Joris Heyman

Under unsaturated conditions, the coexistence of air and water generates complex, dynamically evolving interfacial structures, whose impact on solute mixing, residence times, and reactivity remains poorly understood at the pore scale. Substances transported in the water phase can interact with the air phase at the fluid-fluid interface. In particular, per- and polyfluoroalkyl substances (PFAS) are emerging contaminants of concern that are known to preferentially accumulate at air–water interfaces, where interfacial processes control their retention and mobility in the vadose zone. Darcy-scale models and experimental observations suggest that transient hydrological conditions and interfacial area dynamics can strongly influence PFAS fate. However, the pore-scale mechanisms governing transport toward air–water interfaces and the resulting mixing-limited reactivity remain largely unexplored even under steady flow. This gap limits the development of models capable of upscaling pore-scale interfacial mixing processes and predicting solute fate at larger spatial and temporal scales. We investigate these mechanisms using a Lagrangian particle-tracking approach to resolve solute transport in steady two-dimensional pore-scale flow fields under partial saturation. Solute trajectories are governed by advection, diffusion, and interactions with both fluid–fluid (air–water) and fluid–solid interfaces, enabling direct quantification of interfacial encounter statistics and residence-time distributions. These metrics provide natural descriptors of mixing-limited regimes, in which effective reaction rates are controlled by transport toward interfacial zones rather than intrinsic kinetics, and allow identification of pore-scale features that control the large-scale evolution of solute transport. This study contributes to ongoing efforts to connect pore-scale physical processes with effective models of solute transport in the vadose zone, with direct implications for predicting the fate of reactive contaminants under transient unsaturated conditions.

The salt‐freshwater interface (SFI) in coastal aquifers, where water bodies of differing chemical compositions converge, represents as a hotspot for chemical disequilibrium and biogeochemical reactions. The interplay between variable density flow, mixing and reaction in porous media leads to complex and highly localized reaction patterns across the SFI, which are difficult to visualize, understand and model. In this study, we present a novel experimental setup to quantify mixing‐driven reactions at the SFI in a quasi two‐dimensional laboratory‐scale sand tank. We use a fast irreversible chemiluminescence reaction to obtain high resolution images of reaction intensity at the interface. We show that the reaction rate varies spatially along the interface, with up to a factor of four between minimum and maximum. We present a mechanistic model quantifying the evolution of the reaction rate along the interface by describing the coupled effects of solute dispersion, fluid compression and the change of the interface geometry under different flow rates. This modeling framework captures the spatial distribution of reaction along the SFI observed experimentally. It also explains and predicts the enhancement of the effective reaction rate across the interface when increasing freshwater flow. These findings provide a novel experimental technique and a new modeling framework to image and predict mixing‐driven chemical reactions in coastal aquifers.

Recent studies reveal the central role of chaotic advection in controlling pore-scale processes including solute mixing and dispersion, chemical reactions, and biological activity. These dynamics have been observed in porous media (PM) with a continuous solid phase (such as porous networks) and PM comprising discrete elements (such as granular matter). However, a unified theory of chaotic advection across these continuous and discrete classes of PM is lacking. Key outstanding questions include: (i) topological unification of discrete and continuous PM; (ii) the impact of the non-smooth geometry of discrete PM; (iii) how exponential stretching arises at contact points in discrete PM; (iv) how fluid folding arises in continuous PM; (v) the impact of discontinuous mixing in continuous PM; and (vi) generalised models for the Lyapunov exponent in both PM classes. We address these questions via a unified theory of pore-scale chaotic advection. We show that fluid stretching and folding (SF) in discrete and continuous PM arise via the topological complexity of the medium. Mixing in continuous PM manifests as discontinuous mixing through a combination of SF and cutting and shuffling (CS) actions, but the rate of mixing is governed by SF only. Conversely, discrete PM involves SF motions only. These mechanisms are unified by showing that continuous PM is analogous to discrete PM with smooth, finite contacts. This unified theory provides insights into the pore-scale chaotic advection across a broad class of porous materials and points to design of novel porous architectures with tuneable mixing and transport properties.

Mixing-induced reactions play an important role in a wide range of porous media processes. Recent advances have shown that fluid flow through porous media leads to chaotic advection at the pore scale. However, how this impacts Darcy-scale reaction rates is unknown. Here, we measure the reaction rates in steady mixing fronts using a chemiluminescence reaction in index-matched three-dimensional porous media. We consider two common mixing scenarios for reacting species, flowing either in parallel in a uniform flow or towards each other in a converging flow. We study the reactive properties of these fronts for a range of Péclet numbers. In both scenarios, we find that the reaction rates significantly depart from the prediction of hydrodynamic dispersion models, which obey different scaling laws. We attribute this departure to incomplete mixing effects at the pore scale, and propose a mechanistic model describing the pore-scale deformations of the front triggered by chaotic advection and their impact on the reaction kinetics. The model shows good agreement with the effective Darcy-scale reaction kinetics observed in both uniform and converging flows, opening new perspectives for upscaling reactive transport in porous media.

