Zuankai Wang and the Droplet Electricity Generator — Bioinspired Surfaces and a *Nature* Cover Study from CityU Mechanical Engineering

City University of Hong Kong (CityUHK) Integrated Information Database · 04 Research Module · Materials Science Deep Dive Series For an overview and other landmark breakthroughs, see materials-and-engineering-research.md.

In a nutshell: Professor Zuankai Wang (王鑽開), Chair Professor in CityU’s Department of Mechanical Engineering, built his work on a foundation of bioinspired superhydrophobic surfaces. In February 2020^※, he published a droplet-based electricity generator (DEG) in Nature: a single 100-microlitre water drop falling from a height of 15 cm generated over 140 volts and an instantaneous power density of 50.1 W/m²^※ — thousands of times higher than previous comparable devices — enough to light 100 small LED bulbs simultaneously. Earlier, his team’s “pancake bouncing” research on superhydrophobic surfaces was certified by Guinness World Records^※ for the shortest liquid–solid contact time (February 2018).

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# Who is Zuankai Wang and what does he research at CityU?

Zuankai Wang is currently a Chair Professor in the Department of Mechanical Engineering and Co-Director of the Centre for Nature-Inspired Engineering at the City University of Hong Kong. According to his CityU faculty profile^※, he earned his bachelor’s degree in Mechanical Engineering from Jilin University and his master’s from the Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences. He then moved to the United States, completing a PhD in Mechanical Engineering at Rensselaer Polytechnic Institute (2008) and postdoctoral work in Biomedical Engineering at Columbia University (2009). In November 2022, he took up the post of Vice President (Research and Innovation) and Chair Professor at The Hong Kong Polytechnic University, but the body of work he produced during his CityU years remains one of the most internationally impactful research lines in that department’s history.

His core research questions can be boiled down to three challenges: What is the physical limit of liquid–solid contact time? Can a liquid autonomously choose its spreading direction? And how can the Leidenfrost effect be completely suppressed? These three deceptively fundamental physics questions correspond to starkly different application landscapes — anti-icing coatings, microfluidic chips, and cooling for aerospace/nuclear engineering — and form the intellectual logic behind his drive to push “bioinspired surface science” into practical engineering. According to his lab’s official site^※, the team published over 200 papers during his CityU period, including more than 18 in the Nature and Science family of journals.

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# “Pancake bouncing” and a Guinness record: the shortest liquid–solid contact time

To understand Wang’s droplet electricity breakthrough, it helps to grasp the foundational work that earned him his academic reputation at CityU — the dynamics of droplet bouncing on superhydrophobic surfaces.

In 2014^※, the team published “Pancake bouncing on superhydrophobic surfaces” in Nature Physics. The study found that on a superhydrophobic surface constructed from arrays of sub-millimetre cylindrical pillars decorated with nano-textures, an impacting droplet does not follow the classic “spread — retract — rebound” pathway. Instead, it lifts off the surface as a flattened “pancake” during the spreading phase itself — achieving a contact time of roughly 3.4 milliseconds, roughly three-quarters shorter than conventional designs. This finding overturned the classical understanding that there was a theoretical lower bound on droplet bouncing contact time.

In February 2018^※, the team’s invention of the “most water-repellent surface” (shortest liquid–solid contact time) was formally inducted into the Guinness World Records. It remains one of the very few Guinness certifications achieved by CityU’s mechanical engineering research, and it propelled the concept of “superhydrophobic surfaces” from academic circles into the broader public eye. The physical significance of the work is that reducing liquid–solid contact time means less time for liquid to linger and transfer heat on a surface — with direct benefits for industrial scenarios such as anti-icing, friction reduction, and corrosion resistance.

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# The Droplet Electricity Generator (DEG): how can a single drop light 100 LEDs?

