The Experts below are selected from a list of 51264 Experts worldwide ranked by ideXlab platform
Davide Donadio - One of the best experts on this subject based on the ideXlab platform.
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structure and dynamics of the quasi liquid layer at the surface of ice from molecular simulations
Journal of Physical Chemistry C, 2018Co-Authors: Tanja Kling, Felix Kling, Davide DonadioAbstract:We characterized the structural and dynamical properties of the quasi-liquid layer (QLL) at the surface of ice by molecular dynamics simulations with a thermodynamically consistent water model. Our simulations show that for three low-index ice surfaces, only the outermost molecular layer presents short-range and midrange disorder and is diffusive. The onset temperature for Normal Diffusion is much higher than the glass temperature of supercooled water, although the diffusivity of the QLL is higher than that of bulk water at the corresponding temperature. The underlying subsurface layers impose an ordered template, which produces a regular patterning of the ice/water interface at any temperature and is responsible for the major differences between QLL and bulk water, especially for what concerns the dynamics and the midrange structure of the hydrogen-bonded network. Our work highlights the need for a holistic approach to the characterization of QLL, as a single experimental technique may probe only one spe...
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structure and dynamics of the quasi liquid layer at the surface of ice from molecular simulations
The Journal of Physical Chemistry, 2018Co-Authors: Tanja Kling, Felix Kling, Davide DonadioAbstract:We characterized the structural and dynamical properties of the quasi-liquid layer (QLL) at the surface of ice by molecular dynamics simulations with a thermodynamically consistent water model. Our simulations show that for three low-index ice surfaces, only the outermost molecular layer presents short-range and midrange disorder and is diffusive. The onset temperature for Normal Diffusion is much higher than the glass temperature of supercooled water, although the diffusivity of the QLL is higher than that of bulk water at the corresponding temperature. The underlying subsurface layers impose an ordered template, which produces a regular patterning of the ice/water interface at any temperature and is responsible for the major differences between QLL and bulk water, especially for what concerns the dynamics and the midrange structure of the hydrogen-bonded network. Our work highlights the need for a holistic approach to the characterization of QLL, as a single experimental technique may probe only one specific feature, missing part of the complexity of this fascinating system.
Ralf Metzler - One of the best experts on this subject based on the ideXlab platform.
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probability density of the fractional langevin equation with reflecting walls
Physical Review E, 2019Co-Authors: Thomas Vojta, Sarah Skinner, Ralf MetzlerAbstract:We investigate anomalous Diffusion processes governed by the fractional Langevin equation and confined to a finite or semi-infinite interval by reflecting potential barriers. As the random and damping forces in the fractional Langevin equation fulfill the appropriate fluctuation-dissipation relation, the probability density on a finite interval converges for long times towards the expected uniform distribution prescribed by thermal equilibrium. In contrast, on a semi-infinite interval with a reflecting wall at the origin, the probability density shows pronounced deviations from the Gaussian behavior observed for Normal Diffusion. If the correlations of the random force are persistent (positive), particles accumulate at the reflecting wall while antipersistent (negative) correlations lead to a depletion of particles near the wall. We compare and contrast these results with the strong accumulation and depletion effects recently observed for nonthermal fractional Brownian motion with reflecting walls, and we discuss broader implications.
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crossover from anomalous to Normal Diffusion truncated power law noise correlations and applications to dynamics in lipid bilayers
arXiv: Statistical Mechanics, 2018Co-Authors: Daniel Molinagarcia, Trifce Sandev, Hadiseh Safdari, Gianni Pagnini, Aleksei V Chechkin, Ralf MetzlerAbstract:The emerging diffusive dynamics in many complex systems shows a characteristic crossover behaviour from anomalous to Normal Diffusion which is otherwise fitted by two independent power-laws. A prominent example for a subdiffusive-diffusive crossover are viscoelastic systems such as lipid bilayer membranes, while superdiffusive-diffusive crossovers occur in systems of actively moving biological cells. We here consider the general dynamics of a stochastic particle driven by so-called tempered fractional Gaussian noise, that is noise with Gaussian amplitude and power-law correlations, which are cut off at some mesoscopic time scale. Concretely we consider such noise with built-in exponential or power-law tempering, driving an overdamped Langevin equation (fractional Brownian motion) and fractional Langevin equation motion. We derive explicit expressions for the mean squared displacement and correlation functions, including different shapes of the crossover behaviour depending on the concrete tempering, and discuss the physical meaning of the tempering. In the case of power-law tempering we also find a crossover behaviour from faster to slower superDiffusion and slower to faster subDiffusion. As a direct application of our model we demonstrate that the obtained dynamics quantitatively described the subDiffusion-Diffusion and subDiffusion-subDiffusion crossover in lipid bilayer systems. We also show that a model of tempered fractional Brownian motion recently proposed by Sabzikar and Meerschaert leads to physically very different behaviour with a seemingly paradoxical ballistic long time scaling.
