From 'Firefly Trees' to 'Coating Technology'

To everyone reading this page, if you are not familiar with the phenomenon of a 'firefly tree,' please first search the internet for 'firefly tree' and watch a video of it (for example, https://www.youtube.com/watch?v=Ls6jJnJ2CuQ ). You will see a large number of fireflies gathered on a single large tree, repeatedly flashing in unison with a synchronized period, much like the lights on a Christmas tree, or creating flowing bands of light. This is a phenomenon that occurs because each individual firefly entrains its flashing period with the others. So, why does each firefly synchronize its periodic oscillation with that of other fireflies? For those who answered, 'Because they are living organisms, they adapt to the behavior of other organisms around them,' I have two questions for you.

1) Is it possible to generate a synchronization phenomenon where communication occurs through light, even in an artificial chemical system?

2) Before that, does an artificial chemical system that repeatedly emits light in a time-periodic manner even exist in the first place?

The answer to both is YES.

The H2O2-KSCN-CuSO4-NaOH system is a complex chemical reaction network in which numerous chemical reactions proceed while being coupled with each other. Overall, the oxidation of the substrate KSCN by the oxidizing agent H2O2 proceeds, but during this process, time periods occur where the concentration of the Cu(I) hydrogen peroxide complex periodically increases. This is precisely the same kind of phenomenon as the various periodic phenomena, or biorhythms, that occur in living organisms as organic substrates are oxidized using oxygen as an oxidizing agent. Then, when luminol, a substance that emits light by reacting with the hydrogen peroxide complex, is added to this H2O2-KSCN-CuSO4-NaOH system, time-periodic light emission similar to that of fireflies is observed (Figure 1).  

Now, for a time-periodic phenomenon to occur in this chemical reaction network, a certain condition is necessary. That is, either the oxidizing agent H2O2 and the substrate KSCN must be present in excess, or they must be supplied steadily. Yes, it is the same as how a living organism cannot maintain its biorhythms (in short, it 'dies') unless it constantly receives a supply of oxygen and organic substrates. In 1977, the Nobel Prize in Chemistry was awarded to Ilya Prigogine. This was in recognition of his proposal of the concept of 'dissipative structures and self-organization in non-equilibrium systems.' A chemical system that is open to the outside, like a living organism, is called a non-equilibrium open system. In this non-equilibrium open system, fluctuations grow, and various temporal, spatial, and spatiotemporal rhythms emerge without violating the second law of thermodynamics. Such phenomena are called self-organization. In other words, the various rhythms that spontaneously occur in living organisms are not a 'mystery of life' but rather 'self-organization as the output of a complex chemical reaction network under non-equilibrium open conditions.' And a chemical reaction network that generates lively rhythms responds to external stimuli, just like a living organism, even if it is an artificial chemical system. For example, when the aforementioned H2O2-KSCN-CuSO4-NaOH system with added luminol is alternately irradiated with red and white light, it begins to generate a rhythm that is entrained to that period (Figure 2).  

This year, the movie 'The Imitation Game' was released. Enigma was the code used by the German military during World War II, and this film depicts the turbulent 42-year life of Alan Turing, who succeeded in breaking that code (I will refrain from writing the main reasons for his turbulent life here). Alan Turing, who established the foundation of computer algorithms known as the Turing machine and whose name is immortalized in the Turing Award, considered the world's most prestigious award in the field of computer science, wrote one chemical paper in his lifetime titled 'The Chemical Basis of Morphogenesis.' In chemical experiments based on this theory, it is possible to generate spatial periodic concentration patterns called Turing patterns (Figure 3). This theory of Turing's was published before the aforementioned concept of 'dissipative structures and self-organization in non-equilibrium systems' was proposed. However, looking back, it clearly serves as a model for a complex chemical reaction network under non-equilibrium open conditions.

Now, for those who have read this far, you might be wondering, 'Your affiliation is the Department of Applied Chemistry, but what part of this is applied?' It is true that the research results described so far have no direct connection to industrial technology. However, the various considerations in the process of advancing this research have been extremely useful in the development of industrial technologies, especially those involving surface science. This is because, at an interface placed under non-equilibrium open conditions, its shape fluctuations grow, and various patterns emerge.

When a liquid is continuously supplied to the center of a rapidly rotating substrate, the liquid rarely spreads smoothly; instead, various flow patterns spontaneously emerge (Figure 4). This is because it is a non-equilibrium open system where mechanical energy for rotation and the substance of the liquid are continuously supplied. By changing the rotation speed, the liquid inflow rate, and the shape of the substrate, these flow patterns can form various designs, such as single spirals, double spirals, and undulating spirals.

So, what kind of industrial technology is this phenomenon related to? The bodies of automobiles are painted using a method called rotary atomization coating (Figure 5). At the tip of a painting robot is a disc that rotates at high speed, and paint is supplied to its center through the robot arm. The paint then spreads out on the disc to form a liquid film, is further ejected outward as liquid filaments, and these transition into droplets that adhere to the car body. However, if the size of the mist-like droplets generated here is not uniform, it leads to a decrease in coating transfer efficiency and unevenness in the finished paint job. Believing that the distribution in droplet size is caused by the emergence of flow patterns in the liquid film spreading on the rotating substrate, we researched a disc structure that makes it difficult for flow patterns to occur. This research led to the registration of Patent No. 5830612, 'Bell Cup for Rotary Atomization Electrostatic Coating Apparatus.' This patent was also filed as a PCT application and has been nationalized in the United States, Europe (UK, Germany, France), China, Brazil, Russia, India, Mexico, Thailand, and Indonesia. Furthermore, painting robots equipped with this bell cup have been introduced in factories in India, China, the United States, Mexico, and Brazil, and last year, 700,000 automobiles were produced. We have been informed that CO2 emissions have been reduced by 20% compared to the previous painting method. (For the record, I must state that I did not engage in this research out of a blind belief in the global warming effect of CO2. On the contrary, the trend of explaining everything—whether the polar ice increases or decreases, whether winters are mild or severe—as a result of the global warming effect of CO2 gives me, as a scientist, a strong sense of unease, even a sense of crisis that threatens the political neutrality and independence from authority of academia. Given the fact that after a period of about 200 to 300 million years since the Mesozoic Era when there were no ice sheets on Earth at all, glacial and interglacial periods have repeated in approximately 100,000-year cycles since the beginning of the Quaternary, and that about 5,000 years ago during the Jomon transgression, the sea level was around 10 meters above the current level, I can by no means deny the possibility that the Earth is currently slowly heading toward the next ice age.)  

Also, while drinking wine with a researcher from a cosmetics company, our conversation about joint research became animated as we watched the 'tears of wine' (Figure 6) forming on the liquid surface of the wine that had climbed the walls of the glass above the meniscus. This eventually led to the development of a highly water-resistant sunscreen advertised with the 'Nano Barrier DS formulation.' (DS stands for Dissipative Structure.) This product exhibited high water repellency because a periodic concave-convex structure, like that of a lotus leaf, spontaneously formed on the surface after the sunscreen was applied, under the non-equilibrium open conditions of the applied surface (Figure 6). The series launched in 2003 saw a 32% increase in sales compared to the previous year, and the series launched in 2004 saw a 68% increase.

In my personal view, I do not believe that Applied Chemistry is a field where one simply uses the knowledge of the already established discipline of chemistry to develop various technologies. Since all things in the world are chemical substances, opportunities for new chemical research are scattered across various scenes of natural science phenomena. I believe that Applied Chemistry is about stepping into realms such as life phenomena and industrial production from a chemical standpoint, and not only developing new technologies but also establishing new academic systems related to chemistry.