Oscillatory kinetics coupled to diffusion can produce traveling waves as observed in physical, chemical, and biological systems. We show experimentally that the properties of such waves can be controlled by fluid stretching and compression in a hyperbolic flow. Localized packet waves consisting in a train of parallel waves can develop due to a balance between diffusive broadening and advective compression along the unstable manifold. At a given distance from the stagnation point, the parallel waves transform into planelike waves and smeared waves where the transverse parabolic flow profile disturbs the patterns in the gap width. Once a wave packet has been obtained, it imprints a privileged direction that is maintained even if the compression rate is decreased. The width of the wave packet then scales inversely with the compression rate.

Mixing describes the process by which solutes evolve from an initial heterogeneous state to uniformity under the stirring action of a fluid flow. Fluid stretching forms thin scalar lamellae that coalesce due to molecular diffusion. Owing to the linearity of the advection–diffusion equation, coalescence can be envisioned as an aggregation process. Here, we demonstrate that in smooth two-dimensional chaotic flows, mixing obeys a correlated aggregation process, where the spatial distribution of the number of lamellae in aggregates is highly correlated with their elongation, and is set by the fractal properties of the advected material lines. We show that the presence of correlations makes mixing less efficient than a completely random aggregation process because lamellae with similar elongations and scalar levels tend to remain isolated from each other. We show that correlated aggregation is uniquely determined by a single exponent that quantifies the effective number of random aggregation events. These findings expand aggregation theories to a larger class of systems, which have relevance to various fundamental and applied mixing problems.

Characterizing the spatiotemporal variability of water fluxes at the stream‐groundwater interface is extremely challenging due to the lack of methods for estimating hyporheic flows at different scales. To address this, we demonstrate the potential of Active‐Distributed Temperature Sensing (DTS) methods for measuring and mapping hyporheic flow in a lowland stream. Experiments were conducted by burying a few hundred meters of heatable Fiber‐Optic cables within streambed sediments in a large meander, where permanent stream‐losing conditions are observed along the stream reach. We propose a new methodology to filter ambient temperature variations along the heated section of the DTS cable and to extend the application of Active‐DTS to losing streams. After data processing, the results show that, along lateral and longitudinal stream profiles, both thermal conductivity and water flux values follow normal distributions with relatively small standard deviations. Hyporheic fluxes vary by one order of magnitude. The absence of correlation between water fluxes within the hyporheic zone and streambed topography variations suggests that the variability is mainly controlled by local streambed heterogeneities. This means that the spatiotemporal variability of fluxes may be used as a marker of the variability of streambed hydraulic conductivities. The relatively low spatial variability (one order of magnitude) in hyporheic flow suggests a small variability of streambed properties. This is an important result for calibrating models assessing hyporheic processes, in which the hydraulic conductivity distribution is generally assumed. Additionally, measurements made over three years yield similar estimates showing the remarkable stability of hyporheic flows through time.

Background Ultra-trail running races pose appreciable physiological challenges, particularly for glucose metabolism. Previous studies that yielded divergent results only measured glycaemia at isolated times. Objectives We aimed to explore the impact of an ultra-endurance race on continuously measured glycaemia and to understand potential physiological mechanisms, as well as the consequences for performance and behavioural alertness. Methods Fifty-five athletes (78% men, 43.7 ± 9.6 years) ran a 156-km ultra-trail race (six 26-km laps, total elevation 6000 m). Participants wore a masked continuous glucose monitoring sensor from the day before the race until 10 days post-race. Blood was taken at rest, during refuelling stops after each lap, and after 24-h recovery. Running intensity (% heart rate reserve), performance (lap times), psychological stress, and behavioural alertness were explored. Linear mixed models and logistic regressions were carried out. Results No higher risk of hypo- or hyperglycaemia was observed during the exercise phases of the race (i.e. excluding stops for scientific measurements and refuelling) compared with resting values. Laps comprising a greater proportion of time spent at maximal aerobic intensity were nevertheless associated with more time > 180 mg/dL (P = 0.021). A major risk of hyperglycaemia appeared during the 48-h post-race period compared with pre-race (P < 0.05), with 31.9% of the participants spending time with values > 180 mg/dL during recovery versus 5.5% during resting. Changes in circulating insulin, cortisol, and free fatty acids followed profiles comparable with those usually observed during traditional aerobic exercise. However, creatine phosphokinase, and to a lesser extent lactate dehydrogenase, increased exponentially during the race (P < 0.001) and remained high at 24-h post-race (P < 0.001; respectively 43.6 and 1.8 times higher vs. resting). Glycaemic metrics did not influence physical performance or behavioural alertness. Conclusion Ultra-endurance athletes were exposed to hyperglycaemia during the 48-h post-race period, possibly linked to muscle damage and inflammation. Strategies to mitigate muscle damage or subsequent inflammation before or after ultra-trail races could limit recovery hyperglycaemia and hence its related adverse health consequences. Trial Registration Number NCT05538442 2022–09-21 retrospectively registered. Graphical abstract