Reaction Time Scale Research Articles

Does urea modify microheterogeneous nature of ionic amide deep eutectics? Clues from non-reactive and reactive solute-centered dynamics

The impact of urea on the microheterogeneous nature of acetamide + LiNO3 deep eutectic solvents (DESs) was investigated via following the viscosity coupling of the average timescales of solute rotation, solvation and cis–trans isomerization reaction occurring in such media. Picosecond resolved fluorescence measurements were carried out to access these average relaxation timescales and the solutes employed were the non-reactive probe, coumarin153 (C153), and a reactive probe, trans- 2-[4′-(Dimethylamino) styryl]benzothiazole (DMASBT). DMASBT undergoes fast cis–trans isomerization reaction upon photo-excitation. The coupling to the solution viscosity (η) of the experimentally determined average timescales was then analyzed via the relation, 〈τx〉∝ηp, x denoting rotation, solvation or reaction, and a fractional p considered as solution microheterogeneity reflector. A comparison of the findings between the urea containing and non-urea ionic DESs revealed that addition of urea did not alter the p value for C153 rotation and DMASBT reaction times, while the C153 average solvation times showed a stronger decoupling. This stronger decoupling has been ascribed to the faster rate of solvation upon addition of urea via the participation of the amide group through the collective H-bond excitations. Addition of electrolyte at a fixed amide concentration, on the other hand, makes the fractional viscosity dependence more pronounced for both solvation and rotational dynamics, although the viscosity coupling (p value) of isomerization reaction timescale remains unchanged. DMASBT reveals, through steady state fluorescence measurements, stronger spatial heterogeneity in these solutions than that by C153, and DMASBT better senses the urea or electrolyte induced modulation of spatial heterogeneity.

Survival of polycyclic aromatic hydrocarbon knockout fragments in the interstellar medium

Laboratory studies play a crucial role in understanding the chemical nature of the interstellar medium (ISM), but the disconnect between experimental timescales and the timescales of reactions in space can make a direct comparison between observations, laboratory, and model results difficult. Here we study the survival of reactive fragments of the polycyclic aromatic hydrocarbon (PAH) coronene, where individual C atoms have been knocked out of the molecules in hard collisions with He atoms at stellar wind and supernova shockwave velocities. Ionic fragments are stored in the DESIREE cryogenic ion-beam storage ring where we investigate their decay for up to one second. After 10 ms the initially hot stored ions have cooled enough so that spontaneous dissociation no longer takes place at a measurable rate; a majority of the fragments remain intact and will continue to do so indefinitely in isolation. Our findings show that defective PAHs formed in energetic collisions with heavy particles may survive at thermal equilibrium in the interstellar medium indefinitely, and could play an important role in the chemistry in there, due to their increased reactivity compared to intact or photo-fragmented PAHs.

Time-Resolved Local pH Measurements during CO2 Reduction Using Scanning Electrochemical Microscopy: Buffering and Tip Effects

The electrochemical reduction of CO2 is widely studied as a sustainable alternative for the production of fuels and chemicals. The electrolyte’s bulk pH and composition play an important role in the reaction activity and selectivity and can affect the extent of the buildup of pH gradients between the electrode surface and the bulk of the electrolyte. Quantifying the local pH and how it is affected by the solution species is desirable to gain a better understanding of the CO2 reduction reaction. Local pH measurements can be realized using Scanning Electrochemical Microscopy (SECM); however, finding a pH probe that is stable and selective under CO2 reduction reaction conditions is challenging. Here, we have used our recently developed voltammetric pH sensor to perform pH measurements in the diffusion layer during CO2 reduction using SECM, with high time resolution. Using a 4-hydroxylaminothiophenol (4-HATP)/4-nitrosothiophenol (4-NSTP) functionalized gold ultramicroelectrode, we compare the local pH developed above a gold substrate in an argon atmosphere, when only hydrogen evolution is taking place, to the pH developed in a CO2 atmosphere. The pH is monitored at a fixed distance from the surface, and the sample potential is varied in time. In argon, we observe a gradual increase of pH, while a plateau region is present in CO2 atmosphere due to the formation of HCO3– buffering the reaction interface. By analyzing the diffusion layer dynamics once the sample reaction is turned “off”, we gain insightful information on the time scale of the homogeneous reactions happening in solution and on the time required for the diffusion layer to fully recover to the initial bulk concentration of species. In order to account for the effect of the presence of the SECM tip on the measured pH, we performed finite element method simulations of the fluid and reaction dynamics. The results show the significant localized diffusion hindrance caused by the tip, so that in its absence, the pH values are more acidic than when the tip is present. Nonetheless, through the simulation, we can account for this effect and estimate the real local pH values across the diffusion layer.

