Research

What is Quantum Plumbing? 

Our goal is to understand the laws that govern liquid flows at nanometer scales, where classical and quantum dynamics meet. We use theory and simulations to predict new physics and we design experiments to probe these physics. We then screen our findings for applications at the water-energy nexus.

Methods

Theoretical understanding is a pillar of our research. Theoretical discoveries motivate the design of experiments and experimental results prompt new theoretical questions. Our specialty is in applying field theory methods to nanofluidic systems; in particular, the Keldysh formalism has the unique ability to capture classical and quantum dynamics on equal footing. Below is a video of Nikita explaining how it works during his Inaugural Lecture. 

To understand nanofluidic systems, we must look at them, and to look at them we first need to make them. We use microfabrication and van der Waals assembly techniques to design systems that are tailored to our scientific goals. In addition to established techniques, we design our own tools to probe these systems, such as nano-tribometry and NV-center-based magnetic sensing. 

Hahn echo pusle sequence. From S. Hong et al., MRS Bulletin (2013).

When pen-and-paper theory is not enough, we turn to numerical simulations. In addition to standard molecular dynamics, we employ simulation techniques inspired by strongly correlated electronic systems. In particular, we develop the CTSEG quantum impurity solver within the TRIQS library, and use dynamical mean-field theory to study electronic correlations at solid-liquid interfaces. 

One of the Monte Carlo moves in the CTSEG solver.

 

Projects

At the nanoscale, liquid and electronic flows are intertwined. This is due to the fluctuation-induced quantum friction phenomenon. Quantum friction amounts to the direct transfer of momentum from a flowing liquid to a solid’s electrons, which are thus set in motion: a liquid flow induces an electronic current. Conversely, an electronic current driven through a channel’s solid wall is expected to induce a liquid flow. These phenomena, termed respectively hydro-electronic drag and quantum osmosis, could have significant implications for nanoscale hydroelectricity and osmotic energy harvesting. Yet, they are so far barely understood theoretically, and they have not been directly observed in experiments. 

We seek to address this problem from both sides. Experimentally, we design systems where nanoscale hydro-electronic couplings have microscale consequences, so that can they can be probed in a quantitative and reliable way. Theoretically, we aim at solving the role of channel wall vibrations – phonons – in nanoscale fluid transport, as these are expected to play a key role in the above-mentioned coupling phenomena. The theory further aims at formulating criteria for maximal efficiency of nanoscale hydroelectric conversion. 

Team: Peter Gispert (experiment), Adrien Sutter (theory). 

Selected publications:

The observation of ultrafast water flows through carbon nanotube membranes could be considered the founding experiment of nanofluidics. The very large permeability of carbon nano-conduits is due to exceedingly low friction between water and the carbon wall, but the origin of this low friction is still not understood. The water-carbon interface is one of the few systems where molecular dynamics simulations systematically disagree with experiment: its study is bound to reveal new physics. These physics could be key to designing filtration membranes with reduced frictional losses. 

To understand water flows through carbon nanotubes, we need to reliably measure them. To this end, we develop a quantum sensing approach, based on nitrogen-vacancy centers in diamond, to detect water flows down to femto-liters per second. In parallel, we leverage quantum friction theory to obtain predictions for the structure-permeability relation in carbon nanotubes. 

Team: Killian Rigaux (experiment), Lucas Floquet (experiment), Peter Gispert (theory), Hao Lu (simulations). 

Image courtesy of Mathieu Lizée.

The permeability of a macroscopic pipe is determined by the pipe’s diameter and the viscosity – that is, the internal friction – of the liquid being pushed through. In a nanoscale channel, the permeability is rather determined by the friction between the liquid and the channel wall. Despite the importance of solid-liquid friction in nanofluidics, there is currently no reliable way to measure it on small-scale samples, such as exfoliated 2D material flakes that are often used to build nanofluidic devices. Our lab develops the nanoTM (short for nano-tribometer), an instrument inspired both by the surface force apparatus and the tuning fork AFM, to fill this technological gap. 

Team: Caroline Cramail (experiment), Lucas Floquet (software).

In solid-state physics, water is often considered a featureless dielectric medium. This turns out to be quite far from the truth, as water exhibits an intricate charge fluctuation spectrum in the THz to infrared frequency range. We study how water charge fluctuations couple to electron dynamics in 2D materials – particularly, those materials where electron-electron interactions are strong and produce effects beyond the random phase approximation. Such effects are also beyond the reach of analytical theory. We seek to capture them in a numerical quantum embedding approach, supported by our CTSEG quantum impurity solver. 

Team: Hao Lu (simulations/theory).

Selected publications: