TY  - JOUR
A1  - Wang, Wei
A1  - Metzler, Ralf
A1  - Cherstvy, Andrey G.
T1  - Anomalous diffusion, aging, and nonergodicity of scaled Brownian motion with fractional Gaussian noise: overview of related experimental observations and models
JF  - Physical chemistry, chemical physics : PCCP ; a journal of European chemical societies
N2  - How does a systematic time-dependence of the diffusion coefficient D(t) affect the ergodic and statistical characteristics of fractional Brownian motion (FBM)? Here, we answer this question via studying the characteristics of a set of standard statistical quantifiers relevant to single-particle-tracking (SPT) experiments. We examine, for instance, how the behavior of the ensemble- and time-averaged mean-squared displacements-denoted as the standard MSD < x(2)(Delta)> and TAMSD <<(delta(2)(Delta))over bar>> quantifiers-of FBM featuring < x(2) (Delta >> = <<(delta(2)(Delta >)over bar>> proportional to Delta(2H) (where H is the Hurst exponent and Delta is the [lag] time) changes in the presence of a power-law deterministically varying diffusivity D-proportional to(t) proportional to t(alpha-1) -germane to the process of scaled Brownian motion (SBM)-determining the strength of fractional Gaussian noise. The resulting compound "scaled-fractional" Brownian motion or FBM-SBM is found to be nonergodic, with < x(2)(Delta >> proportional to Delta(alpha+)(2H)(-1) and <(delta 2(Delta >) over bar > proportional to Delta(2H). We also detect a stalling behavior of the MSDs for very subdiffusive SBM and FBM, when alpha + 2H - 1 < 0. The distribution of particle displacements for FBM-SBM remains Gaussian, as that for the parent processes of FBM and SBM, in the entire region of scaling exponents (0 < alpha < 2 and 0 < H < 1). The FBM-SBM process is aging in a manner similar to SBM. The velocity autocorrelation function (ACF) of particle increments of FBM-SBM exhibits a dip when the parent FBM process is subdiffusive. Both for sub- and superdiffusive FBM contributions to the FBM-SBM process, the SBM exponent affects the long-time decay exponent of the ACF. Applications of the FBM-SBM-amalgamated process to the analysis of SPT data are discussed. A comparative tabulated overview of recent experimental (mainly SPT) and computational datasets amenable for interpretation in terms of FBM-, SBM-, and FBM-SBM-like models of diffusion culminates the presentation. The statistical aspects of the dynamics of a wide range of biological systems is compared in the table, from nanosized beads in living cells, to chromosomal loci, to water diffusion in the brain, and, finally, to patterns of animal movements.
Y1  - 2022
U6  - https://doi.org/10.1039/d2cp01741e
SN  - 1463-9076
SN  - 1463-9084
VL  - 24
IS  - 31
SP  - 18482
EP  - 18504
PB  - RSC Publ.
CY  - Cambridge
ER  - 
TY  - JOUR
A1  - Jeon, Jae-Hyung
A1  - Monne, Hector Martinez-Seara
A1  - Javanainen, Matti
A1  - Metzler, Ralf
T1  - Anomalous diffusion of phospholipids and cholesterols in a lipid bilayer and its origins
JF  - Physical review letters
N2  - Combining extensive molecular dynamics simulations of lipid bilayer systems of varying chemical compositions with single-trajectory analyses, we systematically elucidate the stochastic nature of the lipid motion. We observe subdiffusion over more than 4 orders of magnitude in time, clearly stretching into the submicrosecond domain. The lipid motion depends on the lipid chemistry, the lipid phase, and especially the presence of cholesterol. We demonstrate that fractional Langevin equation motion universally describes the lipid motion in all phases, including the gel phase, and in the presence of cholesterol. The results underline the relevance of anomalous diffusion in lipid bilayers and the strong effects of the membrane composition.
Y1  - 2012
U6  - https://doi.org/10.1103/PhysRevLett.109.188103
SN  - 0031-9007
VL  - 109
IS  - 18
PB  - American Physical Society
CY  - College Park
ER  - 
TY  - JOUR
A1  - Metzler, Ralf
A1  - Jeon, Jae-Hyung
A1  - Cherstvy, Andrey G.
A1  - Barkai, Eli
T1  - Anomalous diffusion models and their properties: non-stationarity, non-ergodicity, and ageing at the centenary of single particle tracking
JF  - Physical chemistry, chemical physics : a journal of European Chemical Societies
N2  - Modern microscopic techniques following the stochastic motion of labelled tracer particles have uncovered significant deviations from the laws of Brownian motion in a variety of animate and inanimate systems. Such anomalous diffusion can have different physical origins, which can be identified from careful data analysis. In particular, single particle tracking provides the entire trajectory of the traced particle, which allows one to evaluate different observables to quantify the dynamics of the system under observation. We here provide an extensive overview over different popular anomalous diffusion models and their properties. We pay special attention to their ergodic properties, highlighting the fact that in several of these models the long time averaged mean squared displacement shows a distinct disparity to the regular, ensemble averaged mean squared displacement. In these cases, data obtained from time averages cannot be interpreted by the standard theoretical results for the ensemble averages. Here we therefore provide a comparison of the main properties of the time averaged mean squared displacement and its statistical behaviour in terms of the scatter of the amplitudes between the time averages obtained from different trajectories. We especially demonstrate how anomalous dynamics may be identified for systems, which, on first sight, appear to be Brownian. Moreover, we discuss the ergodicity breaking parameters for the different anomalous stochastic processes and showcase the physical origins for the various behaviours. This Perspective is intended as a guidebook for both experimentalists and theorists working on systems, which exhibit anomalous diffusion.
