A Comment on the Scale Length Validity of the Position Dependent Diffusion Coefficient Representation of Structural Heterogeneity
arXiv:1802.06682 · doi:10.1063/1.5025921
Abstract
Experimental studies of the variation of the mean square displacement (MSD) of a particle in a confined colloid suspension that exhibits density variations on the scale length of the particle diameter are not in agreement with the prediction that the spatial variation in MSD should mimic the spatial variation in density. The predicted behavior is derived from the expectation that the MSD of a particle depends on the system density and the assumption that the force acting on a particle is a point function of position. The experimental data come from studies of the MSDs of particles in narrow ribbon channels and between narrowly spaced parallel plates, and from new data, reported herein, of the radial and azimuthal MSDs of a colloid particle in a dense colloid suspension confined to a small circular cavity. In each of these geometries a dense colloid suspension exhibits pronounced density oscillations with spacing of a particle diameter. We remove the discrepancy between prediction and experiment using the Fisher-Methfessel interpretation of how local equilibrium in an inhomogeneous system is maintained to argue that the force acting on a particle is delocalized over a volume with radius equal to a particle diameter. Our interpretation has relevance to the relationship between the scale of inhomogeneity and the utility of translation of the particle MSD into a position dependent diffusion coefficient, and to the use of a spatially dependent diffusion coefficient to describe mass transport in a heterogeneous system.
arXiv comment16 pages, 7 figures
References in corpus (11)
- Brownian yet non-Gaussian diffusion: from superstatistics to subordination of diffusing diffusivities
- Kinetic Equations for Diffusion in the Presence of Entropic Barriers
- "Diffusing diffusivity": A model for anomalous and "anomalous yet Brownian" diffusion
- Anomalous diffusion and ergodicity breaking in heterogeneous diffusion processes
- Layering and position-dependent diffusive dynamics of confined fluids
- Multiple Excitation of Confined Graphene Plasmons by Single Free Electrons
- Anomalous, non-Gaussian tracer diffusion in heterogeneously crowded environments
- Population splitting, trapping, and non-ergodicity in heterogeneous diffusion processes
- Particle invasion, survival, and non-ergodicity in 2D diffusion processes with space-dependent diffusivity
- Test of the diffusing-diffusivity mechanism using near-wall colloidal dynamics
- Local influence of boundary conditions on a confined supercooled colloidal liquid