Slide electrification: charging of surfaces by moving water drops

Stetten, Amy Z.; Golovko, Dmytro S.; Weber, Stefan A. L.; Butt, Hans‐Jürgen

doi:10.1039/c9sm01348b

Cited by 79 publications

(127 citation statements)

References 71 publications

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“…Handling of liquid droplets has received great interest in a broad range of research areas including microfluidics, enabling chemical reactions, medical analysis, separation and extraction of target analytes, triboelectric generation, and water harvesting. [ 8,45–50 ] The shape‐designable polyhedral LMs/LPs developed in this study show unique handling abilities and should attain great interest due to possible applications including high‐performance microreaction containers and sensors. [ 51,52 ]…”

Section: Resultsmentioning

confidence: 99%

Shape‐Designable Polyhedral Liquid Marbles/Plasticines Stabilized with Polymer Plates

Fujiwara

Geyer

Butt

et al. 2020

Adv Materials Inter

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Polyhedral liquid marbles/plasticines are prepared using (sub)millimeter‐sized polymer plates as a stabilizer and water as an inner liquid. Precise control of size and shape can be successfully performed by tuning the size ratio of the water droplet and the plate, number of plates adsorbed to the droplet, coalescence (jointing) of multiple polyhedral liquid marbles/plasticines, and application of external mechanical stress. Thanks to interfacial jamming of the plates, plastic deformation of the liquid marbles/plasticines is achieved. The authors are able to fabricate liquid marbles/plasticines with various shapes including A–Z letters of alphabet. Liquid marble/plasticine with an aspect ratio exceeding 800, the largest aspect ratio ever reported, is also successfully prepared; the length of the liquid marble/plasticine exceeded 1.5 m. The liquid marbles can be picked up and be piled up on top of each other using tweezers or fingers. Furthermore, Janus‐type liquid marbles/plasticines with different curvatures and different stabilizers in a single liquid marble/plasticine can be fabricated by coalescence (jointing) of near‐spherical and cuboid liquid marbles/plasticines stabilized by plates with different sizes. An internal liquid flow from the near‐spherical liquid marble to the cuboid liquid marble/plasticine immediately after jointing is observed, making this system act as a micropump.

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Section: Resultsmentioning

confidence: 99%

Shape‐Designable Polyhedral Liquid Marbles/Plasticines Stabilized with Polymer Plates

Fujiwara

Geyer

Butt

et al. 2020

Adv Materials Inter

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“…As reported before, negative charges preferentially adsorbed at hydrophobic surfaces. [33][34][35] Previous observations indicated that fluoropolymer materials even spontaneously and permanently adsorb negative charges upon extended contact with water. [25] The highest charge densities had been reported for elevated pH.…”

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Charge Trapping‐Based Electricity Generator (CTEG): An Ultrarobust and High Efficiency Nanogenerator for Energy Harvesting from Water Droplets

et al. 2020

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and droplet-based electricity generator (DEG). [9] However, inherent flaws exist in current approaches. Reverse electrowetting energy harvesting devices always need external voltages. [1] Triboelectric nanogenerator (TENG), [10,11] which was first invented in 2012 by Wang and coworkers, [12,13] has provided a passive energy harvesting approach. But the performance of TENG is limited by the low density and poor stability of surface charges on tribo-layers. High surface charge density could only be achieved in vacuum environment [14] or by utilizing external pumping or excitation sources. [11,15] The droplet energy harvesting efficiency of the conventional TENG was only 0.01%. [5] Recently, Z. K. Wang and coworkers have reported a water dropbased electric generator, DEG, [9] showing significantly enhanced energy harvesting efficiency to 2.2%. Nevertheless, the energy harvesting efficiency of DEG is still limited by the density and stability of charges generated by triboelectrification during drop impact. The maximum surface charge density of DEG displayed around 0.184 mC m −2 (49.8 nC for 2.7 cm 2). [9] The surface charges in DEG were superior stability compared to the conventional TENG, although the charge density still degraded in a harsh environment with 100% humidity. Moreover, the efficiency greatly dropped with increasing salt Strategies toward harvesting energy from water movements are proposed in recent years. Reverse electrowetting allows high efficiency energy generation, but requires external electric field. Triboelectric nanogenerators, as passive energy harvesting devices, are limited by the unstable and low density of tribo-charges. Here, a charge trapping-based electricity generator (CTEG) is proposed for passive energy harvesting from water droplets with high efficiency. The hydrophobic fluoropolymer films utilized in CTEG are pre-charged by a homogeneous electrowetting-assisted charge injection (h-EWCI) method, allowing an ultrahigh negative charge density of 1.8 mC m −2. By utilizing a dedicated designed circuit to connect the bottom electrode and top electrode of a Pt wire, instantaneous currents beyond 2 mA, power density above 160 W m −2 , and energy harvesting efficiency over 11% are achieved from continuously falling water droplets. CTEG devices show excellent robustness for energy harvesting from water drops, without appreciable degradation for intermittent testing during 100 days. These results exceed previously reported values by far. The approach is not only applicable for energy harvesting from water droplets or wave-like oscillatory fluid motion, but also opens up avenues toward other applications requiring passive electric responses, such as diverse sensors and wearable devices.

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“…In many ENGs, the water drops also fulfill a second role: in addition to providing the required initial mechanical energy, the impact process is also responsible for the generation of the trapped surface charge on the hydrophobic surface [6,9,12]. However, like other triboelectric charging mechanisms that have been explored, this process is notoriously difficult to control and dependent on details of materials and process conditions [16][17][18]. The poor stability of surface charges generated by drop impact, tribocharging, and hydrophobicwater contact [19][20][21][22][23][24] compromises the performance and stability of possible devices and has also hampered the development of a quantitative model of the energy conversion process, which is needed to provide guidelines for further improvements of the technology.…”

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confidence: 99%

Energy Harvesting from Drops Impacting onto Charged Surfaces

et al. 2020

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We use a combination of high-speed video imaging and electrical measurements to study the direct conversion of the impact energy of water drops falling onto an electrically precharged solid surface into electrical energy. Systematic experiments at variable impact conditions (initial height; impact location relative to electrodes) and electrical parameters (surface charge density; external circuit resistance; fluid conductivity) allow us to describe the electrical response quantitatively without any fit parameters based on the evolution of the drop-substrate interfacial area. We derive a scaling law for the energy harvested by such "nanogenerators" and find that optimum efficiency is achieved by matching the timescales of the external electrical energy harvesting circuit and the hydrodynamic spreading process.

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Slide electrification: charging of surfaces by moving water drops

Abstract: We investigate the charge separation caused by the motion of a water drop across a hydrophobic, insulating solid surface.

Cited by 79 publications

References 71 publications

Shape‐Designable Polyhedral Liquid Marbles/Plasticines Stabilized with Polymer Plates

Shape‐Designable Polyhedral Liquid Marbles/Plasticines Stabilized with Polymer Plates

Charge Trapping‐Based Electricity Generator (CTEG): An Ultrarobust and High Efficiency Nanogenerator for Energy Harvesting from Water Droplets

Energy Harvesting from Drops Impacting onto Charged Surfaces

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