We work at the frontiers of the unknown

A specialist in soft matter and fluid mechanics, Lydéric Bocquet studies nanofluidics and its unique properties with a view to finding practical applications that support the energy transition.

He has been invited to hold the Technological Innovation Liliane Bettencourt Annual Chair for the 2022–2023 academic year.

Nanofluidics is the study of how fluids behave in channels at the nanoscale. How did you come to be interested in this transdisciplinary field?

Lydéric Bocquet: Originally, I was trained as a theorist. I studied statistical physics—that is, physical systems composed of a large number of particles. Quite early in my career, I became interested in confined fluids and their unique properties, primarily from a theoretical perspective. But I realized that this issue of fluids at the very small scales was notably lacking in well-controlled experimental studies. We can theoretically predict many phenomena, but that’s somewhat pointless if we don’t have the means to grasp the reality of nature (since it’s often far more unexpected than we could ever hope for). The major hurdle was that we didn’t know how to build nanosystems suitable for studying fluid transport at the nanoscale, let alone how to quantify the molecular flows within them. In 2002, when I became a professor at the University of Lyon 1, I decided to shift the focus of my research by developing my own experimental work. I formed a group with a few colleagues focused on what is known as “soft matter,” and then turned my attention to the field of fluid transport in very small systems. My team and I set out to explore fluid flows in nanosystems, such as nanotubes, or more recently, graphene ^[1]; all of these systems have opened up new possibilities for addressing these questions. Previously, the few results in this field focused on membranes, and many were quite surprising—even controversial—for example, regarding the properties of nanotube membranes for water transport. Between 2008 and 2010, together with a few colleagues and some courageous students, we set out to examine this problem by studying flow in single nanotubes. That was the beginning of this adventure, and since then, I have carved out my niche in what is known as “nanofluidics”—the general study of fluid properties at the nanoscale. I have sought to understand the emergent properties, which differ from what we typically observe on larger scales, making this a distinct disciplinary field at the interface between the continua of hydrodynamics and the molecular—or even quantum—nature of condensed matter. We’re working at the frontiers of the unknown, and some of our discoveries are truly unexpected.

You recently worked on what’s called an “ionic neuron.” What is that?

This is a classic example of an unexpected application of nanofluidics. This research began with a theoretical study of the properties of ionic systems in two-dimensional structures. In fact, we’ve only recently been able to fabricate such channels using what are known as “van der Waals assemblies,” developed by the team of Nobel laureate Andre Geim, with whom we collaborate extensively. These assemblies are very interesting because they allow us to fabricate extremely fine channels. Thus, for the first time, we were able to create and perfectly control the flow of fluids or ions through channels that measure only a few angstroms ^[2] —essentially two-dimensional. In general, the properties of two-dimensional systems are very unusual; ions interact very strongly, for example. We had predicted that clusters of ions would agglomerate to form a structure resembling a sort of spaghetti. Through theory and numerical simulations, we saw that it was possible to break up these spaghetti-like structures, but that they could then reassemble. This is a very complex dynamic. However, since these ultra-confined ion filaments are quite large, this process takes time to unfold; and a memory-like phenomenon emerges. This is a property that resembles—or even mimics—that of neurons in our brain. We first demonstrated this theoretically in 2021 by reproducing a few very basic neural functions. Then, in 2022, we moved from theory to experiment and demonstrated these mimicked neural functions in our two-dimensional channels. In particular, using these nanofluidic channels, we were able to replicate, in a very similar way, the functions of synapses, which form the basis of learning in our brains. This is a perfect example of a completely unexpected result: when we confine water at the nanoscale, we observe behaviors that bear no resemblance to what we know at larger scales.

Could these results have applications in the field of artificial intelligence?

We’re still a long way from practical application, but in theory, yes, a new path has opened up. We hope to be able to follow it. The brain functions using ions and water, and it has opted for an ionic circuitry that we want to try to mimic. This hasn’t been achieved yet. However, we’re seeing possibilities emerge with nanofluidic systems. For now, we’re working with a single system that resembles a neuron. The idea would be to create parallel systems that can communicate with one another—in other words, ionic circuits capable of performing ionic computations. This may well be one of the major challenges in this emerging field. Interestingly, there’s a real underlying energy issue at play. Artificial intelligence consumes an enormous amount of energy, whereas the brain operates with astonishing energy efficiency (the equivalent of two bananas a day!).

This field has been booming for several decades. What are its other major challenges today?

The challenges of nanofluidics are shaped by two key factors. The first is that a new field of study is taking shape in the wake of a cascade of new phenomena and discoveries, many of which remain unexplained. From a scientific standpoint, we need to equip ourselves with the tools to understand these new concepts. For example, we now know that quantum phenomena come into play—something that was completely unexpected, particularly in hydrodynamics. This fundamental aspect is very exciting, because we’re dealing with entirely new concepts, and I’m learning something new every day. The second aspect, which I’ve experienced firsthand, is that these properties at the nanoscale have implications not only for various scientific fields but also for technological applications, such as water desalination and osmotic energy. The path from fundamental discoveries to their application in technological fields is short. Some of our early experiments, involving boron-nitrogen nanotubes, showed that very strong electric currents could be generated by creating a salinity gradient between very salty water and fresh water. This is known as “osmotic energy.” We patented the technology and then co-founded a startup, Sweetch Energy, which now employs more than thirty people. The company will also launch an industrial pilot project with the Compagnie nationale du Rhône (CNR) and EDF in 2023. To me, this is a great illustration of the impact of basic science. There are opportunities for rapid transfer between basic research and technological applications, which are in constant interaction.