Simplified modeling combustion chemistry of neat and blended large hydrocarbon fuels with different functional groups

In the present study, a simplified modeling approach based on a two-step reaction scheme (TSRS) was developed to describe the high-temperature combustion chemistry of large hydrocarbon fuels. The characteristic reaction time scale analysis indicated a distinct two-step reaction property for the conversion of large hydrocarbon molecules. Along with the quasi-steady state assumption, TSRS utilized a seven-reaction sub-model to describe the fuel decomposition to a small group of intermediate products, and a detailed foundational fuel combustion chemistry model to describe the oxidation of these decomposed products. In the first half of this paper, the TSRS model was constructed for four fuel components with different functional groups, including n-heptane, iso-octane, methylcyclohexane, and n-butylbenzene, and validated sequentially by their speciation data, and global combustion properties of ignition delay time and laminar flame speed data. Sensitivity analysis and rate of production analysis revealed the primary decomposition paths in the TSRS model. Then, in the second half of the paper, algebraically blending the TSRS models of neat fuel components enabled the construction of surrogate fuel models for real fuels, including a two-component primary reference fuel (PRF), a three-component gasoline surrogate fuel (toluene-PRF), a four-component Chinese jet fuel (RP-3) surrogate, and a four-component diesel surrogate. Predictions from the TSRS-based surrogate models for real fuels were shown to be consistent with the speciation data as well as the global combustion properties, whenever available in the literatures, indicating the robustness and broad applicability of the TSRS approach.

A New Strategy to Regulate the Selectivity of Photo-Mediated Catalytic Reaction.

Here we developed a new method for regulating the selectivity of photo-mediated catalytic reaction by manipulating the surface charge of Au/TiO2 (gold/titanium dioxide) catalysts within chemical reaction timescales. Two kinds of photocatalytic reactions, hydrogenation of acetophenone and benzyl alcohol oxidation, have been applied to investigate the photocatalytic performance over Au/TiO2 catalysts with tunable surface charges. We found that a suitable timescale of switching surface charge on Au would benefit for the enhanced quantum efficiency and play different roles in the selectivity of desired products in hydrogenation and oxidation reactions. Au/TiO2 catalyst under 5 μs flashing light irradiation exhibits much higher selectivity of 1-phenylethanol in the hydrogenation of acetophenone than that under continuous light and 5 s flashing light irradiation; by contrast, Au/TiO2 catalysts under both flashing light and continuous light irradiation exhibit a similar selectivity of benzaldehyde in benzyl alcohol oxidation. Our findings will benefit for a better understanding of electronic structure–mediated reaction mechanism and be helpful for achieving highly efficient photocatalytic systems.

Investigation of Nucleation and Growth at a Liquid-Liquid Interface by Solvent Exchange and Synchrotron Small-Angle X-Ray Scattering.

Hybrid nanomaterials possess complex architectures that are driven by a self-assembly process between an inorganic element and an organic ligand. The properties of these materials can often be tuned by organic ligand variation, or by swapping the inorganic element. This enables the flexible fabrication of tailored hybrid materials with a rich variety of properties for technological applications. Liquid-liquid interfaces are useful for synthesizing these compounds as precursors can be segregated and allowed to interact only at the interface. Although procedurally straightforward, this is a complex reaction in an environment that is not easy to probe. Here, we explore the interfacial crystallization of mithrene, a supramolecular multi-quantum well. This material sandwiches a well-defined silver-chalcogenide layer between layers of organic ligands. Controlling mithrene crystal size and morphology to be useful for applications requires understanding details of its crystal growth, but the specific mechanism for this reaction remain only lightly investigated. We performed a study of mithrene crystallization at an oil-water interfaces to elucidate how the interfacial free energy affects nucleation and growth. We exchanged the oil solvent on the basis of solvent viscosity and surface tension, modifying the dynamic contact angle and interfacial free energy. We isolated and characterized the reaction byproducts via scanning electron microscopy (SEM). We also developed a high-throughput small angle X-ray scattering (SAXS) technique to measure crystallization at short reaction timescales (minutes). Our results showed that modifying interfacial surface energy affects both the reaction kinetics and product size homogeneity and yield. Our SAXS measurements reveal the onset of crystallinity after only 15 min. These results provide a template for exploring directed synthesis of complex materials via experimental methods.