Y1  - 2014
U6  - https://doi.org/10.1039/c4cp03465a
SN  - 1463-9076
SN  - 1463-9084
VL  - 16
IS  - 44
SP  - 24128
EP  - 24164
PB  - Royal Society of Chemistry
CY  - Cambridge
ER  - 
TY  - JOUR
A1  - Metzler, Ralf
A1  - Jeon, Jae-Hyung
A1  - Cherstvy, Andrey G.
A1  - Barkai, Eli
T1  - Anomalous diffusion models and their properties
BT  - non-stationarity, non-ergodicity, and ageing at the centenary of single particle tracking
JF  - physical chemistry, chemical physics : PCCP
N2  - Modern microscopic techniques following the stochastic motion of labelled tracer particles have uncovered significant deviations from the laws of Brownian motion in a variety of animate and inanimate systems. Such anomalous diffusion can have different physical origins, which can be identified from careful data analysis. In particular, single particle tracking provides the entire trajectory of the traced particle, which allows one to evaluate different observables to quantify the dynamics of the system under observation. We here provide an extensive overview over different popular anomalous diffusion models and their properties. We pay special attention to their ergodic properties, highlighting the fact that in several of these models the long time averaged mean squared displacement shows a distinct disparity to the regular, ensemble averaged mean squared displacement. In these cases, data obtained from time averages cannot be interpreted by the standard theoretical results for the ensemble averages. Here we therefore provide a comparison of the main properties of the time averaged mean squared displacement and its statistical behaviour in terms of the scatter of the amplitudes between the time averages obtained from different trajectories. We especially demonstrate how anomalous dynamics may be identified for systems, which, on first sight, appear to be Brownian. Moreover, we discuss the ergodicity breaking parameters for the different anomalous stochastic processes and showcase the physical origins for the various behaviours. This Perspective is intended as a guidebook for both experimentalists and theorists working on systems, which exhibit anomalous diffusion.
KW  - intermittent chaotic systems
KW  - Fokker-Planck equations
KW  - time random-walks
KW  - fluorescence photobleaching recovery
KW  - fluctuation-dissipation theorem
KW  - fractional dynamics approach
KW  - photon-counting statistics
KW  - weak ergodicity breaking
KW  - flight search patterns
KW  - levy flights
Y1  - 2014
U6  - https://doi.org/10.1039/c4cp03465a
SN  - 1463-9076
SN  - 1463-9084
VL  - 2014
IS  - 16
SP  - 24128
EP  - 24164
ER  - 
TY  - JOUR
A1  - Cherstvy, Andrey G.
A1  - Metzler, Ralf
T1  - Anomalous diffusion in time-fluctuating non-stationary diffusivity landscapes
JF  - Physical chemistry, chemical physics : PCCP ; a journal of European chemical societies
N2  - We investigate the ensemble and time averaged mean squared displacements for particle diffusion in a simple model for disordered media by assuming that the local diffusivity is both fluctuating in time and has a deterministic average growth or decay in time. In this study we compare computer simulations of the stochastic Langevin equation for this random diffusion process with analytical results. We explore the regimes of normal Brownian motion as well as anomalous diffusion in the sub- and superdiffusive regimes. We also consider effects of the inertial term on the particle motion. The investigation of the resulting diffusion is performed for unconfined and confined motion.
Y1  - 2016
U6  - https://doi.org/10.1039/C6CP03101C
SN  - 1463-9084
SN  - 1463-9076
VL  - 18
SP  - 23840
EP  - 23852
PB  - RSC Publ.
CY  - Cambridge
ER  - 
TY  - JOUR
A1  - Cherstvy, Andrey G.
A1  - Metzler, Ralf
T1  - Anomalous diffusion in time-fluctuating non-stationary diffusivity landscapes
JF  - Physical chemistry, chemical physics : a journal of European Chemical Societies
N2  - We investigate the ensemble and time averaged mean squared displacements for particle diffusion in a simple model for disordered media by assuming that the local diffusivity is both fluctuating in time and has a deterministic average growth or decay in time. In this study we compare computer simulations of the stochastic Langevin equation for this random diffusion process with analytical results. We explore the regimes of normal Brownian motion as well as anomalous diffusion in the sub- and superdiffusive regimes. We also consider effects of the inertial term on the particle motion. The investigation of the resulting diffusion is performed for unconfined and confined motion.
