What is active matter physics?
A flock of starlings circling the dusk sky like a gigantic black ghost. Or a school of sardines creating massive whirlpools in the sea to fool their enemies. Or, on land rather than at sea, a colony of army ants marching in circles to form a hill-like structure. All of these examples display magnificent collective behavior as if there was a leader somewhere in control. However, there is no leader, no choreographer. How is such neatly ordered collective behavior possible? That is the mystery that Daiki Nishiguchi is tackling. One could be forgiven for thinking that his research pertains to biology and has nothing to do with physics. Yet, “This is active matter physics, a field within non-equilibrium statistical physics,” he clarifies.
Active matter is an object made up of self-propelling elements. As such, active matter physics considers a group or a flock as a single object (matter) and tries to discover the universal laws that govern such diverse collectives. A single starling, a single sardine, or a single army ant is considered a single velocity vector, an arrow. A group of "arrows" is considered "active matter."
“The study of active matter explores the properties of matter that consists of self-propelling elements, unlike matter made up of ordinary atoms and molecules. Each element is using energy to move, be it swimming or flying, and is not being driven from the outside. We investigate the characteristics of matter made up of such non-equilibrium “molecules,” searching for new physical properties. It is exciting to find new properties that have not been observed in conventional materials,” says Nishiguchi.
Non-equilibrium statistical physics is the area of statistical physics (otherwise known as statistical mechanics) concerned with non-equilibrium systems, as the name suggests. If the water inside a cup on a table is the same as the room temperature, it is an equilibrium system (in an equilibrium state). However, pouring hot water and stirring instant coffee with a spoon in a cup creates a non-equilibrium system (in a non-equilibrium state). In short, an equilibrium system is one in which there is no inflow of energy from outside, meaning that temperature, pressure, volume, and other aggregate properties remain constant. A system with a flow of energy that never settles to a constant is in a non-equilibrium state. For example, the atmosphere, with its always-changing wind and temperature, and living cells are non-equilibrium systems. Non-equilibrium statistical physics uses the idea of statistical mechanics to explore this domain of extreme complexity. In other words, active matter is a non-equilibrium system.
Before things get complicated, let us explore what attracted Nishiguchi to active matter physics.
A boy who loved living things and experimentation
Since childhood, Nishiguchi has loved animals.
“I had all sorts of pets. We had a dog of course, but also quail, crawfish, turtles, hamsters, budgies, Java sparrows, carp, and tadpole shrimp. We also had two chickens, often crowing on the balcony. I bought them as chicks at a festival and raised them myself, but their crowing became a nuisance to the neighbors, so my father took them to a farm (laughs). I seem to be attracted to moving things. Non-equilibrium physics is peculiar in that we can actually see fascinating and gorgeous phenomena, moments of the position space captured by a camera. That is where active matter physics has a connection to living things which appealed to me, I think.”
Physics itself has also been of great interest to him since he was a boy.
“As a child, I enjoyed science festivals and science museums. My parents also read science encyclopedias for children to me before bed, so my interest in science continued throughout my childhood. I especially enjoyed physics because I could do experiments myself. When I entered high school, I formed a physics club. My friends and I would do physics experiments for fun after the classes. At that time, I found out about a national physics contest, and I had to jump in and challenge myself. Luckily, I did well and was selected to participate in the International Physics Olympiad. Many university professors were involved in the organization of these events, and that was the first time in my life that I met actual physics researchers. I was captivated by the way they talked about science with such enthusiasm. That is what made me want to become a researcher. After I enrolled in college, it also inspired me to start the UTokyo student circle CAST (CommunicAtors of Science and Technology), which held demonstrations and science shows.”
His interest in statistical physics was sparked while he was studying liberal arts as a freshman at the Komaba campus. Even so, he already had a budding interest in non-equilibrium systems in high school.