Assessing Clinical Change in Individuals Exposed to Repetitive Head Impacts: The Repetitive Head Impact Composite Index.

Background: There is a current lack of any composite measure for the effective tracking and monitoring of clinical change in individuals exposed to repetitive head impacts (RHI). The aim of this study is to create a composite instrument for the purposes of detecting change over time in cognitive and behavioral function in individuals exposed to RHI.Methods: The data to derive the composite instrument came from the Professional Fighters Brain Health Study (PFBHS), a longitudinal study of active and retired professional fighters [boxers and mixed martial arts (MMA) fighters] and healthy controls. Participants in the PFBHS underwent assessment on an annual basis that included computerized cognitive testing and behavioral questionnaires. Multivariate logistic regression models were employed to compare active fighters (n = 117) with controls (n = 22), and retired fighters (n = 26) with controls to identify the predictors that could be used to differentiate the groups over time. In a second step, linear discriminant analysis was performed to derive the linear discriminant coefficients for the three groups by using the predictors from the two separate logistic regression models.Results: The composite scale is a weighted linear value of 12 standardized scores consisting of both current and yearly change scores in domains including: processing speed, choice reaction time, semantic fluency, letter fluency, and Barrett Impulsiveness Scale. Because the weighting of values differed between active and retired fighters, two versions emerged. The mean and standard deviation ratio (MSDR) showed that the new index had better sensitivity compared to the individual measures, with the ratio of MSDR of the new index to that of the existing measures of at least 1.84.Conclusion: With the increasing need for tools to follow individuals exposed to RHI and the potential of clinical trials on the horizon for CTE, the RHICI is poised to serve as an initial approach to a composite clinical measure.

Incorporating Improved Chemical and Electrochemical Reaction Schemes in Electrochemical Engineering Models for Lithium Sulfur Batteries

Lithium sulfur batteries have gained popularity due to the promise of high energy density, predicted to be 2-3 times than of lithium ion batteries.1 The challenges that limit commercial success for lithium sulfur batteries stem from the poor cyclability of lithium metal, insulating sulfur products, and the polysulfide shuttle mechanism where soluble sulfur species react with the lithium metal anode, resulting in permanent active material loss.2 The complexity of the challenges facing the adoption of lithium sulfur batteries has led to a focus on electrolyte engineering, additives, and anode coatings. The complex reaction pathway is sensitive to changes in electrolyte and additives, which can alter the shape of the voltage curve and even capacity.3,4 Understanding the effects of different electrolyte systems is important to characterizing the performance. Various techniques like UV-vis and Raman spectroscopy are useful to identifying the speciation. The complex speciation pathway has shown to include dissociation and disproportionation reactions and stabilization of radical species through UV-vis and Raman spectroscopy.5–10 Physics-based modeling can help elucidate underlying phenomena within batteries. Kumaresan and White11 developed a one-dimensional discharge model for lithium sulfur batteries. The model utilized included 5-step reduction scheme with Sn 2- species (n=8,6,4,2,1). The subsequent models that were developed used a similar reaction scheme or a reduced reaction scheme with only 2 or 3 reactions. The chemistry has not been updated to include the new insights available from spectroscopy work. In this work, the following homogeneous dissociation reaction that has been identified in many sulfur electrochemical systems12–17 is added to the reaction schemeS6 2- ↔ S3 ·- (1)The S6 2- species balance is modified to include a sink for the equilibrium reaction according to reaction (1). The material balance for S3 ·- radical species is added with the only source as the equilibrium reaction; the S3 ·- species is assumed to be electrochemically inactive. When there is significant S3 ·- species, the S6 2- species essentially becomes trapped as the S3 ·- radical species, removing it from the reduction scheme. When the reaction is pushed farther to S3 ·- radical species, the electrolyte system favors the stabilization of this radical species over the S6 2- species. The forward rate of reaction (1) sets the reaction timescale, and the equilibrium constant determines the ratio of S3 ·- and S6 2-. The resulting interplay of these dynamics on the electrochemical behavior of the battery cell will be explored, and the implications on electrolyte engineering will be discussed.Acknowledgments: The authors are thankful to the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy through the Advanced Battery Materials Research (BMR) Program (Battery500 Consortium) for funding of this work.

