Zuankai Wang and the Droplet-Based Electricity Generator – Biomimetic Surfaces and a Nature Cover Study at CityU Mechanical Engineering

City University of Hong Kong (CityUHK) integrated information database · 04 Research module · Materials science deep-dive series For the overview and other landmark breakthroughs see materials-and-engineering-research.md; together with chemist Hua Zhang's phase-engineered nanomaterials, this is one of CityU's signature figures in materials/engineering – one working on biomimetic surfaces, the other on nanocrystal phase engineering.

In one sentence: Zuankai Wang, Chair Professor of Mechanical Engineering at City University of Hong Kong, built his reputation on biomimetic superhydrophobic surface research and in February 2020^※ published his droplet-based electricity generator (DEG) in Nature – a single 100-microlitre drop falling from a height of 15 cm generates over 140 volts, with an instantaneous power density of 50.1 W/m²^※ (thousands of times higher than previous devices), enough to light 100 small LED bulbs simultaneously. His team's superhydrophobic "pancake bouncing" work also holds a Guinness World Record^※ for the shortest liquid–solid contact time (February 2018).

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# Who is Zuankai Wang, and what does he work on at CityU?

Zuankai Wang is Chair Professor in the Department of Mechanical Engineering at City University of Hong Kong and Co-Director of the Centre for Nature-Inspired Engineering. According to his CityU staff profile^※, he received his bachelor's degree in mechanical engineering from Jilin University, a master's from the Shanghai Institute of Microsystem and Information Technology at the Chinese Academy of Sciences, then went on to a PhD in mechanical engineering at Rensselaer Polytechnic Institute (2008) and postdoctoral research in biomedical engineering at Columbia University (2009). In November 2022 he moved to The Hong Kong Polytechnic University as Associate Vice President (Research and Innovation) and Chair Professor, but the body of work he produced during his CityU years remains one of the most internationally influential research lines in that department's mechanical engineering.

Wang's core research questions can be summed up in three: What is the physical time limit for liquid–solid contact? Can a liquid choose its own direction of spreading? And how do you fully suppress the Leidenfrost effect? These three seemingly basic physics questions map onto very different applications – anti-icing coatings, microfluidic chips, and cooling for aviation and nuclear power – and together form the logical core of his push to take "biomimetic surface science" into engineering practice. According to the lab's official page^※, the team published over 200 papers during its CityU years, including more than 18 in the Nature and Science family of journals. Wang himself joined CityU in September 2009 and was promoted to Chair Professor in July 2021^※, during which time he also co-directed the Centre for Nature-Inspired Engineering.

One important point: during his CityU tenure Wang published not just two Nature papers – on droplet electricity generation and the Leidenfrost effect – but a third top-journal paper as well. On 17 September 2021^※, the team published work in Science that solved a liquid-direction problem that had stood since 1804 – a span of 217 years (see the next section but one). These three top-journal papers, spanning 2020 to 2022, mark the most fertile period of his late CityU career.

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# "Pancake bouncing" and the Guinness record: the shortest liquid–solid contact time

To understand Wang's droplet generator breakthrough, you first need to know the foundational work that established his reputation at CityU: the dynamics of droplet bouncing on superhydrophobic surfaces.

In 2014^※, the team published "Pancake bouncing on superhydrophobic surfaces" in Nature Physics. They showed that on a superhydrophobic surface made of nanotextures arrayed over sub-millimetre cylindrical posts, an impacting droplet does not follow the classic "spread – retract – bounce" path. Instead it leaves the surface as a flat "pancake" during the spreading phase itself, with a contact time of only about 3.4 milliseconds – roughly three-quarters shorter than conventional designs. The finding overturned the received view that droplet-bounce contact time had a theoretical lower limit.

In February 2018^※, the team's invention for the "most waterproof surface" (shortest liquid–solid contact time) was officially entered in the Guinness World Records. It remains one of only a handful of Guinness certifications earned by CityU mechanical engineering research, and it brought the concept of "superhydrophobic surfaces" out of the lab and into public view. The physical significance is straightforward: a shorter liquid–solid contact time means less time for liquid to linger and transfer heat, which has direct value for anti-icing, friction reduction and corrosion protection in industrial settings.

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# The droplet-based electricity generator (DEG): why can one drop light 100 LEDs?

# What is the problem?

Before Wang's team's droplet generator work, "triboelectric nanogenerators" (TENGs) harvesting kinetic energy from rain or waves already had a substantial literature – but they suffered a critical bottleneck. Because conventional designs rely on interfacial effects to generate and release charge, charge density stayed stubbornly low, with peak power density below 1 W/m²^※ – far short of what practical use requires.