Y1  - 2016
U6  - https://doi.org/10.1039/c6cp03101c
SN  - 1463-9076
SN  - 1463-9084
VL  - 18
SP  - 23840
EP  - 23852
PB  - Royal Society of Chemistry
CY  - Cambridge
ER  - 
TY  - JOUR
A1  - Jeon, Jae-Hyung
A1  - Leijnse, Natascha
A1  - Oddershede, Lene B.
A1  - Metzler, Ralf
T1  - Anomalous diffusion and power-law relaxation of the time averaged mean squared displacement in worm-like micellar solutions
JF  - New journal of physics : the open-access journal for physics
N2  - 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 0.52-2.5 mu m after short-time normal diffusion turn over to perform anomalous diffusion of the form < r(2)(t)> similar or equal to t(alpha) with alpha approximate to 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.
Y1  - 2013
U6  - https://doi.org/10.1088/1367-2630/15/4/045011
SN  - 1367-2630
VL  - 15
IS  - 4
PB  - IOP Publ. Ltd.
CY  - Bristol
ER  - 
TY  - JOUR
A1  - Wang, Wei
A1  - Cherstvy, Andrey G.
A1  - Liu, Xianbin
A1  - Metzler, Ralf
T1  - Anomalous diffusion and nonergodicity for heterogeneous diffusion processes with fractional Gaussian noise
JF  - Physical review : E, Statistical, nonlinear and soft matter physics
N2  - Heterogeneous diffusion processes (HDPs) feature a space-dependent diffusivity of the form D(x) = D-0|x|(alpha). Such processes yield anomalous diffusion and weak ergodicity breaking, the asymptotic disparity between ensemble and time averaged observables, such as the mean-squared displacement. Fractional Brownian motion (FBM) with its long-range correlated yet Gaussian increments gives rise to anomalous and ergodic diffusion. Here, we study a combined model of HDPs and FBM to describe the particle dynamics in complex systems with position-dependent diffusivity driven by fractional Gaussian noise. This type of motion is, inter alia, relevant for tracer-particle diffusion in biological cells or heterogeneous complex fluids. We show that the long-time scaling behavior predicted theoretically and by simulations for the ensemble-and time-averaged mean-squared displacements couple the scaling exponents alpha of HDPs and the Hurst exponent H of FBM in a characteristic way. Our analysis of the simulated data in terms of the rescaled variable y similar to |x|(1/(2/(2-alpha)))/t(H) coupling particle position x and time t yields a simple, Gaussian probability density function (PDF), PHDP-FBM(y) = e(-y2)/root pi. Its universal shape agrees well with theoretical predictions for both uni- and bimodal PDF distributions.
Y1  - 2020
U6  - https://doi.org/10.1103/PhysRevE.102.012146
SN  - 2470-0045
SN  - 2470-0053
SN  - 1063-651X
SN  - 1539-3755
SN  - 2470-0061
VL  - 102
IS  - 1
SP  - 012146-1
EP  - 012146-16
PB  - American Physical Society
CY  - College Park
ER  - 
TY  - JOUR
A1  - Cherstvy, Andrey G.
A1  - Chechkin, Aleksei V.
A1  - Metzler, Ralf
T1  - Anomalous diffusion and ergodicity breaking in heterogeneous diffusion processes
JF  - New journal of physics : the open-access journal for physics
N2  - We demonstrate the non-ergodicity of a simple Markovian stochastic process with space-dependent diffusion coefficient D(x). For power-law forms D(x) similar or equal to vertical bar x vertical bar(alpha), this process yields anomalous diffusion of the form < x(2)(t)> similar or equal to t(2/(2-alpha)). Interestingly, in both the sub- and superdiffusive regimes we observe weak ergodicity breaking: the scaling of the time-averaged mean-squared displacement <(delta(2)(Delta))over bar> remains linear in the lag time Delta and thus differs from the corresponding ensemble average < x(2)(t)>. We analyse the non-ergodic behaviour of this process in terms of the time-averaged mean- squared displacement (delta(2)) over bar and its random features, i.e. the statistical distribution of (delta(2)) over bar and the ergodicity breaking parameters. The heterogeneous diffusion model represents an alternative approach to non- ergodic, anomalous diffusion that might be particularly relevant for diffusion in heterogeneous media.
Y1  - 2013
U6  - https://doi.org/10.1088/1367-2630/15/8/083039
SN  - 1367-2630
VL  - 15
IS  - 15
PB  - IOP Publ. Ltd.
CY  - Bristol
ER  - 
TY  - JOUR
A1  - Javanainen, Matti
A1  - Hammaren, Henrik
A1  - Monticelli, Luca
A1  - Jeon, Jae-Hyung
A1  - Miettinen, Markus S.
A1  - Martinez-Seara, Hector
A1  - Metzler, Ralf
A1  - Vattulainen, Ilpo
T1  - Anomalous and normal diffusion of proteins and lipids in crowded lipid membranes
JF  - Faraday discussions
N2  - 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.