CRISPR Enzyme Kinetics for Molecular Diagnostics.

CRISPR-diagnostic assays have gained significant interest in the last few years. This interest has grown rapidly during the current COVID-19 pandemic, where CRISPR-diagnostics have been frontline contenders for rapid testing solutions. This surge in CRISPR-diagnostic research prompts the following question: what exactly are the achievable limits of detection and associated assay times enabled by the kinetics of enzymes such as Cas12 and Cas13? To explore this question, we here present a model based on Michaelis-Menten enzyme kinetics theory applied to CRISPR enzymes. We use the model to develop analytical solutions for reaction kinetics and develop back-of-the-envelope criteria to validate and check for consistency in reported enzyme kinetic parameters. We applied our analyses to all studies known to us, which report Michaelis-Menten-type kinetic data for CRISPR-associated enzymes. These studies include all subtypes of Cas12 and Cas13 and orthologs. We found all but one study clearly violate at least two of our three rules and therefore present data that violate basic physical limits. We performed an experimental study of reaction kinetics of LbCas12a with both ssDNA and dsDNA activators and use these data to validate our model and its predicted scaling. The validated model is used to explore CRISPR reaction time scales and the degree of reaction completion for practically relevant target concentrations applicable to CRISPR-diagnostic assays. The results have broad implications for achievable limits of detection and assay times of emerging, amplification-free CRISPR-detection methods.

Hematite Photoanodes for Water Oxidation: Electronic Transitions, Carrier Dynamics, and Surface Energetics

AbstractWe review the current understanding of charge carriers in model hematite photoanodes at different stages. The origin of charge carriers is discussed based on the electronic structure and absorption features, highlighting the controversial assignment of the electronic transitions near the absorption edge. Next, the dynamic evolution of charge carriers is analyzed both on the ultrafast and on the surface reaction timescales, with special emphasis on the arguable spectroscopic assignment of electrons/holes and their kinetics. Further, the competitive charge transfer centers at the solid–liquid interface are reviewed, and the chemical nature of relevant surface states is updated. Finally, an overview on the function of widely employed surface cocatalysts is given to illustrate the complex influence of physiochemical modifications on the charge carrier dynamics. The understanding of charge carriers from their origin all the way to their interfacial transfer is vital for the future of photoanode design.

Time-scales to reach chemical equilibrium in ices at snowline distance around compact objects: the influence of accretion mass in the central object

ABSTRACT In this work, we analyse soft X-ray emission due to mass accretion on to compact stars and its effects on the time-scale to reach chemical equilibrium of eventual surrounding astrophysical ices exposed to that radiation. Reaction time-scales due to soft X-ray in water-rich and pure ices of methanol, acetone, acetonitrile, formic acid, and acetic acid were determined. For accretion rates in the range $\dot{m}=10^{-12}\!-\!10^{-8}\,{\rm M}_\odot$ yr−1 and distances in the range 1–3 LY from the central compact objects, the time-scales lie in the range 10–108 yr, with shorter time-scales corresponding to higher accretion rates. Obtained time-scales for ices at snow-line distances can be small when compared to the lifetime (or age) of the compact stars, showing that chemical equilibrium could have been achieved. Time-scales for ices to reach chemical equilibrium depend on X-ray flux and, hence, on accretion rate, which indicates that systems with low accretion rates may not have reached chemical equilibrium.