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anomalous Diffusion and power law relaxation of the time averaged mean squared displacement in worm like micellar solutions
New Journal of Physics, 2013Co-Authors: Jaehyung Jeon, Ralf Metzler, Natascha Leijnse, Lene B OddershedeAbstract:We report the results of single tracer particle tracking by optical tweezers and video microscopy in micellar solutions. From careful analysis in terms of different stochastic models, we show that the polystyrene tracer beads of size 0.52-2.5µm after short-time Normal Diffusion turn over to perform anomalous Diffusion of the form hr 2 (t)i ' t with 0.3. This free anomalous Diffusion is ergodic and consistent with a description in terms of the generalized Langevin equation with a power-law memory kernel. With optical tweezers tracking, we unveil a power-law relaxation over several decades in time to the thermal plateau value under the confinement of the harmonic tweezer potential, as predicted previously (Phys. Rev. E 85 021147 (2012)). After the subdiffusive motion in the millisecond range, the motion becomes faster and turns either back to Normal Brownian Diffusion or to even faster superDiffusion, depending on the size of the tracer beads.
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anomalous and Normal Diffusion of proteins and lipids in crowded lipid membranes
Faraday Discussions, 2013Co-Authors: Matti Javanainen, Ralf Metzler, Henrik Hammaren, Luca Monticelli, Jaehyung Jeon, Markus S Miettinen, Hector Martinezseara, Ilpo VattulainenAbstract:Lateral Diffusion plays a crucial role in numerous processes that take place in cell membranes, yet it is quite poorly understood in native membranes characterized by, e.g., domain formation and large concentration of proteins. In this article, we use atomistic and coarse-grained simulations to consider how packing of membranes and crowding with proteins affect the lateral dynamics of lipids and membrane proteins. We find that both packing and protein crowding have a profound effect on lateral Diffusion, slowing it down. Anomalous Diffusion is observed to be an inherent property in both protein-free and protein-rich membranes, and the time scales of anomalous Diffusion and the exponent associated with anomalous Diffusion are found to strongly depend on packing and crowding. Crowding with proteins also has a striking effect on the decay rate of dynamical correlations associated with lateral single-particle motion, as the transition from anomalous to Normal Diffusion is found to take place at macroscopic time scales: while in protein-poor conditions Normal Diffusion is typically observed in hundreds of nanoseconds, in protein-rich conditions the onset of Normal Diffusion is tens of microseconds, and in the most crowded systems as large as milliseconds. The computational challenge which results from these time scales is not easy to deal with, not even in coarse-grained simulations. We also briefly discuss the physical limits of protein motion. Our results suggest that protein concentration is anything but constant in the plane of cell membranes. Instead, it is strongly dependent on proteins' preference for aggregation.
Jacqueline H Chen - One of the best experts on this subject based on the ideXlab platform.
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a direct numerical simulation study of flame structure and stabilization of an experimental high ka ch4 air premixed jet flame
Combustion and Flame, 2017Co-Authors: Haiou Wang, Evatt R Hawkes, Jacqueline H ChenAbstract:Abstract In the present work, a direct numerical simulation (DNS) of an experimental high Karlovitz number (Ka) CH4/air piloted premixed flame was analyzed to study the inner structure and the stabilization mechanism of the turbulent flame. A reduced chemical mechanism for premixed CH4/air combustion with NOx based on GRI-Mech3.0 was used, including 268 elementary reactions and 28 transported species. The evolution of the stretch factor, I0, indicates that the burning rate per unit flame surface area is considerably reduced in the near field and exhibits a minimum at x/D = 8. Downstream, the burning rate gradually increases. The stretch factor is different between different species, suggesting the quenching of some reactions but not others. Comparison between the turbulent flame and strained laminar flames indicates that certain aspects of the mean flame structure can be represented surprisingly well by flamelets if changes in boundary conditions are accounted for and the strain rate of the mean flow is employed; however, the thickening of the flame due to turbulence is not captured. The spatial development of displacement speeds is studied at higher Ka than previous DNS. In contrast to almost all previous studies, the mean displacement speed conditioned on the flame front is negative in the near field, and the dominant contribution to the displacement speed is Normal Diffusion with the reaction contribution being secondary. Further downstream, reaction overtakes Normal Diffusion, contributing to a positive displacement speed. The negative displacement speed in the near field implies that the flame front situates itself in the pilot region where the inner structure of the turbulent flame is affected significantly, and the flame stabilizes in balance with the inward flow. Notably, in the upstream region of the turbulent flame, the main reaction contributing to the production of OH, H+O2⇔O+OH (R35), is weak. Moreover, oxidation reactions, H2+OH⇔H+H2O (R79) and CO+OH⇔CO2+H (R94), are influenced by H2O and CO2 from the pilot and are completely quenched. Hence, the entire radical pool of OH, H and O is affected. However, the fuel consumption layer remains comparably active and generates heat, mainly via the reaction CH4+OH⇔CH3+H2O (R93).
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analysis of the contribution of curvature to premixed flame propagation
Combustion and Flame, 1999Co-Authors: Tarek Echekki, Jacqueline H ChenAbstract:In this analysis, the authors attempt to identify the contributions of curvature to the displacement speed by using the governing transport equation for the deficient reactant. They show that the displacement speed is a balance of three components: reaction, Normal Diffusion, and curvature. The contribution of the three components is then evaluated using results from direct numerical simulations (DNS) of an unsteady stoichiometric methane-air flame embedded in a 2-D vortical flow field.
