two materials when they touch and then separate. When water serves as one of the materials, each droplet impact or sliding motion on a surface can create a small charge.
However, continuous flow limits this effect. At the interface, ions in the liquid form an electric double layer that screens the surface and prevents further charge transfer. Once that layer stabilizes, electrical output declines sharply.
Numerous approaches have been tested to counteract this problem. Some use injected air, vibrating surfaces, or pressurized sprays to break continuous flow into droplets. While effective, these methods add complexity and require external energy. A simple, self-sustaining mechanism that maintains efficient charge generation under steady flow has remained elusive.
The researchers employ Plateau–Rayleigh instability, a phenomenon that causes a thin jet of liquid to divide into droplets because surface tension favors shapes with minimal surface area. By adjusting the speed and height of the water jet, they allow a continuous flow to break into droplets just before contacting the solid surface. Each droplet touches and detaches in rapid sequence, renewing the interface and preventing the formation of a persistent electric double layer.
Concept for designing a triboelectric nanogenerator that exploits Plateau-Rayleigh instability (PR-TENG). a) The motion state of fluids with different flow rates. b) The cause of the Plateau-Rayleigh instability of a low-speed liquid jet. c) Comparison of the falling state and contact state of water droplets and a low-speed liquid jet. d) The Isc in three states: droplet, flow, and low-speed liquid jet. e) Comparison with the Isc of droplet TENG and flow TENG based on the induction electrode. (Image: Reprinted with permission from Wiley-VCH Verlag) (click on image to enlarge)
The device itself is simple. A thin film of polytetrafluoroethylene (PTFE) serves as the contact surface, supported by a copper electrode and a rigid substrate. PTFE is hydrophobic and tends to attract electrons when it contacts water. Each droplet transfers electrons to the PTFE and departs slightly positively charged. The copper electrode collects the resulting charge and produces an electrical signal.
Because droplet formation occurs naturally and at high frequency, the surface experiences continuous charging and discharging cycles without mechanical assistance or external power.
At a flow rate of about 320 milliliters per minute, the liquid-jet device produces a short-circuit current exceeding 100 microamperes and operates at approximately 50 hertz. These results represent a substantial improvement compared with conventional droplet-based or continuous-flow devices. The improvement arises from the periodic droplet formation, which repeatedly interrupts charge screening and restores efficient contact electrification.
The authors describe the physical mechanism in detail. As the jet accelerates under gravity, slight variations in its diameter grow because surface tension drives liquid away from narrower regions toward wider ones. The stream pinches off into droplets. When these droplets strike the PTFE film, three charge-transfer processes occur: The first is direct contact electrification between the water and polymer. The second is electrostatic induction within the copper electrode as the surface charge changes. The third is a brief current pulse that appears when successive droplets merge on the surface. High-speed imaging synchronized with electrical measurements confirms these events.
Quantitative comparisons highlight the improvement. The short-circuit current is roughly 120 times higher than that from a single-droplet generator and around 3,000 times higher than that from continuous flow. Surface potential measurements show that under droplet impact, the PTFE surface maintains its ability to exchange charge, whereas continuous flow drives it to rapid saturation. Constant renewal of the contact area keeps the surface active.
Charge measurements of the water before and after impact support this interpretation. Free-falling droplets carry a small positive charge, likely caused by friction in the tubing, while a continuous jet holds a slight negative charge due to its internal double layer. After impact, the jet removes more electrons from the PTFE than isolated droplets can. The authors note that fast air exchange near the interface may enhance electron movement, though any chemical contribution from oxygen remains uncertain.
Device output depends on several controllable parameters. Increasing flow velocity lengthens the smooth portion of the jet and creates droplets with greater impact energy, which increases current. The tilt of the PTFE surface affects spreading and rebound. At shallow angles, droplets merge into thin films, reducing hydrophobicity and limiting charge transfer. An angle near 60 degrees allows droplets to hit and detach quickly, producing the highest current density. At steeper angles, contact area decreases slightly and output falls.
Surface chemistry also influences performance. Plasma treatment that makes PTFE more wettable lowers both the amplitude and frequency of current peaks. Strong hydrophobicity remains essential for effective charge transfer. The properties of the water provide another factor. Low concentrations of dissolved salt improve conductivity and increase current, but high concentrations reduce output by screening the surface. Performance follows a distinct optimum rather than a monotonic trend.
Further experiments reveal that droplet coalescence contributes directly to current generation. When a stationary droplet is placed on the surface, each merging event with an incoming droplet produces a sharp current spike lasting about 50 microseconds. Replacing the stationary droplet with a small metal conductor yields a similar signal, confirming that charge redistribution during coalescence plays an important role. Adding multiple conductors multiplies the number of spikes but lowers their amplitude, showing that total charge divides among them.
The device can perform practical functions. At a flow rate of 200 milliliters per minute, it charges a 1-microfarad capacitor to 4 volts within one minute. The output powers arrays of light-emitting diodes through a rectifier and can illuminate a single diode directly with minimal flicker. The maximum power density reaches about 1,235 milliwatts per square centimeter at an external load of 50 kiloohms, approximately 19 times higher than that of a comparable droplet generator.
The same mechanism can support sensing applications. When combined with a siphon tube, the generator produces a distinct electrical signal whenever rising water triggers droplet formation. The signal stops once the siphon ceases, creating a self-resetting water-level indicator that requires no external power.
This study demonstrates that controlling the physical state of flowing water can overcome a major limitation in triboelectric energy harvesting. By exploiting natural fluid instability to convert a steady jet into a droplet sequence, the researchers achieve efficient charge transfer without mechanical input.
Although the results are from controlled laboratory tests, the device’s simple structure and reliance on inherent fluid dynamics suggest potential for environmental energy harvesting and autonomous monitoring. The work establishes a direct link between hydrodynamic behavior and interfacial charge transport, providing a foundation for practical small-scale power and sensing systems.
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