Y1  - 2013
U6  - https://doi.org/10.1039/c2fd20085f
SN  - 1359-6640
VL  - 161
IS  - 1
SP  - 397
EP  - 417
PB  - Royal Society of Chemistry
CY  - Cambridge
ER  - 
TY  - JOUR
A1  - Safdari, Hadiseh
A1  - Cherstvy, Andrey G.
A1  - Chechkin, Aleksei V.
A1  - Bodrova, Anna
A1  - Metzler, Ralf
T1  - Aging underdamped scaled Brownian motion
BT  - Ensemble- and time-averaged particle displacements, nonergodicity, and the failure of the overdamping approximation
JF  - Physical review : E, Statistical, nonlinear and soft matter physics
N2  - We investigate both analytically and by computer simulations the ensemble- and time-averaged, nonergodic, and aging properties of massive particles diffusing in a medium with a time dependent diffusivity. We call this stochastic diffusion process the (aging) underdamped scaled Brownian motion (UDSBM). We demonstrate how the mean squared displacement (MSD) and the time-averaged MSD of UDSBM are affected by the inertial term in the Langevin equation, both at short, intermediate, and even long diffusion times. In particular, we quantify the ballistic regime for the MSD and the time-averaged MSD as well as the spread of individual time-averaged MSD trajectories. One of the main effects we observe is that, both for the MSD and the time-averaged MSD, for superdiffusive UDSBM the ballistic regime is much shorter than for ordinary Brownian motion. In contrast, for subdiffusive UDSBM, the ballistic region extends to much longer diffusion times. Therefore, particular care needs to be taken under what conditions the overdamped limit indeed provides a correct description, even in the long time limit. We also analyze to what extent ergodicity in the Boltzmann-Khinchin sense in this nonstationary system is broken, both for subdiffusive and superdiffusive UDSBM. Finally, the limiting case of ultraslow UDSBM is considered, with a mixed logarithmic and power-law dependence of the ensemble-and time-averaged MSDs of the particles. In the limit of strong aging, remarkably, the ordinary UDSBM and the ultraslow UDSBM behave similarly in the short time ballistic limit. The approaches developed here open ways for considering other stochastic processes under physically important conditions when a finite particle mass and aging in the system cannot be neglected.
Y1  - 2017
U6  - https://doi.org/10.1103/PhysRevE.95.012120
SN  - 2470-0045
SN  - 2470-0053
VL  - 95
PB  - American Physical Society
CY  - College Park
ER  - 
TY  - JOUR
A1  - Safdari, Hadiseh
A1  - Chechkin, Aleksei V.
A1  - Jafari, Gholamreza R.
A1  - Metzler, Ralf
T1  - Aging scaled Brownian motion
JF  - Physical review : E, Statistical, nonlinear and soft matter physics
N2  - Scaled Brownian motion (SBM) is widely used to model anomalous diffusion of passive tracers in complex and biological systems. It is a highly nonstationary process governed by the Langevin equation for Brownian motion, however, with a power-law time dependence of the noise strength. Here we study the aging properties of SBM for both unconfined and confined motion. Specifically, we derive the ensemble and time averaged mean squared displacements and analyze their behavior in the regimes of weak, intermediate, and strong aging. A very rich behavior is revealed for confined aging SBM depending on different aging times and whether the process is sub- or superdiffusive. We demonstrate that the information on the aging factorizes with respect to the lag time and exhibits a functional form that is identical to the aging behavior of scale-free continuous time random walk processes. While SBM exhibits a disparity between ensemble and time averaged observables and is thus weakly nonergodic, strong aging is shown to effect a convergence of the ensemble and time averaged mean squared displacement. Finally, we derive the density of first passage times in the semi-infinite domain that features a crossover defined by the aging time.
Y1  - 2015
U6  - https://doi.org/10.1103/PhysRevE.91.042107
SN  - 1539-3755
SN  - 1550-2376
VL  - 91
IS  - 4
PB  - American Physical Society
CY  - College Park
ER  - 
TY  - JOUR
A1  - Schulz, Johannes H. P.
A1  - Barkai, Eli
A1  - Metzler, Ralf
T1  - Aging renewal theory and application to random walks
JF  - Physical review : X, Expanding access
N2  - We discuss a renewal process in which successive events are separated by scale-free waiting time periods. Among other ubiquitous long-time properties, this process exhibits aging: events counted initially in a time interval [0, t] statistically strongly differ from those observed at later times [t(a,) t(a) + t]. The versatility of renewal theory is owed to its abstract formulation. Renewals can be interpreted as steps of a random walk, switching events in two-state models, domain crossings of a random motion, etc. In complex, disordered media, processes with scale-free waiting times play a particularly prominent role. We set up a unified analytical foundation for such anomalous dynamics by discussing in detail the distribution of the aging renewal process. We analyze its half-discrete, half-continuous nature and study its aging time evolution. These results are readily used to discuss a scale-free anomalous diffusion process, the continuous-time random walk. By this, we not only shed light on the profound origins of its characteristic features, such as weak ergodicity breaking, along the way, we also add an extended discussion on aging effects. In particular, we find that the aging behavior of time and ensemble averages is conceptually very distinct, but their time scaling is identical at high ages. Finally, we show how more complex motion models are readily constructed on the basis of aging renewal dynamics.
