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Interfaces between air and water are omnipresent in nature and in industrial processes, but we understand surprisingly little about what goes on there. Researchers in Germany now report that a new spectroscopy technique could reshape our understanding of the molecular structure of these aqueous interfaces and the dynamical processes at play. Their discoveries could lead to better models of atmospheric processes and improved electrochemical devices such as batteries, to name but two examples.

When air and water meet, the interface between them strongly influences the behaviour of the first four layers of water. This interfacial water, as it is known, is only 7-8 angstroms thick, and the water below it behaves like a bulk liquid. To study interface effects, researchers therefore need to probe only these four layers and characterize the way their H₂O molecules are oriented.

Probing the H-O-H bending vibration

One way to do this is to observe the bending vibration of the H-O-H structure, as this parameter approximately aligns with the water molecule’s dipole. In particular, one can analyse how the anisotropic bending mode changes with the thickness of the interfacial water. This can be calculated using a factor known as the depth-dependent second-order susceptibility, 𝜒(ଶ)(𝑧).

There is a problem with this approach, however, because it requires the H-O-H bending vibration to originate from the electric dipole of H₂O and to contain only an interfacial dipolar signal. This is not always the case because electric quadrupolar signals from the bulk of the sample, as well as magnetic dipolar signals, can also contribute to the spectra.

These other signals do not provide any information on the orientations of the H₂O dipole. Worse, they can mask the structural information that researchers are looking for. This means they either need to be ruled out or have their contributions to the overall signal removed.

A tuneable visible upconversion

The new technique, developed by Martin Thämer and colleagues in the Nonlinear Interfacial Spectroscopy Group of the Fritz-Haber Institute der Max-Planck-Gesellschaft, involves feeding the 800-nm-wavelenght light output from a Ti:sapphire laser into two independent optical parametric amplifiers. The first amplifier produces mid-infrared light through a process called difference frequency generation (DFG) and the second produces a signal beam that is subsequently doubled in frequency to produce a tuneable visible upconversion.

Using these two beams, the researchers irradiated the surface of a water sample and excited nonlinear vibrations in the water molecules. This excitation generates two new light beams at different visible frequencies. By measuring the differences in the phase and amplitude of these beams, the team was able to isolate the vibrational response of the interfacial water layer and separate it from the bulk-water quadrupole term.

Thämer and colleagues then combined their spectra with high level molecular dynamics simulations developed by their colleagues at the Frei Universität Berlin to determine the precise orientations of the water molecules in the interfacial region.

Traditional description is insufficient

Traditionally, the structure of interfacial water is described in terms of water molecules pointing up or down (the “tilt angle”). However, based on the team’s results, Thämer says this description is insufficient. An additional orientation parameter is required, namely the “water twist angle”, or the molecule’s rotation about the axis of its dipole. “The new picture of the water structure we present is layered one with alternating twist and tilt angles that indeed extends over only four molecular water layers,” Thämer tells Physics World.

Looking ahead, the researchers, who detail their work in Science Advances, say they now plan to study other aqueous interfaces, including charged interfaces and biomolecular systems.

The post Interface between air and water gets a new twist appeared first on Physics World.

A new study is turning an elegant quantum idea into a practical platform for light. In research published in Light: Science & Applications, physicists report “topological Jackiw–Rebbi” states engineered inside photonic van der Waals heterostructures—systems built by stacking atomically thin materials with precisely tailored interfaces. The result is a photonic phenomenon that echoes how particles can be trapped at boundaries where the rules of their governing equations change.

The work builds on the classic Jackiw–Rebbi mechanism, originally proposed for relativistic particles. In those models, a localized mode appears when a mass term flips sign, effectively creating a domain wall. The authors translate this concept into optics by designing photonic structures where an effective “mass” characterizing light propagation changes across an interface.

What makes the platform especially compelling is the van der Waals architecture. Unlike conventional fabrication routes that can introduce disorder, the layered nature of these materials enables sharp, controllable boundaries and tunable photonic band structures. By adjusting the stacking and interlayer optical responses, the researchers create conditions in which topological modes become robust against typical imperfections.

The study emphasizes that these states are not merely conventional guided resonances. Instead, they are tied to topology: the light modes are associated with an invariant property of the band structure. As a consequence, the Jackiw–Rebbi-like photonic states are expected to persist even when the system’s details vary, so long as the relevant symmetry and band configuration remain intact.

Topological protection is more than a slogan. In photonics, it can translate into reduced sensitivity to fabrication tolerances and a pathway toward devices that rely on stable boundary transport. The paper therefore positions these localized states as building blocks for future optical components that route signals along designed interfaces.

The authors demonstrate the concept theoretically and connect it to experimental observables by focusing on how the interface should host modes localized near the domain wall region. Such localization is a hallmark of the Jackiw–Rebbi scenario, now realized in a controllable, designer photonic environment.

Beyond fundamental interest, the approach could influence the design of reconfigurable photonic circuits. Because van der Waals stacks can be combined and adjusted, the same topology-driven logic may be extended to new material systems, wavelengths, and device geometries.

Overall, the report signals a broader trend: using topological field-theory ideas to engineer light–matter behavior in atomically precise platforms, potentially enabling the next generation of robust, interface-based photonic technologies.

Subject of Research: Topological photonic states (Jackiw–Rebbi) in van der Waals heterostructures.

Article Title: Topological Jackiw-Rebbi states in photonic Van der Waals heterostructures.