The Effect of Pressure on NOx Entitlement and Reaction Timescales in a Premixed Axial Jet-In-Crossflow

Abstract This paper investigates the pressure dependency of a lean premixed jet injected into a lean vitiated crossflow with an experimentally verified detailed chemistry computational fluid dynamics (CFD) model and 53 species considered. Experimental data were taken in an axially staged combustor with an optically accessible test section, allowing the use of particle image velocimetry (PIV) and CH* chemiluminescence techniques as well as point measurement of species concentration, temperature, and pressure. The experimental data cases at one, three, and five atmospheres were selected to describe the flame stabilization dependency on pressure and gain the required knowledge for an extrapolation to engine condition. Simulated exit nitrogen oxide levels were validated with experimental emission data, and a global emission trend for the NO reduction at elevated pressure and constant turbine inlet temperature level was defined. The nitrogen oxide benefit at elevated operating pressure was justified with the significantly smaller flame surface area: the analysis of the simulated spanwise and top-view profiles showed a relatively short receded core flame with nitrogen oxide production in the center at high pressure relative to a longer and larger shear layer flame at atmospheric condition that produced NO toward the inner and outer side of the flame. Decomposition of the Damköhler number revealed the strong influence of the reaction timescales with higher reaction rates at elevated pressure, along with a moderate influence of the turbulent timescales, showing higher turbulence intensity in the lee-side recirculation zone at lower pressure.

A second-order dynamic adaptive hybrid scheme for time-integration of stiff chemistry

A dynamic adaptive hybrid integration (AHI) scheme of second-order accuracy (AHI2) is proposed for time-integration of chemically reacting flows involving stiff chemistry. AHI2 is extended from a first-order AHI method (AHI1) developed in a previous study, which showed that when significant radical sources are present in the non-chemical source terms, splitting the chemical and the transport sub-systems may incur O(1) errors unless the splitting time steps are comparable to or smaller than that required for explicit integration. As such, the transport term needs to be carried during the integration of stiff chemistry to avoid the large splitting errors. In AHI, fast species and reactions that may induce stiffness are treated implicitly, while the non-stiff variables and source terms, including slow reactions and the mixing term, are treated explicitly. The separation of fast-slow chemistry is performed on-the-fly based on analytically evaluated timescales for species and reactions, such that the complexity of the implicit core in the governing equations is minimized at each time step and the time-integration can be performed with high efficiency. The newly developed AHI2 scheme combines the midpoint scheme and the trapezoidal rule to achieve second-order accuracy. The second-order scheme is tested with a toy problem, as well as auto-ignition and unsteady perfectly stirred reactors (PSR) with detailed chemistry. Results show that AHI2 can significantly improve accuracy compared with AHI1. It was further found that AHI2 can accurately predict extinction of unsteady PSRs while the Strang splitting scheme fails to control the error, showing the necessity not to split the chemistry and transport source terms for prediction of extinction or forced-ignition problems involving significant radical sources. Further analysis of numerical efficiency shows that for auto-ignition AHI2 reduces computational cost primarily through the reduction in the number of variables to be solved implicitly, and the time-saving increases with the mechanism size, reaching approximately 70% for the 111-species USC-Mech II compared with a fully implicit scheme. For unsteady PSR involving homogeneous mixing, AHI2 achieved speedup factors of 20 to 30 compared with the Strang splitting scheme. Furthermore, sparse matrix techniques are integrated into AHI2 (AHI2-S) to achieve high computational efficiency. It is shown that the computational cost of AHI2-S is overall linearly proportional to the mechanism size and is comparable to that of evaluating reaction rates using CHEMKIN-II subroutines. It is further shown that AHI2-S achieves a speed-up factor of around two compared with the efficient fully implicit sparse solver LSODES with analytic Jacobian.

Modulation of Riverine Concentration‐Discharge Relationships by Changes in the Shape of the Water Transit Time Distribution

The concentrations of weathering‐derived solutes in rivers and their covariance with discharge are thought to reflect reactive‐transport processes in hillslopes and to reveal the sensitivity of solute fluxes to climatic change. It is expected that discharge‐driven changes in water transit times play some role in setting concentration‐discharge (C‐Q) relationships, but knowledge gaps remain. To explore the specific role of changes in the shape of the transit time distribution with discharge, we combine models to simulate C‐Q relationships for major cations and Si as example solutes with contrasting affinities to partition into secondary phases. The model results are compared with an analysis of C‐Q relationships using the Global River Chemistry Database. We find that changes in the shape of the transit time distribution with discharge can produce a range of cation‐Q and Si‐Q relationships that encompasses most of the range observed in real catchments, including positive Si‐Q relationships and variable cation to Si ratios. We find that C‐Q relationships (characterized by power law exponents) can remain approximately constant, even as the Damköhler Number (ratio of transport time scale to reaction time scale) is varied over 3 orders of magnitude. So, in our model analysis, C‐Q relationships are as sensitive to hydrologic variability as they are to reaction rates. Additionally we find that, depending on the storage‐discharge relationship, changes in rainfall patterns can influence C‐Q relationships. Altogether, our results suggest ways in which C‐Q relationships may be nonstationary in response to climatic change and/or vary in space and time due to catchment hydrologic properties.