“A scientist researching granular materials came to give a lecture at our high school that caught my attention. When people think of physics, they perhaps think of superconductivity, cosmology, or elementary particles, but I realized then that there were many cutting-edge areas of physics besides the well-known ones. After entering university, I conducted experiments and presented my research on granular materials at the May Festival. You could think of granular materials as a collection of a multitude of beads. For example, if you oscillate the granular materials up and down a lattice pattern will spontaneously appear. Because of this force applied from the outside, you can observe various patterns arise. If these granular materials started moving on its own, it could be considered an active matter. This connection led me to active matter physics.”
After completing his doctoral studies, Nishiguchi worked as a researcher at the Institut Pasteur in France for about two years. The Institut Pasteur is a mecca for research on microorganisms and infectious diseases. Why did Nishiguchi, a physicist, go to a research institute for biology and medicine?
“I was doing experiments with bacteria, and I felt the need to learn more about microorganisms, which was one of the reasons I wanted to put myself in an environment such as the Institut Pasteur, where microbiological research is very active. Moreover, France is also deeply engaged in active matter physics research. So, I knew I had to go to France one time at least.”
Nishiguchi recalls the impactful two years spent at the Institut Pasteur. The experiments he was able to conduct with pathogenic bacteria and cultured human cells would have been difficult at the Department of Physics and the experience broadened his horizons significantly.
Complicated enough for one, even more so for many
It is often said that “statistical physics attempts to bridge the microscopic and macroscopic worlds.” The behavior of matter in the microscopic world of atoms and molecules can be accurately described by quantum mechanics. However, the behavior of the macroscopic world, which is a collection of the microscopic world, has an utterly different order and structure that cannot be predicted from microscopic behavior alone. Statistical physics attempts to find universal laws that connect these two worlds at different scales. It is an intricate area of research. That is why Nishiguchi says, “It is complicated enough for one individual (or atom), even more so for many.”
“Statistical physics reveals that different substances, which behave completely differently in the microscopic world, exhibit the same universal properties in the macroscopic world. Of course, I am interested in the universal laws that connect the microscopic and macroscopic worlds, but I am more interested in the universal laws underlying various macroscopic systems. That intrigues me the most.”
Nishiguchi also shows how the study of active matter, a field of statistical physics, plays an essential role in unraveling the mysteries of the world of living things that he loves.
“We can use quantum mechanics to describe the atomic or molecular scale, Newtonian mechanics for the human scale, and the theory of relativity for scales much greater than that. In short, there is an appropriate theory for each. For example, when considering the everyday world, we can establish and use a theoretical framework without unnecessarily involving quantum mechanics. In biology, for example, there are various scales, such as gene regulation, the movement of a cell, a group of cells, tissue, an organism, a group of organisms, and ecosystems. However, it is quite difficult to create a scale-specific, isolated theory. For example, when cell movement changes the shape of the tissue, the boundary conditions of how the cells are confined change, which affect the movement of the cells, creating many feedback loops. In other words, you cannot neatly separate levels of cells or tissue, because they are often connected. Active matter physics has successfully extracted and created effective theoretical descriptions for the simple movement or swarming of, for example, a group of cells.”
As already mentioned, instead of starlings or sardines, Nishiguchi uses easy-to-handle bacteria for his experiments, such as Bacillus subtilis and Escherichia coli, along with non-living particles (colloids). These are extremely small glass beads, half covered with metal (called Janus particles). When placed in a solution and a voltage is applied, the electrical charge generated on the colloid causes it to move around. That is why they are called self-propelled colloids. By altering the voltage and frequency, we can control the movements of this type of colloid. This way, we can observe under a microscope how bacteria and colloids placed in diverse environments form groups and move around. Various patterns and structures emerge, just like in his previous experiments with granual materials. Nishiguchi is struck by their beauty.
“I write papers about beautiful phenomena that emerge when I conduct experiments based on my ideas. Later, I introduce a video of the phenomena at lectures and conferences. I also like explaining the beauty of the principles of physics and mathematics that underlie the observed phenomena and the mathematical and statistical physics analyses that we do. Then, when the audience watches the video again, people are moved by a deeper sense of beauty. To be able to share that kind of wonder with others is what makes me most happy in my research.”
