Non-universal tracer diffusion in crowded media of non-inert obstacles

Abstract

We study the diffusion of a tracer particle, which moves in continuum space between a lattice of excluded volume, immobile non-inert obstacles. In particular, we analyse how the strength of the tracer-obstacle interactions and the volume occupancy of the crowders alter the diffusive motion of the tracer. From the details of partitioning of the tracer diffusion modes between trapping states when bound to obstacles and bulk diffusion, we examine the degree of localisation of the tracer in the lattice of crowders. We study the properties of the tracer diffusion in terms of the ensemble and time averaged mean squared displacements, the trapping time distributions, the amplitude variation of the time averaged mean squared displacements, and the non-Gaussianity parameter of the diffusing tracer. We conclude that tracer-obstacle adsorption and binding triggers a transient anomalous diffusion. From a very narrow spread of recorded individual time averaged trajectories we exclude continuous type random walk processes as the underlying physical model of the tracer diffusion in our system. For moderate tracer-crowder attraction the motion is found to be fully ergodic, while at stronger attraction strength a transient disparity between ensemble and time averaged mean squared displacements occurs. We also put our results into perspective with findings from experimental single-particle tracking and simulations of the diffusion of tagged tracers in dense crowded suspensions. Our results have implications for the diffusion, transport, and spreading of chemical components in highly crowded environments inside living cells and other structured liquids.

Ergodicity breaking and particle spreading in noisy heterogeneous diffusion processes

“…Our results for noisy HDPs could also be useful for the description of nano-objects trapped in dynamical temperature fields [55] and of particles in strong temperature gradients [56]. Another field of relevance is the tracer diffusion in heterogeneous assemblies of distributed obstacles [57] mimicking features of the cell cytoplasm [8] and diffusion on chemically and mesoscopically periodically patterned solid-liquid interfaces [58]. On a macroscopic scale, water diffusion in subsurface hydrology applications is to be mentioned [12], as well as tracer motion in porous heterogeneous media [59].…”

Nonergodic subdiffusion from transient interactions with heterogeneous partners

“…Finally, we discuss the deviation from Gaussianity of the propagator P (x, t), which describes the probability of finding the prey at position x at time t [22][23][24][25][26]. A standard measurement of such deviation is given by the functional [27] …”

Probing Non-Gaussianity in Confined Diffusion of Nanoparticles

“…In addition, in contrast to some theoretical studies suggesting diffusivity memory as a key feature of short-time dynamics, 20 the measured displacement autocorrelation does not show positive correlation between successive motions. Notably, new studies relevant to the interaction between NPs and surrounding structures were performed recently, 22,23 which is helpful to better understand the physical origin. Furthermore, we noticed that the non-Gaussian behavior was also reported in glassy systems, 42,43 where the non-Gaussian parameter α is up to 1−5, which is 1 order of magnitude greater than our results.…”

“…20,21 The theoretical models considering particle restriction could not predict the features of higher moment of displacement. 13,18 Other physical influences such as the interaction between particle and surrounding molecules 22,23 and the dynamics in the heterogeneous structures 17,20,24,25 were also addressed, yet no consensus has been reached. Thus, the purpose of this study is to identify the physical source of nonGaussianity in confined diffusion and clarify its characteristics.…”