© Illustration by Denis Kormann / EPFL
How liquids, and water in particular, behave at scales of a few nanometers is one of the big gaps in modern physics.
Our body contains an intricate system of tiny vessels through which blood, water and other molecules flow. When the size of the pipes shrinks to the nanoscale, where only a few molecules can fit side-by-side, the classical laws of physics governing the behavior of water are influenced by the atomic structure of the walls. "It's not that classical hydrodynamics breaks down, but rather that it gets mixed with the condensed matter physics of the solid walls," says Nikita Kavokine, tenure-track assistant professor and leader of the EPFL Quantum Plumbing Lab.
How liquids, and water in particular, behave at scales of a few nanometers is one of the big gaps in modern physics. For example, in some experiments it has been observed that water flows through carbon nanotubes orders of magnitude faster than expected. Scientists are trying to understand phenomena that biology has mastered after millions of years of evolution. "At the nanometer scale, our body leverages specific properties of water to filter molecules with high energy efficiency," explains Kavokine. Aqua-porins, for example, are protein channels embedded in cell membranes that use these molecular-scale interactions to let water pass while blocking ions and other molecules.
An unknown mechanism
At the nanoscale, water molecules interact with the electrons in the channel wall through electromagnetic coupling. "It is here where things get quantum. And that's why we talk about quantum plumbing," explains Kavokine. Researchers have found that these interactions between water and the channel boundaries create a new friction mechanism. In this mechanism, water molecules and electrons in the channel wall push each other, creating a source of energy dissipation. However, the details of this mechanism are still being investigated. "It is paradoxical that we understand so much about how electrons behave at small scales, but so little about liquids," notes Kavokine.
Since friction is a force, there exists a net transfer of momentum to the electrons in the wall, leading to a sort of hydro-electronic drag, where the flow of water pulls electrons along the surface. This additional momentum generates an electric current flowing through the solid. This current could provide a new way of converting hydraulic energy at the nanoscale. In the future, this small-scale hydroelectric energy conversion could be used for energy recovery in all sorts of filtration processes without relying on any dissolved ions. This mechanism could also help in water treatment and energy harvesting from salinity gradients where seawater meets freshwater.
From single nanochannels to artificial organs
The experimental and technical implementation of a single nanochannel, essential to perform the studies, is challenging. "There are several groups working on this problem around the world, but we still have too few experiments to draw firm conclusions," says Kavokine. Fabricating a single nanochannel that functions like a biological ion channel is already a complex task. But interesting applications will require building large networks that integrate thousands, maybe millions, of these channels on a chip and within a controlled architecture. "In the next few years, we will begin to face this engineering challenge of scaling fundamental nanofluidics to something useful," explains Kavokine.
But the field is still young: nanofluidics emerged only about 20 years ago. Researchers are just figuring out the building blocks of how liquids flow through nanochannels, both theoretically and experimentally. "The equations we have now are not the right language to describe what we observe in our experiments," admits Kavokine. That is why part of the work carried out at the Quantum Plumbing Lab focuses on figuring out new theoretical frameworks that help understand the observations.
Building nanotubes is just one of many projects Kavokine's Quantum Plumbing Lab is working on. Other projects seek to explore the inner structure of carbon nanotubes using quantum sensing. This would allow researchers to understand why water flows with almost no friction through them.
In the long term, the scale-up of single-channel devices can lead to the fabrication of artificial nanofluidic networks that mimic natural ones. This would be the first step towards the creation of new high-efficiency computing architectures based on water and salt. However, there is still a long way to go before researchers can implement these networks. "We are just figuring out how it all works. My dream is to build something like an artificial brain or a kidney that rival their biological analogues in terms of energy efficiency," explains Kavokine.
A recipe to mix water and oil
But not all of water's unusual properties occur only at the nanoscale. Some phenomena that remain unexplained can be observed in everyday life. For instance, it is well known that, under normal conditions, water and oil do not mix. They can, however, form emulsions. The mechanism underlying the stability of oil nanodrops in water was uncovered by Sylvie Roke, professor and director of the EPFL Laboratory for Fundamental BioPhotonics, and her team. Using optical techniques, the researchers created microscopic oil droplets capable of mixing with water. Their findings showed that the explanation for this observation lies in the transfer of small amounts of electric charge from water to oil across the interface between the two liquids via weak hydrogen bonding interactions.
Yet the interface between water and oil can reveal many other processes. Roke's team, for example, also explained how basicity influences the movement of oil droplets in water. The mechanism is linked to the pH-dependent conductivity of bulk water. This general mechanism helps explain a wide range of pH dependent processes in biology, chemistry and nanotechnology.
Understanding water at the molecular level, however, requires scientists to observe how hydrogen bonds form and organize into networks. A new method developed by Roke and her collaborators allows researchers to directly measure interacting hydrogen bonds in bulk water. "We can now measure charge transfer, nuclear quantum effects and other elusive phenomena directly at the atomistic scale where it occurs. It is almost like measuring the unmeasurable," says Roke. The technique provides direct access into molecular couplings and can help elucidate many other molecular properties of water, and other liquids.
