Diffusion Spreadability as a Probe of the Microstructure of Complex Media Across Length Scales

Abstract

Consider the time-dependent problem of mass transfer of a solute between two phases and assume that the solute is initially distributed in one phase (phase 2) and absent from the other (phase 1). We desire the fraction of total solute present in phase 1 as a function of time, ${\cal S}(t)$, which we call the {\it spreadability}, since it is a measure of the spreadability of diffusion information as a function of time. We derive exact direct-space formulas for ${\cal S}(t)$ in any Euclidean space dimension $d$ in terms of the autocovariance function as well as corresponding Fourier representations of ${\cal S}(t)$ in terms of the spectral density. We derive closed-form general formulas for the short- and long-time behaviors of the spreadability in terms of crucial small- and large-scale microstructural information, respectively. The long-time behavior of ${\cal S}(t)$ enables one to distinguish the entire spectrum of microstructures that span from hyperuniform to nonhyperuniform media. For hyperuniform media, disordered or not, we show that the "excess" spreadability, ${\cal S}(\infty)-{\cal S}(t)$, decays to its long-time behavior exponentially faster than that of any nonhyperuniform two-phase medium, the "slowest" being antihyperuniform media. The stealthy hyperuniform class is characterized by an excess spreadability with the fastest decay rate among all translationally invariant microstructures. Moreover, we establish a remarkable connection between the spreadability and an outstanding problem in discrete geometry, namely, microstructures with "fast" spreadabilities are also those that can be derived from efficient "coverings" of space. We also identify heretofore unnoticed remarkable links between the spreadability ${\cal S}(t)$ and NMR pulsed field gradient spin-echo amplitude as well as diffusion MRI measurements.