Frédéric Caupin

Episodically in Earth's history, marine basins became hydrologically restricted, forming “salt giants,” yet the mechanisms driving their formation remain debated. The modern Dead Sea, undergoing restriction from anthropogenic inflow diversion, shows rapid lake‐level decline and salt precipitation. We present a physical model coupling energy and mass fluxes in hypersaline waterbodies. The model reproduces the observed ∼3°C Dead Sea water‐column warming since 1979. By 2100, the Dead Sea will drop ∼85 m, and warm by ∼5°C–∼8°C depending on global CO ₂ emissions. A fully restricted eastern Mediterranean under modern climate conditions would undergo 20°C warming and 2.5 km drawdown within a few millennia. Once isolated, large basins enter a positive feedback loop between warming and water loss that drives precipitation of halite and potash salts without requiring hot, arid climates. Because paleobrine warming tracks brine shrinkage, vertical paleotemperature trends in evaporites may record drawdown rather than paleoclimate.

Dynamic properties of nanoparticles under high pressure are poorly understood due to experimental challenges. Here, we use X-ray photon correlation spectroscopy (XPCS) to investigate the diffusion dynamics of polyethylene glycol (PEG)-functionalized gold nanoparticles dispersed in water at pressures up to 6 kbar. Complementary small-angle X-ray scattering reveals no measurable pressure-induced changes in the gold core size, confirming structural stability across the studied pressure range. XPCS measurements show a systematic slowing down of nanoparticle dynamics with increasing pressure, with relaxation rates retaining the expected q2-dependence characteristic of Brownian motion. The extracted diffusion coefficients decrease continuously with pressure and align with predictions based on the pressure-dependent viscosity of water and the Stokes–Einstein relation, assuming a constant hydrodynamic particle size. Within experimental uncertainty, no additional contribution from pressure-induced ligand-shell modifications is observed in these dilute dispersions.

Fluid phase equilibrium depends on the external constraints imposed on a system. In a closed system with fixed volume, depending on the average density, a vapor bubble may be stable, metastable, or unstable, with respect to the homogeneous liquid phase. In the case where the bubble is metastable, we study its lifetime, i.e., the average waiting time needed to observe bubble collapse, and the corresponding lifetime of the homogeneous liquid. For the smallest systems, we predict the possibility to observe phase flipping, when the fluid oscillates between states with and without bubble. We provide an example of phase flipping in a simulation of a Lennard-Jones fluid.

Advanced XPCS reveals microsecond diffusion of PEGylated gold nanoparticles in water, quantitatively matching Stokes-Einstein predictions, and accessing intrinsic Brownian timescales even in supercooled water.

Fluid phase equilibrium depends on the external constraints imposed on a system. In a closed system with fixed volume, depending on the average density, a vapor bubble may be stable, metastable, or unstable, with respect to the homogeneous liquid phase. In the case where the bubble is metastable, we study its lifetime, i.e. the average waiting time needed to observe bubble collapse, and the corresponding lifetime of the homogeneous liquid. For the smallest systems, we predict the possibility to observe phase flipping, when the fluid oscillates between states with and without bubble. We provide an example of phase flipping in a simulation of a Lennard-Jones fluid.

Water shows numerous thermodynamic, dynamic, and structural anomalies. Recent experiments [Eichler et al., Phys. Rev. Lett. 134, 134101 (2025)], based on measurements of shear and bulk viscosities of liquid water up to 1.6 GPa, have reported the existence of a minimum in the variation of the structural relaxation time τα with pressure at room temperature. Here, we investigate this and related properties with molecular dynamics simulations of the TIP4P/2005 water model, performed at extreme pressures commensurate with the experiments. Specifically, we compute dynamic (self-diffusion, shear and bulk viscosities, and structural relaxation time) and structural (oxygen–oxygen radial distribution function, structure factor, and translational order parameter) properties down to 220 K and up to 2.7 GPa. We find good agreement with the experimental observations and confirm the existence of a minimum in τα. The microscopic information obtained from the simulations suggests that this anomaly is related to the sudden reorganization of the hydrogen bond network induced by pressurization.

The fate of a metastable substance is governed by rare nucleation events, whose full microscopic details still remain elusive despite over 150 years of study. Experimental nucleation rates often differ by many orders of magnitude from theory and simulations, limiting predictive power. In this Perspective, after a general overview on nucleation, we focus on crystallization in three representative systems: metastable water, colloidal suspensions, and Lennard-Jones liquids. The latter, which are well realized by rare-gas liquids, provide a touchstone for nucleation theory. Recent femtosecond X-ray diffraction experiments on supercooled argon and krypton deliver accurate crystal nucleation statistics and direct insight into structural defects such as stacking faults. These advances establish rare-gas liquids as uniquely well-controlled systems bridging experiment, simulation, and theory, and pave the way toward a more complete microscopic understanding of nucleation.

Fluid polyamorphism is the existence of multiple fluid-fluid phase transitions in a single-component substance. It can occur due to interconversion between two alternative molecular or supramolecular states. In this work, we investigate a special ("degenerate") case of fluid polyamorphism, in which all three characteristic parameters of the interconversion equilibrium constant, i.e. the changes of energy, entropy, and volume, are zero. This feature of interconversion is typical for the Ising spin model of ferromagnets but is also observed in a polyamorphic chiral fluid mixture of interconverting enantiomers. To investigate the consequences of interconversion's degeneration for fluid polyamorphism, we have performed a meanfield analysis and 3D Monte Carlo simulations of a compressible binary lattice with interconverting species (referred to as the "blinking-checkers model"), which generally demonstrates the existence of both liquid-gas and liquid-liquid transitions. By tuning the interaction parameters, we have demonstrated that, in the degenerate interconverting case, a coupling between the fraction of interconversion (a vector-like nonconserved order parameter) and the total density (a scalar conserved order parameter) may produce a symmetrical tricritical point. At this point, the line of second-order transitions between two fluids, "disordered" (with 50:50 interconversion) and "ordered" (with temperature and pressure dependent interconversion fraction), is terminated by first-order transitions between the fluid states. This point exhibits the typical features of symmetrical tricritical points as observed in a superfluid mixture of helium isotopes and in some magnetic materials. We also show that the transition between the ordered and disordered fluid could occur in either the liquid or the gaseous phase.

We demonstrate an advanced scattering method for accessing the 3D reciprocal space of crystalline structures forming in a rapidly supercooled noble-gas liquid using a combination of femtosecond X-ray diffraction and X-ray cross-correlation analysis. The preservation of angular information from the scattering signal allows probing the structure factor along selected directions in reciprocal space and identifying signatures undetectable in azimuthally integrated scattering curves. Therefore, more information from serial diffraction experiments on stochastic crystallization processes can be retrieved despite the inherent variation of the crystal orientation and morphology for each single probe. We also demonstrate how different features in the correlation maps can be associated with certain forms of stacking faults, which enables studying such defects in situ and disentangling them from simultaneous changes in crystal size and temperature.