Bio-inspired superwettable materials: an interview with Lei Jiang

Scientists have been aware of the phenomenon of superwettability for more than two centuries. In 1805, British scientist Thomas Young introduced the concept of the contact angle to evaluate the wettability of liquid on a solid material surface. Superwettable materials have only macroscopic materials, which provided a framework for the design of superwettable materials. In this recent NSR interview, Jiang discussed the theory and applications of this field over the past two decades, and reflected upon innovative scientific research in general.

INTERVIEW but there were Chinese scientists who did not approve of my research ideas. They deemed that the existing superhydrophobic materials were good enough and that we did not need to design new materials based on the binary cooperative complementary principle. NSR: Your team has investigated diverse kinds of natural superwettable materials. Do these natural materials share similar microstructures and superwettability mechanisms? Jiang: Each of the biological materials that I have worked on has the basic property of superwettability, as well as its own unique properties, which means that they are usually multifunctional materials. For example, mosquito compound eyes are superhydrophobic, and they also possess an antireflection property; the reverse side of a poplar leaf is also superhydrophobic, while at the same time it is totally reflecting; spider silk is superhydrophilic and can be applied to droplet collecting; and fish scales are superoleophobic underwater, which can prevent biological adhesion and reduce drag. All of these diverse properties can be explained by the microstructures and chemical compositions of the materials. NSR: What are the new theoretical breakthroughs in this field? Jiang: Firstly, the cooperative effect of micro-/nanostructures and surface energy are the key to superwettability. Secondly, the arrangement and orientation of micro-and nanostructures may control the wetting state and motion of the liquid. Thirdly, micro-/nanoscale roughness can enhance the wettability of materials, and the enhancements on hydrophilicity and hydrophobicity are equivalent. Fourthly, the threshold contact angle for roughness enhancement is determined by surface gradients of the micro-/nanostructures. Young's equation indicates that the intrinsic wetting threshold (materials with intrinsic contact angles below this threshold can be structurally modified to be superhydrophilic, and materials with intrinsic contact angles above this threshold can only be modified to be superhydrophobic) is at 90 • . However, we found in our experiments that 90 • is not the right answer. Materials with intrinsic contact angles above 65 • and below 90 • can be modified to be superhydrophobic materials. The first three points are new theories that we have added to the textbook, and by revealing the fourth point, we are going to rewrite the textbook. NSR: Is the theoretical understanding of superwettability complete? Jiang: No, not yet. We still need to understand the mechanism of superwettability at the molecular level and probably make some extensions to Young's equation. To achieve these goals, we can use Raman spectroscopy and second-or third-order nonlinear optical spectroscopy to detect the interaction of hydrophobic and hydrophilic bonds on the surfaces of charged metal materials. Atomic force microscopes can also be applied to confirm whether the interaction between the liquid and the surface is attractive or repulsive.

THE BIG MARKETS FOR SUPERWETTABLE MATERIALS
NSR: What are the applications of superwettable materials?
Each of the biological materials that I have worked on has the basic property of superwettability, as well as its own unique properties, which means that they are usually multifunctional materials.
-Lei Jiang Jiang: There are lots of applications, and there is a big market behind our work. Millions of self-cleaning neckties made of superhydrophobic material have been sold. Millions of square meters of self-cleaning superhydrophilic glass have been installed in buildings. Several hundred units of oil-water separation equipment have also been sold. A direct inkjet computer-toplate printing technique has been developed that applies superwettable materials. In addition, we have looked into the mechanism of electrical power generation by the electric eel, which uses sodium/potassium channels on its cell membranes, and developed high-performance energy-conversion devices, which can be used in solar cells. After investigating how traditional Chinese paint brushes could allow low-viscosity ink to be finely manipulated, we have developed an instrument that is able to paint 2D materials finely and easily, which could be used in materials laboratories. NSR: Could you explain how you progress from the observation of nature to developing a new material? Jiang: Actually, this process may be divided into the three steps proposed by Chairman Mao: 'Discovery', 'Invention' and 'Creation'. The first step is 'discovery': we study the phenomenon of nature, find out the relationship between microstructures and material properties, and learn the hidden mechanisms. Then we continue to 'invention': we use physical and chemical strategies to reproduce the structures and properties in the lab. After that, we further improve the structures and endow the materials with new properties. That is 'creation'. Then, we can loop back to the first step and look for new materials in nature. That is what we have done in the field of nature-inspired superwettable materials. Let me give you an example. How can cacti live in the desert? We took this question from nature and began to look for the answers. We found that the cactus spines could condense water droplets from the air and then transport the water to the cactus body. After this 'discovery', we immediately set out to reproduce the nanostructure of cactus spines in the lab. That is 'invention'. Then, we started to ask more questions. If this structure is able to condense water droplets from air, can we use it to accumulate oil droplets from water? This is an important question, because if we can do it, we would be able to separate small oil droplets dispersed in domestic wastewater. We could also accumulate oil from oil wells that have worked for many years and can now only pump out liquids containing little oil. We constructed copper cones mimicking the structure of cactus spines and made it possible to separate oil from water. That is indeed 'creation'. This creation has big market prospects, because it can separate surfactants from wastewater from household baths and washing machines. Large amounts of surfactants are released into natural water systems every day. These surfactants are absorbed by microorganisms and transferred and accumulated through the food chain, greatly interfering with human health. NSR: To transfer a technology from the laboratory to the market, scientists have to apply for patents and may be directly involved with commercial companies. Do you have some experience of this? Jiang: Patents are undoubtedly necessary. But we have to be aware of the fact that most of the patents applied for today are short-lived and only a small percentage transfer into industrial products. One of the reasons there are so many is that in the graduation standards of many universities, patents applied for by graduate students are counted as equivalent to academic papers.
I do not suggest that scientists run companies by themselves, that it is very difficult. In scientific research, one discovery leads to one article. However, technology transfer is much more difficult. You only have to meet with one unsolvable problem, and no product emerges. It is almost impossible for a scientist to do basic research and company operations at the same time.
However, it is feasible to sell your technology to other companies. The fee for my first technology transfer deal was 60 million RMB, and the whole negotiation took only 30 minutes. I had a cup of cool water, a cup of hot water, a piece of cloth and a piece of glass in front of me. I poured the cool water onto the cloth; all the water ran off immediately and the cloth remained dry. When I held the glass above the hot water, the untreated half became fogged immediately while the superhydrophilic half remained transparent. That was convincing enough for the negotiation. At that time, they wanted to give us three researchers some personal reward. I refused that, and instead took 12 million RMB research funding into my laboratory account at the Institute of Chemistry, at the Chinese Academy of Sciences. With this money, I purchased instruments including a field emission scanning electron microscope, a Raman microscopy, a Raman spectrometer, an infrared spectrometer, an ultraviolet spectrometer and so on. This greatly improved the efficiency of my lab. At that time, students in other labs of the institute had to wait 14 days for the public instruments of the institute platform. But the waiting time in my lab was less than 1 hour, and the students could finish all the experiments in 1 day.