# How does the "field-effect transistor" structure break through?

On 5 February 2020^※, Wang's team, in collaboration with Professor Xiao Cheng Zeng of the University of Nebraska–Lincoln and Professor Zhong Lin Wang of the Beijing Institute of Nanoenergy and Nanosystems at the Chinese Academy of Sciences, published a paper in Nature (Vol. 578, No. 7795, pp. 392–396, DOI: 10.1038/s41586-020-1985-6) titled "A droplet-based electricity generator with high instantaneous power density", proposing a droplet generator whose structure mimics a field-effect transistor (FET).

The device's core construction: an indium tin oxide (ITO) substrate overlaid with a polytetrafluoroethylene (PTFE, i.e. Teflon) film, plus an aluminium electrode. PTFE is a "permanent electret" material that holds surface charge over long periods. The key innovation lies here: as water droplets keep hitting the PTFE surface, charge accumulates to saturation; when a droplet spreads wide enough, it naturally "bridges" the otherwise disconnected ITO and aluminium electrodes, closing the circuit and releasing all the stored charge in an instant – converting an effect confined to the surface interface into a bulk effect and fundamentally breaking the charge-density bottleneck.

# What are the key numbers?

The table below summarises the core performance figures of the droplet generator (all taken from CityU's official press releases and the original paper):

CityU's official press release^※ quotes Wang's research vision: "One drop of rain can light up 100 small LED bulbs." The demonstration, recorded on video in the lab, became the signature image of the study as it travelled through more than 50 media outlets worldwide.

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# Why is this "biomimetic" research, and what does it have to do with nature?

The droplet generator's technical path is tightly connected to biomimetic logic – it is not a bolt from the blue.

Wang's team has long drawn design inspiration from superhydrophobic structures in nature: the micro/nano-papillae on lotus leaves (which make water droplets roll off without wetting), the gradient conical structures on cactus spines (which direct water mist toward a point), and the lubricated rim of the pitcher plant (which makes droplets slide in one direction) – all of these are prototypes the team has studied. The droplet generator accumulates charge efficiently precisely because PTFE's superhydrophobic character ensures droplets spread and bounce in a controlled way, maximising charge accumulation per impact. In a November 2022 team review^※, Wang published a review in Nano Research Energy titled "Bio-inspired water-driven electricity generators: from fundamental mechanisms to practical applications", systematically mapping the full chain from biological prototypes to power-generating devices, covering rain-driven, evaporation-driven, and wave-driven forms among others.

What is more, "liquid–solid contact time control" on superhydrophobic surfaces and droplet electricity generation are two faces of the same body of knowledge: the former seeks to get droplets off a surface as quickly as possible (anti-icing/corrosion scenarios), while the latter needs precise control of droplet spreading speed to maximise charge release – a deep understanding of liquid–solid interfacial dynamics is the shared foundation of both.

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# The directional-transport trilogy: from Janus droplets to capillary ratchets, and how a 217-year problem was solved

In one sentence: The droplet generator's efficient charge accumulation rests on techniques the team had been mastering since 2016 for "getting liquids to choose their own direction of spreading" – a research line running through nearly a decade that yielded Wang's third Nature/Science paper of his CityU tenure in 2021.

# Step one: directional transport of high-temperature Janus droplets (2016)

The Nature Physics paper "Directional transport of high-temperature Janus droplet mediated by structural topography"^※, by Jing Li, Youmin Hou, Yahua Liu, Chonglei Hao, Minfei Li, Manoj Chaudhury, Shuhuai Yao and Zuankai Wang, found that on a patterned structured surface a high-temperature droplet can be induced into a hybrid state between "Leidenfrost levitation" and "contact boiling", giving back to a droplet – which would otherwise be directionless as it floats on its own vapour cushion – a controllable direction of motion. The work drew a dedicated Nature News & Views commentary and was the team's first demonstration that "structural topography can dictate the direction of high-temperature droplet motion".

# Step two: flipping transport on Janus pillars

The team then published in Science Advances "Tip-induced flipping of droplets on Janus pillars: From local reconfiguration to global transport"^※, showing a more refined mechanism: on an array of Janus (two-faced) micropillars, a droplet first undergoes a local "flip" triggered at the pillar tips – a microscopic reconfiguration – and these local flips accumulate into directional transport at the macroscopic scale. The work connects "a single droplet's change of posture" with "a population of droplets' choice of direction" through the same structural logic.