Y1  - 2014
U6  - https://doi.org/10.1103/PhysRevX.4.011028
SN  - 2160-3308
VL  - 4
IS  - 1
PB  - American Physical Society
CY  - College Park
ER  - 
TY  - JOUR
A1  - Schulz, Johannes H. P.
A1  - Barkai, Eli
A1  - Metzler, Ralf
T1  - Aging effects and population splitting in single-particle trajectoryaverages
JF  - Physical review letters
N2  - We study time averages of single particle trajectories in scale-free anomalous diffusion processes, in which the measurement starts at some time t(a) > 0 after initiation of the process at t = 0. Using aging renewal theory, we show that for such nonstationary processes a large class of observables are affected by a unique aging function, which is independent of boundary conditions or the external forces. Moreover, we discuss the implications of aging induced population splitting: with growing age ta of the process, an increasing fraction of particles remains motionless in a measurement of fixed duration. Consequences for single biomolecule tracking in live cells are discussed.
Y1  - 2013
U6  - https://doi.org/10.1103/PhysRevLett.110.020602
SN  - 0031-9007
SN  - 1079-7114
VL  - 110
IS  - 2
PB  - American Physical Society
CY  - College Park
ER  - 
TY  - JOUR
A1  - Metzler, Ralf
A1  - Sanders, L.
A1  - Lomholt, M. A.
A1  - Lizana, L.
A1  - Fogelmark, K.
A1  - Ambjoernsson, Tobias
T1  - Ageing single file motion
JF  - The European physical journal
Y1  - 2014
U6  - https://doi.org/10.1140/epjst/e2014-02333-5
SN  - 1951-6355
SN  - 1951-6401
VL  - 223
IS  - 14
SP  - 3287
EP  - 3293
PB  - Springer
CY  - Heidelberg
ER  - 
TY  - JOUR
A1  - Kruesemann, Henning
A1  - Schwarzl, Richard
A1  - Metzler, Ralf
T1  - Ageing Scher-Montroll Transport
JF  - Transport in Porous Media
N2  - We study the properties of ageing Scher-Montroll transport in terms of a biased subdiffusive continuous time random walk in which the waiting times between consecutive jumps of the charge carriers are distributed according to the power law probability with . As we show, the dynamical properties of the Scher-Montroll transport depend on the ageing time span between the initial preparation of the system and the start of the observation. The Scher-Montroll transport theory was originally shown to describe the photocurrent in amorphous solids in the presence of an external electric field, but it has since been used in many other fields of physical sciences, in particular also in the geophysical context for the description of the transport of tracer particles in subsurface aquifers. In the absence of ageing () the photocurrent of the classical Scher-Montroll model or the breakthrough curves in the groundwater context exhibit a crossover between two power law regimes in time with the scaling exponents and . In the presence of ageing a new power law regime and an initial plateau regime of the current emerge. We derive the different power law regimes and crossover times of the ageing Scher-Montroll transport and show excellent agreement with simulations of the process. Experimental data of ageing Scher-Montroll transport in polymeric semiconductors are shown to agree well with the predictions of our theory.
KW  - Anomalous diffusion
KW  - Ageing
KW  - Scher-Montroll transport
Y1  - 2016
U6  - https://doi.org/10.1007/s11242-016-0686-y
SN  - 0169-3913
SN  - 1573-1634
VL  - 115
SP  - 327
EP  - 344
PB  - Springer
CY  - New York
ER  - 
TY  - JOUR
A1  - Krüsemann, Henning
A1  - Godec, Aljaz
A1  - Metzler, Ralf
T1  - Ageing first passage time density in continuous time random walks and quenched energy landscapes
JF  - Journal of physics : A, Mathematical and theoretical
N2  - We study the first passage dynamics of an ageing stochastic process in the continuous time random walk (CTRW) framework. In such CTRW processes the test particle performs a random walk, in which successive steps are separated by random waiting times distributed in terms of the waiting time probability density function Psi (t) similar or equal to t(-1-alpha) (0 <= alpha <= 2). An ageing stochastic process is defined by the explicit dependence of its dynamic quantities on the ageing time t(a), the time elapsed between its preparation and the start of the observation. Subdiffusive ageing CTRWs with 0 < alpha < 1 describe systems such as charge carriers in amorphous semiconducters, tracer dispersion in geological and biological systems, or the dynamics of blinking quantum dots. We derive the exact forms of the first passage time density for an ageing subdiffusive CTRW in the semi-infinite, confined, and biased case, finding different scaling regimes for weakly, intermediately, and strongly aged systems: these regimes, with different scaling laws, are also found when the scaling exponent is in the range 1 < alpha < 2, for sufficiently long ta. We compare our results with the ageing motion of a test particle in a quenched energy landscape. We test our theoretical results in the quenched landscape against simulations: only when the bias is strong enough, the correlations from returning to previously visited sites become insignificant and the results approach the ageing CTRW results. With small bias or without bias, the ageing effects disappear and a change in the exponent compared to the case of a completely annealed landscape can be found, reflecting the build-up of correlations in the quenched landscape.