ADVANCES IN NANOMATERIAL RESEARCH
NSR: During the last two decades, what important achievements have Chinese scientists made in the field of nanomaterials? Jiang: In this field, we are developing quickly. Some of the fundamental achievements include aggregation-induced emission materials by Benzhong Tang, graphdiyne-based materials by Yuliang Li, single-atom catalysis materials by Tao Zhang and Xinhe Bao, and work on nanostructures by Dongyuan Zhao and Yadong Li. We have exceeded the United States in the number of research articles. However, we have to be aware that fundamental and innovative works accomplished by Chinese scientists are still limited. NSR: Is it a good time for young scientists to enter this field? Jiang: It is definitely a good time, but it is also a bad time. The field of nanomaterials is very hot at the moment. Large amounts of research funding are entering this field, and many expensive and powerful experimental instruments are available. As a result, many researchers are easily overwhelmed by the instrumentation and forget to think about the real scientific problems. The more resources we put into the instruments, the more difficult it is to make truly innovative and important discoveries. So, the quick development of this field is a double-edged sword. True science is not based on money, but on minds and ideas. NSR: So, what is your definition of true science? How can young scientists find truly promising research areas? Jiang: There are two kinds of true science. One is to create new knowledge and the other is to create new applications. As for choosing a research area, I propose that the most important thing is to learn from nature. Good directions are often not chosen by the researchers, but by nature itself. For instance, cacti are undoubtedly able to live in the desert. So, unless given up easily by the researchers, a study on the survival mechanisms of cacti will definitely have a good result. We need to find the true research subjects, not create fake ones or just follow the steps of somebody else.
Another key point is to select according to your ability. Some topics, for instance to decipher the mechanism of information transfer in the human brain, are good science topics but cannot be easily accomplished by several scientists in a short time. For these topics, one needs to break a huge scientific question into smaller ones. You can begin with small research subjects such as working on the information transfer mechanism of a single neuron, the interaction mechanism between neuron and the environment, or simply the molecular mechanism of an ion channel on the neuron plasma membrane. These small topics can be solved, and many scientists are indeed working on them.

SCIENTIFIC COOPERATION AND GRADUATE EDUCATION
NSR: You have cooperated with scientists with diverse disciplinary backgrounds. Are there any difficulties in communicating with them? Jiang: No. Disciplinary boundaries are not a big problem for me. I majored in physics in my undergraduate years, and then transferred to physical chemistry and became familiar with chemical knowledge. I also self-learned biology. I have a wide range of interests. NSR: You spent seven years in Japan, accepting scientific training and performing scientific research. How were you influenced by these experiences?
Many researchers are easily overwhelmed by the instrumentation and forget to think about the real scientific problems.
-Lei Jiang INTERVIEW Jiang: My Japanese mentor Akira Fujishima and my Chinese mentor Tiejin Li had completely different characters. Li was a scientist with integrity and gentleness, and hardly ever joked. He had a wide range of research interests and was able to research many subjects. However, Fujishima was completely different. He chose one direction and dug into it for his whole career, from the basic science to its applications. In this regard, I am much like Fujishima. I concentrate on superwettable materials and never considered changing direction. In fact, the spirit of concentration is one of the significant causes of the rapid rise of modern Japanese science. NSR: How do you train your own graduate students? Jiang: I have many excellent students. Many of them are already funded by the National Science Fund for Distinguished Young Scholars, the Excellent Young Scientists Fund, or the Young Overseas High-Level Talents Introduction Plan. At the beginning of their training, I would spend time in emotional exchanges with them, because it is only when students became emotionally close to you that you can guide them effectively. The ancient Chinese military strategist Sun Tzu's saying, 'Never punish estranged subordinates', brilliantly explains this principle. In scientific training, I give the students enough freedom to search for ways to fix scientific problems by themselves. At the same time, I am strict with them and always pushing them to move forward. I believe it to be the only way to rapidly cultivate scientific thinking.