The Quantum Plumber

In the current era, physics has split into two worlds: classical and quantum. Classical physics, which describes things on large scales — such as planets — begins to break down when describing things at the nanoscale level, where physicists must use quantum physics to describe anything smaller. This has created two disciplines so disparate that many physicists have become specialists that work from only one point of view.

But sometimes the classical and quantum worlds collide. Theoretical physicist Nikita Kavokine is one such scientist working to understand what happens when quantum dynamics affects physics on larger scales. This work has led to surprising insights into what happens on nanoscales when liquids and solids interact, such as Kavokine’s recent discovery that electrons in the walls of carbon nanotubes create friction with water flowing through them — a finding that solves a decades-old mystery.

Kavokine is a research fellow at the Flatiron Institute’s Center for Computational Quantum Physics in New York City and a fellow at the Max Planck – New York Center for Non-equilibrium Quantum Phenomena. Prior to his fellowship, he earned his bachelors, masters and doctorate degrees at École Normale Supérieure in Paris.

Kavokine recently spoke to the Simons Foundation about his work and its many applications. The conversation has been edited for clarity.

You recently made a big discovery about quantum friction. Can you explain that breakthrough?

Many of us are familiar with the concept of friction that we were taught in our high school physics class, which explains why there is a difference between pushing a box across thick, plush carpet versus a smooth wooden floor. It turns out there is also friction on the quantum level. Quantum friction is basically a type of friction that occurs between a liquid and a solid that comes from the quantum dynamics of the electrons in the solid. In our recent Nature paper, we proposed that this quantum friction can solve the long-standing puzzle in fluid dynamics where we counterintuitively find that water flowing through tiny carbon nanotubes flows faster in smaller pipes.

This work was exciting because it bridged fluid dynamics and condensed matter physics. Previously, people studying hydrodynamics considered walls as just walls. They didn’t care what the walls were made of. And the condensed matter physicists, who study what happens inside those walls, only looked at the complicated dynamics of electrons inside solid materials. But people haven’t really been looking at what happens at the interface between these two.

In fact, what happens at that interface becomes important when you start looking at flows of liquids in tiny pipes, such as carbon nanotubes. We demonstrated that at small scales, the fluid flow is really coupled to the electron dynamics inside the walls. Essentially, the water molecules and the electrons inside the solid wall push and pull on each other.

What practical applications does this finding have?

This mostly has relevance for membrane technology applications, such as filtration or seawater desalination. However, our findings also have applications in certain energy production methods which exploit differences in water salinity. When you’re filtering water, you have to push the water through tiny pores in a membrane. If there’s a lot of friction, that push requires more force and energy. Knowing how friction is determined at very small scales can help us design materials with lower frictional properties, which can help save energy and thus cost when using them.

Already there are several collaborations of researchers working to experimentally verify our predictions, including a group at the Max Planck Institute for Polymer Research in Mainz, Germany. They’ve been able to shine lasers at a solid-liquid interface and determine some very fine properties of the water molecules. With this technique, they directly observed one of the elementary building blocks of the quantum friction mechanism: the interaction between vibrations of the water molecules and a particular vibration of electrons in the solid graphene. This is something that was at the heart of the quantum friction mechanism that we predicted.

Why do you think that people previously haven’t put these two fields together?

First, experiments on nanoscale fluid flows have started only about 20 years ago, and there was no obvious need to put the two fields together before then. Second, I think people working in these two fields come from very different backgrounds, so there are few who understand them both. I come from the fields of fluid dynamics and statistical mechanics and had to learn a lot of the quantum and condensed matter physics required for this work on the spot. Actually, this is precisely why I applied to the Flatiron Institute. I wanted to work with people that specialize in the study of electron dynamics and see if we could work together to find more connections with fluid dynamics.

I believe this work could represent a new research area at the intersection of quantum physics and fluid dynamics. I’m hoping that more people will get involved and we will discover more exciting new things.

Why do you think it’s important to study these things on such tiny scales? And how does that translate into our everyday lives?

First, I think that there is a fundamental aspect. This is part of the bigger picture of how we want to understand how the world is built and what are the physical principles governing it. As physicists, we are intrinsically excited by the possibility of finding new ways to think about these nanoscale fluid systems.

Then there is also a very practical side to this research. In the past, there has been a very short path between a fundamental discovery, like this one, and innovation and applications. In my former lab, there was a fundamental discovery in 2013 concerning water flow through boron nitride nanotubes, and already that idea has been picked up by a startup company that has raised several million euros to begin building a clean power plant on the Rhone River in France, which is very exciting.

Personally, I’m motivated by the fact that we get to use very abstract but beautiful theoretical techniques to describe something very concrete, such as water flowing through a tiny pipe.

This Q&A is part of Flatiron Scientist Spotlight, a series in which the institute’s early-career researchers share their latest work and contributions to their respective fields.