Liquid–liquid phase separation driven compartmentalization of reactive nucleoplasm

The nucleus of eukaryotic cells harbors active and out of equilibrium environments conducive to diverse gene regulatory processes. On a molecular scale, gene regulatory processes take place within hierarchically compartmentalized sub-nuclear bodies. While the impact of nuclear structure on gene regulation is widely appreciated, it has remained much less clear whether and how gene regulation is impacting nuclear order itself. Recently, the liquid–liquid phase separation emerged as a fundamental mechanism driving the formation of biomolecular condensates, including membrane-less organelles, chromatin territories, and transcriptional domains. The transience and environmental sensitivity of biomolecular condensation are strongly suggestive of kinetic gene-regulatory control of phase separation. To better understand kinetic aspects controlling biomolecular phase-separation, we have constructed a minimalist model of the reactive nucleoplasm. The model is based on the Cahn–Hilliard formulation of ternary protein–RNA–nucleoplasm components coupled to non-equilibrium and spatially dependent gene expression. We find a broad range of kinetic regimes through an extensive set of simulations where the interplay of phase separation and reactive timescales can generate heterogeneous multi-modal gene expression patterns. Furthermore, the significance of this finding is that heterogeneity of gene expression is linked directly with the heterogeneity of length-scales in phase-separated condensates.

Theoretical Constraints on Fe Reduction Rates in Upland Soils as a Function of Hydroclimatic Conditions

AbstractPeriods of high soil wetness promote anaerobic processes such as iron (Fe) reduction within soil microsites, with implications for organic matter decomposition, the fate of pollutants, and nutrient cycling. Here we discuss potential Fe reduction rates emerging from an interplay between the timescales of the internal reactions (Fe oxidation and reduction) and external forcings (length of oxic vs. anoxic conditions), and under no organic substrate and microbial population limitations. We compute the upper bound on Fe reduction and the theoretical maximum reduction rate, which would be reached under “resonant conditions,” whereby the timescales of external forcings match the internal timescales of the redox reactions. The variability of soil oxygen is then linked to rainfall frequency and intensity through soil moisture dynamics, allowing us to determine the hydroclimatic conditions that generate oxic/anoxic cycles that most favor Fe reduction. These predictions are applied to an aseasonal tropical (Luquillo, Puerto Rico, USA) and a seasonal subtropical (Calhoun, SC, USA) humid forests. We show that the tropical site maintains a high potential for Fe reduction throughout the year, due to rapid and frequent transitions between predicted oxic and anoxic microsite conditions, with a potential to reduce up to 1,800 mmol kg−1 soil of Fe per year, while a less humid and seasonal climate in the subtropical site limits maximum reduction rates to 60 mmol kg−1 year−1. This analysis paves the way for a global identification of hot spots of potential Fe reduction using readily available hydroclimatic observations.

Bayesian decision-making under stress-preserved weighting of prior and likelihood information

A rich body of empirical work has addressed the question of how stress changes the way we memorize, learn, and make high-level decisions in complex scenarios. There is evidence that stress also changes the way we perceive the world, indicating influences on decision-making at lower levels. Surprisingly, as of yet, little research has been conducted in this domain. A few studies suggest that under stress, humans tend to eschew existing knowledge, and instead focus on novel input or information from bottom-up. Decision-making in the perceptual domain has been modeled with Bayesian frameworks. Here, existing knowledge about structures and statistics of our environment is referred to as prior, whereas sensory data are termed likelihood. In this study, we directly assessed whether stress, as induced by the socially evaluated cold pressure task (SECPT), would modulate low-level decisions, specifically the weight given to sensory information, and how people reacted to changes in prior and sensory uncertainty. We found that while the stress-inducing procedure successfully elicited subjective stress ratings aswell as stress relevant physiological paramters, it did not change participants’ average reliance on sensory information. Furthermore, it did not affect participants’ sensitivity to changes in prior and sensory uncertainty, with both groups able to detect it and modulate their behavior accordingly, in a way predicted by Bayesian statistics. Our results suggest that, contrary to our predictions, stress may not directly affect lower-level sensory-motor decisions. We discuss the findings in context of time scales of the stress reaction, linked to different neural and functional consequences.