Eiji Yamamoto - One of the best experts on this subject based on the ideXlab platform.
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langevin equation with fluctuating diffusivity a two state model
Physical Review E, 2016Co-Authors: Tomoshige Miyaguchi, Takuma Akimoto, Eiji YamamotoAbstract:Recently, anomalous subDiffusion, aging, and scatter of the Diffusion coefficient have been reported in many single-particle-tracking experiments, though the origins of these behaviors are still elusive. Here, as a model to describe such phenomena, we investigate a Langevin equation with diffusivity fluctuating between a fast and a slow state. Namely, the diffusivity follows a dichotomous stochastic process. We assume that the sojourn time distributions of these two states are given by power laws. It is shown that, for a nonequilibrium ensemble, the ensemble-averaged mean-square displacement (MSD) shows transient subDiffusion. In contrast, the time-averaged MSD shows Normal Diffusion, but an effective Diffusion coefficient transiently shows aging behavior. The propagator is non-Gaussian for short time and converges to a Gaussian distribution in a long-time limit; this convergence to Gaussian is extremely slow for some parameter values. For equilibrium ensembles, both ensemble-averaged and time-averaged MSDs show only Normal Diffusion and thus we cannot detect any traces of the fluctuating diffusivity with these MSDs. Therefore, as an alternative approach to characterizing the fluctuating diffusivity, the relative standard deviation (RSD) of the time-averaged MSD is utilized and it is shown that the RSD exhibits slow relaxation as a signature of the long-time correlation in the fluctuating diffusivity. Furthermore, it is shown that the RSD is related to a non-Gaussian parameter of the propagator. To obtain these theoretical results, we develop a two-state renewal theory as an analytical tool.
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distributional behaviors of time averaged observables in the langevin equation with fluctuating diffusivity Normal Diffusion but anomalous fluctuations
Physical Review E, 2016Co-Authors: Takuma Akimoto, Eiji YamamotoAbstract:We consider the Langevin equation with dichotomously fluctuating diffusivity, where the Diffusion coefficient changes dichotomously over time, in order to study fluctuations of time-averaged observables in temporally heterogeneous Diffusion processes. We find that the time-averaged mean-square displacement (TMSD) can be represented by the occupation time of a state in the asymptotic limit of the measurement time and hence occupation time statistics is a powerful tool for calculating the TMSD in the model. We show that the TMSD increases linearly with time (Normal Diffusion) but the time-averaged Diffusion coefficients are intrinsically random when the mean sojourn time for one of the states diverges, i.e., intrinsic nonequilibrium processes. Thus, we find that temporally heterogeneous environments provide anomalous fluctuations of time-averaged diffusivity, which have relevance to large fluctuations of the Diffusion coefficients obtained by single-particle-tracking trajectories in experiments.
Haiou Wang - One of the best experts on this subject based on the ideXlab platform.
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a direct numerical simulation study of flame structure and stabilization of an experimental high ka ch4 air premixed jet flame
Combustion and Flame, 2017Co-Authors: Haiou Wang, Evatt R Hawkes, Jacqueline H ChenAbstract:Abstract In the present work, a direct numerical simulation (DNS) of an experimental high Karlovitz number (Ka) CH4/air piloted premixed flame was analyzed to study the inner structure and the stabilization mechanism of the turbulent flame. A reduced chemical mechanism for premixed CH4/air combustion with NOx based on GRI-Mech3.0 was used, including 268 elementary reactions and 28 transported species. The evolution of the stretch factor, I0, indicates that the burning rate per unit flame surface area is considerably reduced in the near field and exhibits a minimum at x/D = 8. Downstream, the burning rate gradually increases. The stretch factor is different between different species, suggesting the quenching of some reactions but not others. Comparison between the turbulent flame and strained laminar flames indicates that certain aspects of the mean flame structure can be represented surprisingly well by flamelets if changes in boundary conditions are accounted for and the strain rate of the mean flow is employed; however, the thickening of the flame due to turbulence is not captured. The spatial development of displacement speeds is studied at higher Ka than previous DNS. In contrast to almost all previous studies, the mean displacement speed conditioned on the flame front is negative in the near field, and the dominant contribution to the displacement speed is Normal Diffusion with the reaction contribution being secondary. Further downstream, reaction overtakes Normal Diffusion, contributing to a positive displacement speed. The negative displacement speed in the near field implies that the flame front situates itself in the pilot region where the inner structure of the turbulent flame is affected significantly, and the flame stabilizes in balance with the inward flow. Notably, in the upstream region of the turbulent flame, the main reaction contributing to the production of OH, H+O2⇔O+OH (R35), is weak. Moreover, oxidation reactions, H2+OH⇔H+H2O (R79) and CO+OH⇔CO2+H (R94), are influenced by H2O and CO2 from the pilot and are completely quenched. Hence, the entire radical pool of OH, H and O is affected. However, the fuel consumption layer remains comparably active and generates heat, mainly via the reaction CH4+OH⇔CH3+H2O (R93).