KW  - first passage
KW  - random walks
KW  - anomalous diffusion
Y1  - 2015
U6  - https://doi.org/10.1088/1751-8113/48/28/285001
SN  - 1751-8113
SN  - 1751-8121
VL  - 48
IS  - 28
PB  - IOP Publ. Ltd.
CY  - Bristol
ER  - 
TY  - JOUR
A1  - Chechkin, Aleksei V.
A1  - Kantz, Holger
A1  - Metzler, Ralf
T1  - Ageing effects in ultraslow continuous time random walks
JF  - The European physical journal : B, Condensed matter and complex systems
N2  - In ageing systems physical observables explicitly depend on the time span elapsing between the original initiation of the system and the actual start of the recording of the particle motion. We here study the signatures of ageing in the framework of ultraslow continuous time random walk processes with super-heavy tailed waiting time densities. We derive the density for the forward or recurrent waiting time of the motion as function of the ageing time, generalise the Montroll-Weiss equation for this process, and analyse the ageing behaviour of the ensemble and time averaged mean squared displacements.
Y1  - 2017
U6  - https://doi.org/10.1140/epjb/e2017-80270-9
SN  - 1434-6028
SN  - 1434-6036
VL  - 90
PB  - Springer
CY  - New York
ER  - 
TY  - JOUR
A1  - Cherstvy, Andrey G.
A1  - Chechkin, Aleksei V.
A1  - Metzler, Ralf
T1  - Ageing and confinement in non-ergodic heterogeneous diffusion processes
JF  - Journal of physics : A, Mathematical and theoretical
N2  - We study the effects of ageing-the time delay between initiation of the physical process at t = 0 and start of observation at some time t(a) > 0-and spatial confinement on the properties of heterogeneous diffusion processes (HDPs) with deterministic power-law space-dependent diffusivities, D(x) = D-0 vertical bar x vertical bar(alpha). From analysis of the ensemble and time averaged mean squared displacements and the ergodicity breaking parameter quantifying the inherent degree of irreproducibility of individual realizations of the HDP we obtain striking similarities to ageing subdiffusive continuous time random walks with scale-free waiting time distributions. We also explore how both processes can be distinguished. For confined HDPs we study the long-time saturation of the ensemble and time averaged particle displacements as well as the magnitude of the inherent scatter of time averaged displacements and contrast the outcomes to the results known for other anomalous diffusion processes under confinement.
KW  - stochastic processes
KW  - anomalous diffusion
KW  - ageing
KW  - weak ergodicity breaking
Y1  - 2014
U6  - https://doi.org/10.1088/1751-8113/47/48/485002
SN  - 1751-8113
SN  - 1751-8121
VL  - 47
IS  - 48
PB  - IOP Publ. Ltd.
CY  - Bristol
ER  - 
TY  - JOUR
A1  - Godec, Aljaz
A1  - Metzler, Ralf
T1  - Active transport improves the precision of linear long distance molecular signalling
JF  - Journal of physics : A, Mathematical and theoretical
N2  - Molecular signalling in living cells occurs at low copy numbers and is thereby inherently limited by the noise imposed by thermal diffusion. The precision at which biochemical receptors can count signalling molecules is intimately related to the noise correlation time. In addition to passive thermal diffusion, messenger RNA and vesicle-engulfed signalling molecules can transiently bind to molecular motors and are actively transported across biological cells. Active transport is most beneficial when trafficking occurs over large distances, for instance up to the order of 1 metre in neurons. Here we explain how intermittent active transport allows for faster equilibration upon a change in concentration triggered by biochemical stimuli. Moreover, we show how intermittent active excursions induce qualitative changes in the noise in effectively one-dimensional systems such as dendrites. Thereby they allow for significantly improved signalling precision in the sense of a smaller relative deviation in the concentration read-out by the receptor. On the basis of linear response theory we derive the exact mean field precision limit for counting actively transported molecules. We explain how intermittent active excursions disrupt the recurrence in the molecular motion, thereby facilitating improved signalling accuracy. Our results provide a deeper understanding of how recurrence affects molecular signalling precision in biological cells and novel medical-diagnostic devices.
KW  - noise in biochemical signalling
KW  - Brownian motion
KW  - active transport
KW  - linear response theory
KW  - fluctuation-dissipation theorem
KW  - generalised Langevin equation
KW  - recurrence
Y1  - 2016
U6  - https://doi.org/10.1088/1751-8113/49/36/364001
SN  - 1751-8113
SN  - 1751-8121
VL  - 49
PB  - IOP Publ. Ltd.
CY  - Bristol
ER  - 
TY  - JOUR
A1  - Kar, Prathitha
A1  - Cherstvy, Andrey G.