Strong hydroclimatic controls on vulnerability to subsurface nitrate contamination across Europe

Subsurface contamination due to excessive nutrient surpluses is a persistent and widespread problem in agricultural areas across Europe. The vulnerability of a particular location to pollution from reactive solutes, such as nitrate, is determined by the interplay between hydrologic transport and biogeochemical transformations. Current studies on the controls of subsurface vulnerability do not consider the transient behaviour of transport dynamics in the root zone. Here, using state-of-the-art hydrologic simulations driven by observed hydroclimatic forcing, we demonstrate the strong spatiotemporal heterogeneity of hydrologic transport dynamics and reveal that these dynamics are primarily controlled by the hydroclimatic gradient of the aridity index across Europe. Contrasting the space-time dynamics of transport times with reactive timescales of denitrification in soil indicate that ~75% of the cultivated areas across Europe are potentially vulnerable to nitrate leaching for at least one-third of the year. We find that neglecting the transient nature of transport and reaction timescale results in a great underestimation of the extent of vulnerable regions by almost 50%. Therefore, future vulnerability and risk assessment studies must account for the transient behaviour of transport and biogeochemical transformation processes.

Climbing up the memory staircase: Equatorial zoom-whirl orbits

The nonlinear, or null gravitational memory effect for zoom-whirl orbits around fast spinning Kerr black holes for extreme mass ratio systems has a staircase structure. The latter is characterized by a periodic fast increase in the memory during the whirl phases of the orbit, and near constant levels when the orbit is near apoapsis. We study the contribution of different spherical harmonic modes to this effect, and discuss the relative importance thereof. Because the hereditary memory effect increases in magnitude linearly in the number of periapsis passages on a dynamical time scale, while the oscillatory parent gravitational waves are sourced by an orbit that evolves on a much longer radiation reaction time scale, the magnitude of the former relative to the latter increases with time. We also consider orphan memory and the memory effect for extreme mass ratio inspirals. We then discuss the prospect of detection with the Laser Interferometer Space Antenna (LISA).

Effect of bioclogging on the nitrate source and sink function of a hyporheic zone

Nitrogen dynamic in a hyporheic zone (HZ) is mediated by microbes in the form of biofilms attached to sediment grains. Previous studies have neglected the influence of microbial effects on hyporheic nitrate function despite the fact that biofilm-induced clogging can significantly reduce sediment permeability and affect both residence time distribution and biochemical reaction rates. In this study, to assess how the development of bioclogging affects the nitrate source and sink function of HZs, we established a numerical model by coupling water flow, reactive solute transport, and microbial metabolism to simulate nitrogen cycling in a dune-shaped streambed. The classic Damköhler number (DaO2) with an average reaction timescale was not appropriate to determine hyporheic function for the context of biofilm growth induced streambed permeability and biogeochemical reaction rates changing with time. We proposed a new Damköhler number (DaO2∗) with a correction factor (the ratio between current bacteria and initial bacteria) to identify hyporheic function while accounting for the dynamic process of microbial growth. Our results demonstrated that bioclogging layers with aerobic bacteria aggregation acted as a nitrate source system with DaO2∗ ≫ 1, whereas deeper streambed with negligible microbiota survival severed as a nitrate sink with DaO2∗ < 1 when VO2 = 1 h−1. The threshold value of net nitrate source or sink system was influenced by maximum reaction rate of oxygen, and it was in a general range of 1–50 (the range of VO2 was in 1–3 h−1). Threshold transition times from nitrate source to sink were evaluated from the spectrum of flow paths and had strong spatiotemporal variability due to microbial growth. The threshold times decreased over time and stabilized between 1.0 and 3.7 h (for VO2 = 1 h−1) in a dune-shaped streambed when biofilm layers formed.