Article References: Randerson, S.A., Bouteyre, P., Hu, X. et al. Light Sci Appl 15, 323 (2026). https://doi.org/10.1038/s41377-026-02392-5

Image Credits: AI Generated

DOI: 10.1038/s41377-026-02392-5

Keywords:

It might soon be “game over” for the video game controller. Yale researchers have developed a new kind of

In the ever-evolving landscape of energy storage technologies, the lithium–sulfur (Li–S) battery stands out for its promise to deliver unmatched energy density and reduced costs compared to traditional lithium-ion batteries. However, progress toward commercial viability has been hampered by complex electrochemical challenges, particularly those involving interfacial charge transfer dynamics. A new study, recently published in Nature, unveils groundbreaking insights into the intricate reaction pathways occurring at solid–liquid interfaces within Li–S batteries, illuminating mechanisms that govern lithium sulfide deposition under realistic, lean electrolyte conditions.

The difficulty in understanding and controlling the conversion of lithium polysulfides (LiPSs) during battery cycling stems largely from the dynamic and highly concentrated nature of reactant distributions at the electrode–electrolyte interface. In these environments, the balance between surface-bound and dissolved species dictates the overall electrochemical performance. The new research deploys advanced in situ liquid-cell electron microscopy techniques to directly visualize these sophisticated concentration gradients, a feat that has eluded scientists due to the liquid phase’s complexity and the scales involved.

What emerges from this study is a revelation of concentration-driven phase segregation phenomena right at the interfacial layers, termed high-concentration interfacial layers (HCILs). These HCILs form as LiPSs aggregate in the immediate vicinity of the electrode surface during cycling, dramatically altering the local reaction environment. This segregation is not merely a passive occurrence; it actively directs charge-transfer dynamics by establishing competitive conditions between surface-mediated and solution-based pathways. The delicate interplay in these zones influences where and how lithium sulfide (Li₂S) nucleates and grows, directly impacting the battery’s capacity retention and cycling stability.

Complementing the microscopy observations, computational explorations using density functional theory (DFT) provide a molecular-level understanding of how LiPS clusters modify their electronic structure upon aggregation. The study identifies that the molecular geometries and orbital hybridizations evolve as LiPSs coalesce, resulting in enhanced electronic interactions that facilitate charge transfer through the concentrated clusters. This phenomenon fundamentally challenges previously held models which treated LiPS species largely as isolated entities, offering a new paradigm that accounts for collective behavior in highly concentrated solutions.

The practical implications of these mechanistic insights are far-reaching. By comprehending how HCILs govern interfacial reaction pathways, the researchers have engineered electrodes that strategically balance surface and solution reactions amidst high LiPS concentrations. These meticulously optimized electrodes enable impressive fast-charging capabilities—reaching rates up to 4 C with current densities of 26.8 mA cm⁻²—without compromising energy density, which itself surges beyond 400 Wh kg⁻¹. Such milestones represent a substantial leap forward in the achievable performance metrics for Li–S battery technology.

This study’s significance extends beyond the immediate Li–S chemistry, addressing a broader challenge in electrochemical energy conversion: capturing and controlling complex interfacial phenomena under dynamically varying concentrations. The in situ liquid-cell electron microscopy method establishes a powerful approach for observing reactive species and phase evolution directly in their native electrochemical environment. As such, it holds promise to transform research across a host of energy storage and catalysis systems where interfacial reactions dictate overall function.

Notably, this work also clarifies why lean electrolyte conditions—essential for practical high-energy devices—have historically been so difficult to optimize. The rich detail uncovered concerning phase segregation and charge transfer competition suggests that managing HCIL formation and stability may be the key to unlocking robust Li₂S deposition and dissolution cycles. Hence, this strategy could mitigate common pitfalls like cathode passivation and capacity fading, which have limited long-term operational viability.

The fusion of experimental visualization with robust theoretical modeling provides a comprehensive framework that future researchers and engineers can exploit to rationally design electrode interfaces. Moving past empirical trial-and-error, the ability to predict and control molecular aggregation behavior and its effects on electronic properties opens new avenues for tuning battery chemistry on an atomic scale. Such sophistication could accelerate the sustainable deployment of lithium–sulfur batteries within electric vehicles, grid storage, and portable electronics.

Moreover, the study emphasizes the profound impact of molecular-level phenomena on macroscopic performance metrics such as charging speed and energy density. By showing that the electrochemical environment at the nanoscale directly modulates reaction kinetics and phase evolution, this research bridges a critical knowledge gap between fundamental science and technological application. It highlights the necessity for integrated approaches combining real-time imaging, theoretical insight, and materials engineering.

Looking ahead, the insights gained are poised to influence material selection, electrolyte formulation, and cell architecture design. Tailoring these elements to favor desirable HCIL characteristics could lead to batteries that not only charge rapidly and store vast amounts of energy but also maintain stability over many cycles. Furthermore, the foundational principles elucidated may inspire innovations beyond lithium–sulfur systems, potentially transforming a range of emerging battery chemistries where interface complexity rules.

In conclusion, this remarkable study marks a pivotal advance in understanding and harnessing the subtleties of interfacial charge transfer within lithium–sulfur batteries. By unraveling the competitive reactions shaped by concentration gradients and molecular aggregation, it charts a pathway toward high-performance batteries ready for real-world demands. As energy storage solutions continue to underpin the clean energy transition, such elegant fusion of fundamental science and engineering innovation will be indispensable for propelling next-generation technologies.

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Subject of Research: Charge transfer and reaction mechanisms at solid–liquid interfaces in lithium–sulfur batteries.

Article Title: Revealing competitive interfacial reactions in high-energy Li–S batteries.

Article References:
Zhou, S., Pei, F., Zheng, Q. et al. Revealing competitive interfacial reactions in high-energy Li–S batteries. Nature (2026). https://doi.org/10.1038/s41586-025-09473-2

Image Credits: AI Generated

DOI: https://doi.org/10.1038/s41586-025-09473-2