# Step three: the three-dimensional capillary ratchet ends a 217-year problem (2021)

On 17 September 2021^※, the team published "Three-dimensional capillary ratchet-induced liquid directional steering" in Science (Vol. 373, No. 6561, pp. 1344–1348, DOI: 10.1126/science.abg7552) – Wang's third top-journal paper of his CityU tenure, following the 2020 droplet generator in Nature. According to a South China Morning Post feature on "high-impact research"^※, the study solved a problem that had stood unresolved since 1804 – 217 years – namely the long-held assumption that the direction of a liquid's spreading on a surface is determined only by surface structure, not by the liquid's own properties.

The team designed a three-dimensional biomimetic surface inspired by the leaves of the Araucaria tree (the Araucaria Leaf-Inspired Surface, ALIS), covered with three-dimensional capillary ratchets bearing barb-like structures, with the spacing between ratchet teeth comparable to the liquid's capillary length (on the order of millimetres). Surface tension thus becomes the deciding variable in spreading direction: high-surface-tension liquids such as water are "pinned" by the ratchet tips and can only spread backwards; low-surface-tension liquids such as ethanol overcome the pinning and spread forwards; water–ethanol mixtures spread in both directions, with the ratio adjustable. Wang told the SCMP^※: "For the first time, we have achieved directional control over the transport of liquids, solving a problem that has existed since 1804." The mechanism is thought to hold promise for microfluidic design, enhanced heat transfer and smart liquid steering.

Together, these three steps form the "upstream" knowledge on which the droplet generator rests: without an understanding of how solid–liquid interfacial topography governs the direction of liquid motion, there would have been no "droplet precisely bridges electrodes" structural design in the FET-style DEG.

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# After Nature: the second paper – the 266-year Leidenfrost problem

The droplet generator was not Wang's only Nature paper during his CityU years. On 27 January 2022^※, the team published "Inhibiting the Leidenfrost effect above 1,000 °C for sustained thermal cooling" (Nature 601, 568–572, 2022), solving a problem that had stood since 1756 – 266 years.

The Leidenfrost effect is the phenomenon whereby a liquid touching an extremely hot surface instantly evaporates to form a vapour layer that "lifts" the liquid, leaving it suspended above the surface and causing heat transfer efficiency to plummet. The effect severely hampers thermal management in aircraft engines and nuclear reactors. Wang's team designed the "Structured Thermal Armor (STA)": thermally conductive pillars serve as heat bridges, an embedded superhydrophilic membrane absorbs the liquid, and U-shaped microchannels vent the vapour – a three-layer structure working in concert to raise the Leidenfrost point from roughly 550°C to 1,150°C^※ – 600°C higher than the previous record – while the survival time of water droplets on the surface shortened about 50-fold relative to comparison samples, to just 0.33 seconds.

This result and the droplet generator belong to the same research lineage – "actively regulating the liquid–solid interface through biomimetic surfaces" – the former chasing charge conversion efficiency, the latter heat transfer efficiency. Together they showcase the depth of CityU mechanical engineering in micro/nano interfacial science.

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# Practical applications: from umbrellas to wave energy

# Near-term applications

The droplet generator's range of applications is extremely wide simply because it generates electricity from any liquid–solid contact. CityU's official press release^※ lists several candidate directions:

Researchers acknowledged in interviews that the current prototype's overall energy conversion efficiency is about 5% – a far cry from large-scale hydroelectric power. But for distributed, small-scale energy harvesting (such as powering low-power sensors), the 50.1 W/m² peak instantaneous power density is already practically meaningful. Wang has extended the vision further, to "harvesting the mechanical energy of a beating heart" – pointing toward a long-term direction in wearable and implantable medical devices.

# Scaling challenges

Any device based on liquid–solid friction faces challenges of material durability and integration cost. Follow-up research published in Nature Communications in 2025^※ shows the team has begun exploring integrating DEG arrays with micro-supercapacitors to improve energy storage and output stability in large-scale deployment – the engineering step that must be crossed on the road from laboratory to application.

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# After moving to PolyU: the droplet generator keeps evolving (2023–2024)

In one sentence: In November 2022^※ Wang moved to The Hong Kong Polytechnic University as Associate Vice President (Research and Innovation); the droplet generator research did not stop with the team's relocation, and within two years set new performance records twice.

After stepping down as Chair Professor in CityU's Department of Mechanical Engineering, Wang was appointed Kwok Family Professor of Biomimetic Engineering and Chair Professor in the Department of Mechanical Engineering at PolyU, and director of its Centre for Biomimetic Science and Engineering (per PolyU's senior management team page^※). This also means that ownership of the droplet-generator research line and its subsequent output has, since late 2022, belonged to PolyU rather than CityU – but the technical lineage is a direct extension of the CityU work, which is why it is worth documenting here.