A1  - Metzler, Ralf
T1  - Acceleration of bursty multiprotein target search kinetics on DNA by colocalisation
JF  - Physical chemistry, chemical physics : a journal of European Chemical Societies
N2  - Proteins are capable of locating specific targets on DNA by employing a facilitated diffusion process with intermittent 1D and 3D search steps. Gene colocalisation and coregulation-i.e. the spatial proximity of two communicating genes-is one factor capable of accelerating the target search process along the DNA. We perform Monte Carlo computer simulations and demonstrate the benefits of gene colocalisation for minimising the search time in a model DNA-protein system. We use a simple diffusion model to mimic the search for targets by proteins, produced initially in bursts of multiple proteins and performing the first-passage search on the DNA chain. The behaviour of the mean first-passage times to the target is studied as a function of distance between the initial position of proteins and the DNA target position, as well as versus the concentration of proteins. We also examine the properties of bursty target search kinetics for varying physical-chemical protein-DNA binding affinity. Our findings underline the relevance of colocalisation of production and binding sites for protein search inside biological cells.
Y1  - 2017
U6  - https://doi.org/10.1039/c7cp06922g
SN  - 1463-9076
SN  - 1463-9084
VL  - 20
IS  - 12
SP  - 7931
EP  - 7946
PB  - Royal Society of Chemistry
CY  - Cambridge
ER  - 
TY  - JOUR
A1  - Guggenberger, Tobias
A1  - Chechkin, Aleksei
A1  - Metzler, Ralf
T1  - Absence of stationary states and non-Boltzmann distributions of fractional Brownian motion in shallow external potentials
JF  - New journal of physics : the open-access journal for physics
N2  - We study the diffusive motion of a particle in a subharmonic potential of the form U(x) = |x|( c ) (0 < c < 2) driven by long-range correlated, stationary fractional Gaussian noise xi ( alpha )(t) with 0 < alpha <= 2. In the absence of the potential the particle exhibits free fractional Brownian motion with anomalous diffusion exponent alpha. While for an harmonic external potential the dynamics converges to a Gaussian stationary state, from extensive numerical analysis we here demonstrate that stationary states for shallower than harmonic potentials exist only as long as the relation c > 2(1 - 1/alpha) holds. We analyse the motion in terms of the mean squared displacement and (when it exists) the stationary probability density function. Moreover we discuss analogies of non-stationarity of Levy flights in shallow external potentials.
KW  - diffusion
KW  - Boltzmann distribution
KW  - fractional Brownian motion
Y1  - 2022
U6  - https://doi.org/10.1088/1367-2630/ac7b3c
SN  - 1367-2630
VL  - 24
IS  - 7
PB  - Dt. Physikalische Ges.
CY  - [Bad Honnef]
ER  - 
TY  - JOUR
A1  - Sereshki, L. E.
A1  - Lomholt, M. A.
A1  - Metzler, Ralf
T1  - A solution to the subdiffusion-efficiency paradox inactive states enhance reaction efficiency at subdiffusion conditions in living cells
JF  - epl : a letters journal exploring the frontiers of physics
N2  - Macromolecular crowding in living biological cells effects subdiffusion of larger biomolecules such as proteins and enzymes. Mimicking this subdiffusion in terms of random walks on a critical percolation cluster, we here present a case study of EcoRV restriction enzymes involved in vital cellular defence. We show that due to its so far elusive propensity to an inactive state the enzyme avoids non-specific binding and remains well-distributed in the bulk cytoplasm of the cell. Despite the reduced volume exploration capability of subdiffusion processes, this mechanism guarantees a high efficiency of the enzyme. By variation of the non-specific binding constant and the bond occupation probability on the percolation network, we demonstrate that reduced nonspecific binding are beneficial for efficient subdiffusive enzyme activity even in relatively small bacteria cells. Our results corroborate a more local picture of cellular regulation.
Y1  - 2012
U6  - https://doi.org/10.1209/0295-5075/97/20008
SN  - 0295-5075
VL  - 97
IS  - 2
PB  - EDP Sciences
CY  - Mulhouse
ER  - 
TY  - JOUR
A1  - Schwarzl, Maria
A1  - Godec, Aljaz
A1  - Oshanin, Gleb
A1  - Metzler, Ralf
T1  - A single predator charging a herd of prey: effects of self volume and predator-prey decision-making
JF  - Journal of physics : A, Mathematical and theoretical
N2  - We study the degree of success of a single predator hunting a herd of prey on a two-dimensional square lattice landscape. We explicitly consider the self volume of the prey restraining their dynamics on the lattice. The movement of both predator and prey is chosen to include an intelligent, decision making step based on their respective sighting ranges, the radius in which they can detect the other species (prey cannot recognise each other besides the self volume interaction): after spotting each other the motion of prey and predator turns from a nearest neighbour random walk into directed escape or chase, respectively. We consider a large range of prey densities and sighting ranges and compute the mean first passage time for a predator to catch a prey as well as characterise the effective dynamics of the hunted prey. We find that the prey's sighting range dominates their life expectancy and the predator profits more from a bad eyesight of the prey than from his own good eye sight. We characterise the dynamics in terms of the mean distance between the predator and the nearest prey. It turns out that effectively the dynamics of this distance coordinate can be captured in terms of a simple Ornstein–Uhlenbeck picture. Reducing the many-body problem to a simple two-body problem by imagining predator and nearest prey to be connected by an effective Hookean bond, all features of the model such as prey density and sighting ranges merge into the effective binding constant.
