Scientific wonders behind the mesmerising beauty of nature

While visiting an amusement park, a student of Prof. WANG Zuankai, Associate Vice President (Research and Innovation) and Chair Professor of Nature-Inspired Engineering, stumbled upon a fallen Araucaria tree leaf. On close examination, the pattern of the leaf surface revealed both the artistry and science of nature. Deeply inspired by the leaf structure, Prof. Wang and his students embarked on a voyage of scientific discovery. They studied the ways in which specific curvatures and ratchets of the Araucaria leaves direct fluid flows on the leaf surface. In doing so, they made the world’s first discovery about how the flow of liquids deposited on the same surface can be steered—a scientific question that had remained unresolved since it was first posed in 1804. The team’s study, “Three-dimensional capillary ratchet-induced liquid directional steering”, was published in the renowned Science journal. The team also employed 3D printing to produce a surface that mimics the leaf structure, for use in cooling of electronic devices.

Fascinated by the beauty of Mother Nature, Prof. Wang has been answering classic scientific questions about the principles underlying natural phenomena which have perplexed researchers for centuries. Whether the question involves the shortest contact time between a solid and liquid, the steering of directional liquid flow, fundamentally inhibiting the Leidenfrost effect, or another interesting puzzle, Prof. Wang’s scientific inquiry is not simply a matter of intellectual curiosity. His research on surface and interface science and engineering is highly practical, bringing new solutions to thermal cooling and energy harvesting.

In this issue, PAIR looks at ground-breaking studies by Prof. Wang that have answered important scientific questions. These breakthroughs have stemmed from his interest in a very fundamental element of life: water.

Leidenfrost effect: Droplets dancing on heated surfaces

Many of us have encountered the Leidenfrost effect in everyday living. When water is sprinkled onto a hot frying pan, the water droplets glide like beads across the heated surface before evaporating. This tells us that that the pan is hot enough for cooking. The phenomenon was first observed by German physicist Johann Gottlob Leidenfrost in 1759.

The Leidenfrost effect, also known as film boiling, occurs when a liquid comes in close contact with a solid which is significantly hotter than the liquid’s boiling point. A vapour film is formed under the liquid droplet, enabling it to move on the surface. This physical phenomenon, despite its fascinating appeal, reduces the transfer of heat at high temperatures, hence making the liquid cooling of a hot surface ineffective. Although the Leidenfrost effect has been known for more than two centuries, scientists have struggled to counteract it. Prof. Wang’s innovation, structured thermal armour (STA), brings new hope to the perplexed science community. The study “Inhibiting the Leidenfrost effect above 1,000°C for sustained thermal cooling” was published in the journal Nature.

After four years of effort, Prof. Wang and his teammates developed a new strategy to completely break the Leidenfrost effect, even at temperatures over 1,200 °C. STA is a material with three components, thermally conductive steel pillars, a thermally insulating membrane, and underground channels. The pillars act as thermal bridges for transferring heat, and the membrane spreads the liquid while absorbing and evaporating it. The liquid is then removed by the underground channels. STA is the fruit of an interdisciplinary pursuit, combining expertise in surface science, hydro- and aero-dynamics, thermal cooling, materials science, physics, energy and engineering.

The fabrication of flexible STA allows it to be applied and attached easily to other materials, enabling liquid thermal cooling with ultra-high efficiency. The innovation is profoundly impacting industries such as aerospace, energy and electronics. Cooling under extreme conditions in aircraft, turbines, nuclear power plants and computer chips is crucial for performance and safety.

Prof. Wang was named one of the ten winners of the 2023 Falling Walls Science Breakthroughs of the Year (Engineering & Technology), for his scientific work that “breaks the wall to the Leidenfrost effect”.

Beetle-inspired cooling ceramic tiles that inhibit the Leidenfrost effect

The STA marks a triumph over the Leidenfrost effect, but this success has not quenched Prof. Wang’s thirst for innovation. Aiming to solve the challenge presented by the Leidenfrost effect, his team, including Prof. Christopher CHAO, PolyU Vice President (Research and Innovation), and researchers from the City University of Hong Kong, has invented a cooling ceramic that achieves highly efficient light scattering and a near-perfect solar reflectivity of 99.6%. Their transformative research, “Hierarchically structured passive radiative cooling ceramic with high solar reflectivity”, was published in the journal Science.

The material marks another natured-inspired invention by Prof. Wang. The Cyphochilus beetle, a genus native to Southeast Asia, is one of the whitest insects known in the world. The stark whiteness of these beetles helps them survive in the wild, acting as camouflage against fungi. To engineers, this “bio-whiteness” offers a great advantage in cooling, just as white paints on housing surfaces can reduce heat.

To achieve the new cooling ceramic material, Prof. Wang’s team carefully studied the beetle specimen at very fine levels using electron microscopes, to uncover the principles underlying the species’ excellent ability to scatter light, which contributes to their whiteness. The team discovered that the beetles have tiny teardrop-shaped scales that are arranged in such a way as to produce a highly porous exoskeleton filled with air bubbles. The combination of scales and air bubbles explains the beetles’ “near-perfect” reflectivity. This discovery inspired the team’s intricate design of a ceramic material that contains pores throughout: a “hierarchical porous structure”. When it was tested on house roofs, the material exhibited many superior features that facilitate effective thermal management and energy efficiency in various sectors, particularly the construction industry.