Do these two aspects of your team—basic research and innovation—coexist without the demands of one derailing the other?

This is something we pay very close attention to. On the one hand, these two aspects are in constant dialogue, since they are components of a single team: Micromégas, at the École normale supérieure. However, it’s true that their priorities and goals differ. The sole objective of the team’s basic research arm is to generate knowledge for the advancement of science. The innovation arm, on the other hand, aims to file patents, secure industrial contracts, and potentially develop startups. The two subgroups are therefore very different. Getting them to interact and collaborate is incredibly valuable—particularly seeing the fundamental phenomena we observe translate into technological innovations for desalination or energy. This remains a daily challenge, as the objectives—and the scales involved—remain distinct. To achieve successful technology transfer, it’s not enough to desalinate water at the nanotube scale based on a new phenomenon, however fascinating it may be: we must scale up this phenomenon to purify liters, or even cubic meters, of water. It’s a completely different perspective—and therefore a considerable challenge… but a truly rewarding one.

What challenges must a researcher overcome to develop experimental tools?

The challenges differ, once again, between the basic research team and the innovation team. For the former, the goal is to develop new instruments that enable access to previously unattainable measurements. For example, between 2010 and 2016, we were focused on measuring what happens inside a single nanotube. As you can imagine, it would take a billion years to fill a glass of water with a single nanotube. So we asked ourselves how to measure a flow of water on that scale. Since we couldn’t use standard tools to do this, we had to develop our own approaches. This is fairly typical of our work: we build systems and devise new tools—both experimental and theoretical. For example, the results we obtained in 2016 were difficult to understand within the usual frameworks; so we hypothesized that a quantum phenomenon was at work. This was quite unexpected, since it’s hard to imagine, at first glance , water—a relatively classical substance at room temperature—in a “quantum state.” So, we sketched out a new theoretical framework to understand what we were observing. I’m also currently developing a project with two other colleagues: one from Cambridge and the other from the Max Planck Institute in Mainz. Each brings their own expertise to develop new, shared tools that align with these new theoretical frameworks. When it comes to innovation, the challenges largely lie in scaling up. Once again, how do we move from a fundamental discovery made at the nanoscale to an application that benefits society? To do this, we need to make the transition from the single nanocanal in our experiments to a material that can be used to manufacture hundreds of thousands of square meters of membrane. We must therefore reinvent the process, starting from a fundamentally established blueprint. This is exactly what the startup Sweetch Energy has done. It manufactures membranes from bio-based materials on a scale of square meters that replicate the same phenomena observed in our nanotubes. Achieving this scaling up is extremely complex. The first stage takes place in the laboratory. We move from nanotubes to membranes measuring a few square centimeters—and there’s already a world of difference between these two orders of magnitude—and then to the even larger scale of the device itself.

This year, you are a visiting scholar at the Collège de France’s Technological Innovation Liliane Bettencourt Annual Chair. What are you hoping to gain from this experience?

Above all, there’s the joy of teaching, of passing on knowledge, and the excitement of knowing that it can inspire a calling, a passion, or simply an interest in the audience. Through this chair, I also hope to show the general public that French research is striving to remain at the highest scientific level, while being fully aware of the challenges associated with the energy transition. New solutions can emerge. These disciplinary fields and this somewhat relentless and often risky approach that my colleagues and I have been pursuing for fifteen years are moving in this direction. The startup Sweetch Energy and all the applications of our work are rooted in this desire to take action to transform our approaches in line with the climate changes we are experiencing.

You’ve also taught at the University of Lyon and the École normale supérieure. What role should teaching play in a researcher’s life?

It depends a great deal on personal inclinations; personally, I’ve always loved teaching. I believe that teaching crystallizes thought and catalyzes research. I enjoy explaining things to others so that I can learn better myself, and I find that teaching forces us to question ourselves, to explore little-known or uncharted territory. In short, to step outside our comfort zone. That’s the very definition of research, and we see it to some extent in teaching as well. We’re inevitably led to teach things we don’t know very well, and this aspect of my career has enriched me immensely. In particular, I’ve published articles that stem directly from my teaching—even some that are a bit anecdotal—since I’ve done a lot of work on the physics of everyday life. For example, I published a paper on the physics of ricochets, which originated from an exercise I’d developed in response to a question from my son, who was eight at the time (he’s twenty-eight now). I’ve also worked on cooking potatoes, waxing skis, and sliding on snow, among other things. I initially explored these topics for teaching purposes. They eventually evolved into particularly interesting research subjects. I must also say that I’ve been fortunate to work with exceptional doctoral students who are willing to take risks and explore uncharted territory together to advance the discipline. We pass on knowledge, but we also gain so much in return.

Interview by William Rowe-Pirra, journalist

Glossary

[1] Graphene: a two-dimensional material consisting of carbon atoms arranged in a regular pattern.

[2] Angstrom: An angstrom is a unit of measurement equal to 0.1 nanometer, or one-tenth of a billionth of a meter. It is used to take measurements at the atomic scale.