Building a medaka school
Of course, Nishiguchi's work is not about finding beautiful patterns. Nishiguchi's research is to solve the mystery of why self-propelled objects, be it living or non-living, form collectives. That is why they naturally develop an ordered structure and what the underlying mathematical principles are. Therefore, what Nishiguchi does is not easy to encapsulate, as his research spans several levels and dimensions. Take the bacteria E. coli, for example, which propel themselves by rotating their flagella, and tends to move towards wall-like objects. This tendency can be explained by applying the principles of fluid dynamics to the current that E. coli creates when it propels itself in the water. In this way, each individual's movement can be clarified without statistical physics.
Nishiguchi’s research has clarified the mechanisms of what sparks individuals to form a collective. However, his research does not end there.
“I am more interested in the group because I expect to discover universal laws of emerging collective behavior. However, to do that, we need to understand the movement of a single individual or element. When the movement of a single individual changes, it is crucial to understand how much it affects the universality of the macrostructure, whether universality disappears altogether, or to what extent we can manipulate the individual while maintaining macro universality. In this sense, I think that studying the individual while deepening our understanding of the collective is not only vital but complementary.”
When the collective is in focus, the research methods shift toward non-equilibrium statistical physics, or in other words, to the statistical mechanics approach. Considering a single bacterium as a velocity vector, setting the conditions, and simulating collectives on a computer based on mathematical models already proposed by the pioneers of active matter physics are also important “experiments.” In this way, Nishiguchi can create collectives at will. In the case of living bacteria, manipulating the conditions is done by varying the height of the fluid layer that confines the bacteria, obstacles, and other geometrical features.
However, instead of an orderly collective motion, what is often observed in experiments is merely many small groups moving around in a disorderly manner called bacterial or active turbulence. However, Nishiguchi discovered that when he placed a number of microscopic columns with a diameter of about 20 µm in this turbulent flow, stable eddies formed between the columns. He then set out to explain and further develop the phenomenon by combining the viewpoints of areas such as fluid mechanics and topology.
“The world is full of non-equilibrium systems. So, understanding active matter will surely lead to an understanding of developmental biology, and it will also lead to an understanding of life, including human bodies. Research in that direction is springing up, and it is the direction I am interested in going. I am doing research on cultured human cells to achieve that. By adapting the experiments I have been doing with bacteria to cultured human cells, I hope to contribute to research that will lead to understanding physiological phenomena and developmental biology. Another direction is related to my love of animals. However, from the point of view of physics, I genuinely would like to discover the physical constraints on the ecology of living things and on collective behavior, as well as the universal laws that govern such behavior.”
When asked for advice for high school and undergraduate students, Nishiguchi shares his own experiences.
“I believe that if you keep a wide range of interests, one day you will unexpectedly find what truly captures your attention. That is how I ended up researching active matter physics. If you cast a wide net, keep studying and learning various things, I am sure that will lead to something valuable.”
At home, he is the father to a one-year-old girl.
“I show my daughter my favorite bacteria videos. She watches them eagerly. I am just worried about her banging on the computer screen," Nishiguchi grins.
“My wife is a researcher, also working on non-equilibrium systems. We often talk about research at home. I show her videos of successful experiments, explaining what we did, hoping she finds them beautiful as well.”
When asked about his hobbies, he replies like this.
“I know this may sound a bit contrived, but I like aquariums and zoos and often visit them. I never get tired of staring at schools of sardines (laughs). The other day I went with my daughter and explained to her all about them (laughs). I am now even wondering if it would be possible to keep fish at home and do experiments. A medaka school, if you will. I am already planning the experiments I could do with my daughter as her elementary school research project.”
By the way, Dr. Giorgio Parisi, a professor at Sapienza University of Rome, winner of the 2021 Nobel Prize in Physics, is also working on active matter physics and has been studying flocks of starlings. A school of medaka is not to be taken lightly.
※Year of interview: 2023
Photography･Video／ Junichi Kaizuka
▼Listen to an interview where Dr. Daiki Nishiguchi explains his research and life as a researcher, exclusively on the Rigakuru podcast.