KW  - first passage process
KW  - diffusion
KW  - predator-prey model
Y1  - 2016
U6  - https://doi.org/10.1088/1751-8113/49/22/225601
SN  - 1751-8113
SN  - 1751-8121
VL  - 49
PB  - IOP Publ. Ltd.
CY  - Bristol
ER  - 
TY  - JOUR
A1  - Herrmann, Carl J. J.
A1  - Metzler, Ralf
A1  - Engbert, Ralf
T1  - A self-avoiding walk with neural delays as a model of fixational eye movements
JF  - Scientific reports
N2  - Fixational eye movements show scaling behaviour of the positional mean-squared displacement with a characteristic transition from persistence to antipersistence for increasing time-lag. These statistical patterns were found to be mainly shaped by microsaccades (fast, small-amplitude movements). However, our re-analysis of fixational eye-movement data provides evidence that the slow component (physiological drift) of the eyes exhibits scaling behaviour of the mean-squared displacement that varies across human participants. These results suggest that drift is a correlated movement that interacts with microsaccades. Moreover, on the long time scale, the mean-squared displacement of the drift shows oscillations, which is also present in the displacement auto-correlation function. This finding lends support to the presence of time-delayed feedback in the control of drift movements. Based on an earlier non-linear delayed feedback model of fixational eye movements, we propose and discuss different versions of a new model that combines a self-avoiding walk with time delay. As a result, we identify a model that reproduces oscillatory correlation functions, the transition from persistence to antipersistence, and microsaccades.
Y1  - 2017
U6  - https://doi.org/10.1038/s41598-017-13489-8
SN  - 2045-2322
VL  - 7
PB  - Nature Publ. Group
CY  - London
ER  - 
TY  - JOUR
A1  - Herrmann, Carl J. J.
A1  - Metzler, Ralf
T1  - A self-avoiding walk with neural delays as a model of fixational eye movements
JF  - Scientific reports
N2  - Fixational eye movements show scaling behaviour of the positional mean-squared displacement with a characteristic transition from persistence to antipersistence for increasing time-lag. These statistical patterns were found to be mainly shaped by microsaccades (fast, small-amplitude movements). However, our re-analysis of fixational eye-movement data provides evidence that the slow component (physiological drift) of the eyes exhibits scaling behaviour of the mean-squared displacement that varies across human participants. These results suggest that drift is a correlated movement that interacts with microsaccades. Moreover, on the long time scale, the mean-squared displacement of the drift shows oscillations, which is also present in the displacement auto-correlation function. This finding lends support to the presence of time-delayed feedback in the control of drift movements. Based on an earlier non-linear delayed feedback model of fixational eye movements, we propose and discuss different versions of a new model that combines a self-avoiding walk with time delay. As a result, we identify a model that reproduces oscillatory correlation functions, the transition from persistence to antipersistence, and microsaccades.
Y1  - 2017
U6  - https://doi.org/10.1038/s41598-017-13489-8
SN  - 2045-2322
VL  - 7
SP  - 1
EP  - 17
PB  - Springer Nature
CY  - London
ER  - 
TY  - JOUR
A1  - Grebenkov, Denis S.
A1  - Metzler, Ralf
A1  - Oshanin, Gleb
T1  - A molecular relay race: sequential first-passage events to the terminal reaction centre in a cascade of diffusion controlled processes
JF  - New Journal of Physics (NJP)
N2  - We consider a sequential cascade of molecular first-reaction events towards a terminal reaction centre in which each reaction step is controlled by diffusive motion of the particles. The model studied here represents a typical reaction setting encountered in diverse molecular biology systems, in which, e.g. a signal transduction proceeds via a series of consecutive 'messengers': the first messenger has to find its respective immobile target site triggering a launch of the second messenger, the second messenger seeks its own target site and provokes a launch of the third messenger and so on, resembling a relay race in human competitions. For such a molecular relay race taking place in infinite one-, two- and three-dimensional systems, we find exact expressions for the probability density function of the time instant of the terminal reaction event, conditioned on preceding successful reaction events on an ordered array of target sites. The obtained expressions pertain to the most general conditions: number of intermediate stages and the corresponding diffusion coefficients, the sizes of the target sites, the distances between them, as well as their reactivities are arbitrary.
KW  - diffusion
KW  - reaction cascade
KW  - first passage time
Y1  - 2021
U6  - https://doi.org/10.1088/1367-2630/ac1e42
SN  - 1367-2630
VL  - 23
PB  - IOP - Institute of Physics Publishing
CY  - Bristol
ER  -