First, the game-changing innovation reflects light very well. Second, because the material is created from polymers and alumina, it has outstanding resistance to UV degradation and high temperatures exceeding 1,000 °C. Third, it quickly absorbs water droplets while inhibiting the Leidenfrost effect; thus, the ceramic draws in and evaporates water effectively to provide rapid cooling. The material is also eco-friendly—it can be ground into power and remade ten times without degradation of its performance.

Shattering the limit of time: Achieving the shortest contact time between solid and liquid

As the saying goes, simplicity is the ultimate sophistication. To Prof. Wang, his scientific endeavours are essentially about unravelling the laws of nature by focusing on a seemingly “simple” thing—water.

According to a Chinese proverb, “the lotus grows in muddy water, but emerges untainted by the mud” (出淤泥而不染). While this saying is used to describe people who maintain a good heart despite a very bad environment, it is a scientific fact that lotus leaves possess the marvellous ability to repel dirt.

The Lotus effect describes the unique superhydrophobic property of lotus leaves. This property gives aquatic plants their water-repellent and self-cleaning abilities. Normally, when a drop of water strikes a surface, part of it splashes away. In the case of a lotus leaf, however, the water droplet spreads out, recoils, and then bounces upward. Until recently, the contact time—from the start of droplet spread to the end of droplet recoil—had been limited to a certain minimum value.

Further reducing the contact time would be like breaking Usain Bolt’s 100-meter sprint record of 9.58 seconds. This question was hovering in Prof. Wang’s mind. Over a two-year period, Prof. Wang and his team developed a kind of superhydrophobic material that allows water droplets to spread on impact and then leave the surface in a flattened, pancake shape without retracting, hence shortening the contact time between water droplets and solid surfaces by about 80%. This phenomenon, which the team called “pancake bouncing”, was published in a Nature Physics journal paper titled “Pancake bouncing on superhydrophobic surfaces”. The work was also recognised in the Guinness World Records.

A water droplet that powers light bulbs

Water is a ubiquitous resource which many of us take for granted. To Prof. Wang, however, the substance is a fountain of scientific inspiration and creativity. Despite its simple H₂O molecular structure, water has complex physical properties. These have driven Prof. Wang’s team to unveil the flows, motions, speeds, forms, and shapes of water—and also its secret power.

One droplet hitting a surface can generate enough electricity to power 100 LED light bulbs, Prof. Wang’s team found. In the paper “A droplet-based electricity generator with high instantaneous power density” published in Nature journal, the team described their breakthrough in developing a kind of droplet-based electricity generator that converts the kinetic energy in water droplets into electrical energy.

The scientists created a surface made from an inner layer of aluminium electrode coated with indium tin oxide (ITO) and an outer layer of polytetrafluoroethylene (PTFE), a type of electret material that can store charges on its surface and effectively release them. The two components are separated. As a result, as a water droplet strikes the PTFE surface, the charges induced from the droplet bridge the originally disconnected components, forming a closed-loop electrical system.

What is even more stunning, as demonstrated by the experiments, is that a 100-microlitre drop of water released from a height of 15 cm can generate as much as 140 V of electricity, a voltage that is sufficient to light up 100 small LED bulbs. The invention opens a new avenue to water-based energy harvesting, bringing sustainability solutions to the energy crisis.

Behind the scientific venture into microscopic marvels

Over the years, Prof. Wang’s curiosity and his dedication to understanding the physical world, answering century-old scientific questions and bringing nature-inspired solutions to critical real-world challenges have attracted wide recognition, in addition to his publication success in reputable journals. This May, he was awarded the 2024 Nukiyama Memorial Award from The Heat Transfer Society of Japan. This prestigious honour is bestowed on one scientist every two years in the field of thermal science and engineering, in recognition of outstanding scientific and engineering contributions and achievements that yield major insights into boiling phenomena and technological advances. 

Prof. Wang has received numerous other prestigious awards and fellowships: Croucher Senior Research Fellow (2023), RGC Senior Research Fellow (2022), Green Tech Award (2021), Xplorer Prize (2020), Hall of Fame (Advanced Engineering Materials, 2019), and so on. His innovations have also won a Gold Medal, and a Gold Medal with Congratulations of Jury, at the International Exhibition of Inventions of Geneva.

Behind the scientific glory are Prof. Wang’s deep love for science and nature, and his resilience in overcoming the many setbacks along the scientific journey, just as a water droplet “bounces up” after striking a lotus leaf. The scientist was fired twice during his years of graduate school. At his lowest point, an unexpected observation of the spreading dynamic of a water droplet gave him the “Eureka!” moment, rekindling his creativity and interest in research and propelling him to “see something big from the small”. Many of Prof. Wang’s revolutionary outputs originated from a 9.6-square-meter laboratory that is just big enough for a desk. The entire experience has taught him that top-notch research is not simply about making scientific discoveries with cutting-edge equipment; it is also the generation of new knowledge with minimal resources. In other words, it is to see, think and wonder.