In 2024, the team published in the journal Droplet^※ "A droplet-based electricity generator incorporating Kelvin water dropper with ultrahigh instantaneous power density" (first author Yang Li; Wang among corresponding authors), introducing the self-induction discharge mechanism of the classic 19th-century electrostatics device, the "Kelvin water dropper", into the DEG structure. The instantaneous power density per droplet impact reached 105 W/m² – more than double the 50.1 W/m² reported in the 2020 Nature paper. In March of the same year, the team published "Drinking-Bird-Enabled Triboelectric Hydrovoltaic Generator"^※ in Device, a Cell Press journal (first author Hao Wu), combining the evaporation-driven rocking motion of the children's toy "drinking bird" with triboelectric nanogeneration to produce a sustained voltage above 100 V under natural evaporation. This path no longer depends on the transient pulse of a droplet impact; instead it stretches the generation cycle from "a hundred milliseconds per drop" to "hours of continuous evaporation", pointing to an alternative avenue for environmental energy harvesting.

The table below summarises the three generations of droplet/water-kinetic power generation technology (bases per each paper):

A caution: these three generations come from different papers with different test conditions (impact-type instantaneous pulses vs. sustained evaporation-driven), so they should not be converted or compared directly. But together they make one point clear: droplet/water-kinetic power generation was not a one-off "CityU-era" achievement for Wang Zuankai, but a long-term research direction he has continued to invest in since leaving CityU.

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# Major academic honours of Zuankai Wang (during his CityU tenure)

The table below summarises Wang's principal honours during his time at City University of Hong Kong (source: lab official page^※ and CityU official announcement^※):

The BOCHK Science and Technology Innovation Prize citation explicitly notes that Wang "solved three century-old scientific problems in liquid–solid interactions and the Leidenfrost effect" (per the STIP laureate page^※) – covering exactly the three research lines described in this article: pancake bouncing, droplet electricity generation and the Leidenfrost effect. The award came in 2022, just before his move to PolyU, and recognised research accumulated during his CityU years. For a broader comparative list of CityU named chairs and academician honours, see named-centres-and-honours.md; for related patents and entrepreneurial translation, see output-and-startups.md.

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# Research lineage in summary: biomimetics → superhydrophobicity → energy harvesting

Stringing these results together, a clear path of knowledge accumulation emerges:

1. Superhydrophobic micro/nano structures (early 2010s): drawing on biological structures such as the lotus leaf, studying the spreading, bouncing and directional control of droplets on superhydrophobic surfaces.
2. Pancake bouncing and the Guinness record (2014/2018): discovering that droplets can leave a surface in "pancake" form with a minimal contact time (~3.4 ms), overturning the classical lower-limit theory of contact time, and earning official international certification.
3. Droplet directional-transport trilogy (2016/2021): from topographical steering of high-temperature Janus droplets (Nature Physics 2016), to flipping transport on Janus pillars (Science Advances), to the three-dimensional capillary ratchet solving the 217-year liquid-steering problem (Science 2021) – achieving spontaneous directional choice of spreading without external force. These three papers form one of the main bodies of his top-journal output in his late CityU years.
4. The droplet generator (2020): converting knowledge of droplet dynamics on superhydrophobic surfaces into energy harvesting, breaking TENG's power-density limit with an FET-style structure, published as a Nature cover paper.
5. Leidenfrost breakthrough (2022): the Structured Thermal Armor pushing liquid–solid interface engineering into ultra-high-temperature scenarios, another Nature paper.
6. Synthesis and applications (2022 onward): the Nano Research Energy review unifies the whole research line, and the team continues exploring DEG scaling and medical applications.
7. PolyU-era iteration (2023–2024): after moving to PolyU, the team pushed instantaneous power density to 105 W/m² (K-DEG) via the Kelvin water dropper mechanism, and opened a continuous-generation path (DB-THG) with the "drinking bird" evaporation-driven mechanism – proof that the droplet/water-kinetic power generation line has kept its growth curve since he left CityU.

Note on sources and figures: the instantaneous power density of "50.1 W/m²" and "thousands of times higher than previous devices" are peak values under laboratory conditions as reported in the original paper (Nature 578:392–396, 2020). In practice, collection efficiency is affected by droplet frequency, surface area, integration method and other factors; usable power output remains at the R&D stage (efficiency ~5%). The 2024 follow-up K-DEG (105 W/m²) and DB-THG (sustained voltage over 100 V) come from different papers with different test conditions and cannot be directly added to or compared against the 2020 figures. When citing specific numbers, please return to the original papers to verify test conditions and the publishing institution (CityU or PolyU).

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