# What was the problem?

Before the Wang team’s droplet electricity work, there had been extensive research into triboelectric nanogenerators (TENGs) that harvest energy from natural water kinetic sources such as rain or waves. But a critical bottleneck persisted: because conventional designs relied on interfacial effects to generate and release charges, the charge density remained persistently low, with peak power densities below 1 W/m²^※ — far from the levels needed for practical use.

# How did the “field-effect transistor” architecture break the deadlock?

On 5 February 2020^※, the Wang team, collaborating with Professor Xiao Cheng Zeng (曾曉成) of the University of Nebraska–Lincoln and Academician Zhong Lin Wang (王中林) of the Beijing Institute of Nanoenergy and Nanosystems at the Chinese Academy of Sciences, published “A droplet-based electricity generator with high instantaneous power density” in Nature (Vol. 578, No. 7795, pp. 392–396, DOI: 10.1038/s41586-020-1985-6). The paper proposed a droplet generator modelled on the structure of a field-effect transistor (FET).

The device’s core structure consists of an indium tin oxide (ITO) substrate, overlaid with a PTFE (Teflon) film, plus an aluminium electrode. PTFE is a “permanent electret” material that can retain surface charges for extended periods. The key innovation is this: as water droplets repeatedly strike the PTFE surface, charges accumulate until saturation. When a spreading droplet becomes wide enough, it naturally “bridges” the two originally disconnected electrodes — the ITO and the aluminium — forming a closed circuit. All the stored charge is released in an instant, transforming what was previously an effect confined to the surface interface into a bulk-phase effect and fundamentally breaking through the charge-density bottleneck.

# What are the key figures?

The table below summarises the DEG’s core performance indicators (all drawn from CityU’s official press releases and the original paper):

The CityU press release^※ quoted Wang’s research vision: “A single raindrop can light 100 small LED bulbs.” This demonstration, publicly recorded in a laboratory video, became the iconic image through which the research was communicated by more than 50 media outlets worldwide.

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# Why call this “bioinspired” research, and what’s the connection to nature?

The DEG’s technical approach is tightly interwoven with bioinspired logic; it was by no means an isolated breakthrough.

Wang’s team has long drawn design inspiration from superhydrophobic structures in nature: the micro- and nano-scale papillae on a lotus leaf’s surface (which make water droplets roll off without adhering), the gradient conical needles of a cactus spine (which guide fog droplets to converge directionally), and the lubricated rim of a pitcher plant’s peristome (which makes liquid slide off in one direction) — all of these have served as biological prototypes for the team’s research. The reason the DEG can accumulate charges so efficiently is precisely that the PTFE’s superhydrophobic properties ensure water droplets spread and detach in a specific manner, maximising the charge accumulation efficiency of each impact. According to a November 2022 team review^※, Wang published a comprehensive review in Nano Research Energy titled “Bio-inspired water-driven electricity generators: from fundamental mechanisms to practical applications”, systematically mapping the entire chain from biological prototypes to power-generating devices, covering rain-driven, evaporation-driven, and wave-driven forms.

Moreover, the “liquid–solid contact time control” of superhydrophobic surfaces and droplet electricity generation are in fact two sides of the same body of knowledge: the former pursues getting droplets off the surface as fast as possible (for anti-icing/anti-corrosion scenarios), while the latter demands precise control over droplet spreading speed to maximise charge release — a deep understanding of liquid–solid interface dynamics is the shared intellectual bedrock of both.

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# After Nature: the second paper — a 266-year-old Leidenfrost problem

The droplet electricity generator was not Wang’s only full-article publication in Nature during his time at CityU. On 27 January 2022^※, the team again published in Nature, this time with “Inhibiting the Leidenfrost effect above 1,000 °C for sustained thermal cooling” (Nature 601, 568–572, 2022), solving a challenge that had persisted for 266 years since its identification in 1756.

