EPFL Studies Water Behavior at a Few Nanometers Scale

Water flows through carbon nanotubes orders of magnitude faster than expected, a phenomenon that is prompting scientists at EPFL to re-examine fundamental physics at the nanoscale. The behavior of liquids, particularly water, within channels only a few molecules wide is proving to be a significant gap in modern understanding, influenced by the atomic structure of the confining 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. Researchers are discovering that electromagnetic coupling between water molecules and channel walls creates a novel friction mechanism, which could provide a new means of converting hydraulic energy at the nanoscale.

Nanoscale Water Flow Reveals Hydro-Electronic Drag

The behavior of water confined to spaces just a few nanometers wide presents a significant challenge to established physics, prompting researchers to re-evaluate classical hydrodynamic models. At this scale, water molecules interact with the electrons within the channel walls through electromagnetic coupling, a process Kavokine describes as researchers finding that these interactions create a new friction mechanism. Water molecules and electrons exert force on each other, resulting in energy dissipation. This creates 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 momentum transfer creates an electric current and could provide a new way of converting hydraulic energy at the nanoscale. Kavokine suggests this could lead to energy recovery in filtration processes, eliminating the need for dissolved ions, and even facilitate water treatment and energy harvesting from salinity gradients.

The experimental realization of single nanochannels, crucial for these studies, remains technically demanding; Kavokine notes that several groups are working on this problem around the world, but there are still too few experiments to draw firm conclusions. Beyond single channels, the future lies in fabricating large networks of these structures, integrating thousands or millions onto a chip, but the theoretical frameworks currently available are inadequate. “The equations we have now are not the right language to describe what we observe in our experiments,” admits Kavokine, prompting a parallel effort to develop new theoretical models to explain these observations.

Aqua-Porins and Quantum Interactions in Nanochannels

Biological systems have long demonstrated an innate ability to manipulate water at the nanoscale, a proficiency now inspiring investigations at the École polytechnique fédérale de Lausanne (EPFL). Researchers are increasingly focused on how water behaves when confined to channels only a few molecules wide, revealing interactions not predicted by classical physics. Aqua-porins, protein channels found in cell membranes, exemplify this biological mastery; these structures efficiently filter water while actively blocking ions and other molecules, using molecular-scale interactions. Researchers have found that these interactions between water and the channel boundaries create a new friction mechanism, where water molecules and electrons in the channel wall push each other, creating a source of energy dissipation. Kavokine notes, “It is here where things get quantum.” This could provide a new way of converting hydraulic energy at the nanoscale, offering possibilities for energy recovery in filtration processes and even harvesting energy from salinity gradients. Currently, fabricating functional single nanochannels presents a significant experimental challenge, with multiple research groups worldwide attempting to refine the process.

Optical Techniques Measure Stability of Oil-Water Emulsions

Sylvie Roke, professor and director of the EPFL Laboratory for Fundamental BioPhotonics, and her team have recently elucidated a key factor governing the stability of oil-water emulsions, a phenomenon observed in everyday life yet previously lacking a complete molecular explanation. Utilizing advanced optical techniques, the researchers successfully created microscopic oil droplets capable of stable suspension in water, revealing that the stability stems from the transfer of electric charge. This charge transfer occurs from water to oil across the interface between the two liquids, facilitated by weak hydrogen bonding interactions. Roke’s team also explained how basicity influences the movement of oil droplets within water, linking it to the pH-dependent conductivity of the bulk water itself. This general mechanism, they suggest, provides insight into a broad range of pH-dependent processes spanning biology, chemistry, and nanotechnology.

Beyond simply mixing oil and water, the team developed a novel method to directly measure interacting hydrogen bonds within bulk water, offering unprecedented access to molecular couplings. “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, highlighting the technique’s potential for unraveling other complex molecular properties of liquids. The ability to observe these interactions at such a granular level is crucial for understanding water’s behavior at a molecular level, and could have implications for diverse fields. Investigations into water behavior are also occurring at EPFL’s Quantum Plumbing Lab, where researchers are attempting to bridge the gap between classical physics and the quantum realm as it applies to liquid behavior, and ultimately, to create artificial nanofluidic networks that mimic natural systems.

The pursuit of efficient filtration systems is drawing inspiration from an unexpected source: the bizarre behavior of water flowing through carbon nanotubes. Recent experiments demonstrate water travels through these nanoscale tubes at velocities significantly exceeding predictions based on classical hydrodynamics, prompting a re-evaluation of fundamental physics at the smallest scales.