The Leidenfrost effect refers to the phenomenon where a liquid, upon touching an extremely hot surface, instantly vaporises to form a vapour layer that “lifts” the liquid, suspending it above the hot surface and causing the heat transfer efficiency to plummet. This phenomenon severely constrains thermal management in aero-engines and nuclear reactors. The “Structured Thermal Armor (STA)” designed by Wang’s team deploys a three-layer synergy: thermally conductive pillars acting as heat bridges, an embedded superhydrophilic membrane to absorb liquid, and U-shaped microchannels to vent vapour. Together, these raised the Leidenfrost point from roughly 550°C to 1,150°C^※ — a 600°C improvement over previous records — while simultaneously reducing the survival time of water droplets on the surface by roughly a factor of 50 compared to control samples, to just 0.33 seconds.

This achievement and the DEG belong to the same research theme of “actively manipulating liquid–solid interfaces using bioinspired surfaces” — the only difference being that the former pursued charge-conversion efficiency while the latter targets heat-transfer efficiency. Together, they showcase the depth of accumulated expertise in micro- and nano-scale interface science within CityU’s mechanical engineering discipline.

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# Real-world application potential: from umbrellas to wave energy

# Near-term feasible application scenarios

The application landscape for the DEG is remarkably broad, owing to its basic principle of “solid–liquid contact generates electricity.” The CityU press release^※ listed several candidate directions:

In interviews, the researchers have candidly acknowledged that the current prototype’s overall energy conversion efficiency is roughly 5%, a long way from large-scale hydropower. But for distributed, small-scale energy harvesting — such as powering low-power-consumption sensors — a peak instantaneous power density of 50.1 W/m² is already practically meaningful. Wang has extended the long-term vision to “harvesting energy from the mechanical motion of a heartbeat” — pointing towards the extended horizon of wearable/implantable medical devices.

# Scalability challenges

Any electricity-generating device based on liquid–solid friction faces challenges of material durability and integration cost. A 2025 follow-up study published in Nature Communications^※ indicated that researchers have begun exploring the integration of DEG arrays with micro-supercapacitors to enhance energy storage and output stability during large-scale deployment — an engineering step that must be navigated on the road from lab to application.

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

The table below summarises the major honours Wang received during his appointment at City University of Hong Kong (source: lab official site^※ and CityU official announcements^※):

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# Research trajectory in summary: bioinspired → superhydrophobic → energy harvesting

Stringing the above achievements together reveals a clear path of knowledge accumulation:

1. Superhydrophobic micro/nano-structures (early 2010s): Using biological structures such as the lotus leaf as prototypes, research into the spreading, bouncing, and directional control of droplets on superhydrophobic surfaces.
2. Pancake bouncing and Guinness Record (2014/2018): Discovery that droplets can detach from a surface in a “pancake” shape with the shortest contact time (~3.4 ms), overturning the classical lower-bound theory on contact time, with official international certification.
3. Directional droplet transport (2016/2017): Topological liquid diodes and Janus droplet directional transport research, enabling a liquid to autonomously choose its spreading direction without external driving force — published in Nature Physics and Science Advances.
4. Droplet Electricity Generator (2020): Translating knowledge of droplet dynamics on superhydrophobic surfaces into energy harvesting; the FET-type architecture shattered the TENG power-density ceiling, resulting in a Nature cover paper.
5. Leidenfrost effect breakthrough (2022): Structured Thermal Armor pushed the defence of liquid–solid interface problems into ultra-high-temperature regimes, leading to another full article in Nature.
6. Synthesis and application expansion (2022–): The Nano Research Energy review unified the entire research line; the subsequent team continues to explore DEG scalability and medical applications.

Source caveat: The instantaneous power density figure of “50.1 W/m²” and “thousands of times higher than prior devices” are both peak data reported in the original paper (Nature 578:392–396, 2020) under laboratory conditions. In real-world settings, collection efficiency is influenced by droplet frequency, surface area, integration methods, and other factors, and practical power remains at the R&D stage (efficiency ~5%). When citing specific figures, please check testing conditions against the original paper.

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