8 research outputs found
New homogenization approaches for stochastic transport through heterogeneous media
The diffusion of molecules in complex intracellular environments can be
strongly influenced by spatial heterogeneity and stochasticity. A key challenge
when modelling such processes using stochastic random walk frameworks is that
negative jump coefficients can arise when transport operators are discretized
on heterogeneous domains. Often this is dealt with through homogenization
approximations by replacing the heterogeneous medium with an
homogeneous medium. In this work, we present a new class
of homogenization approximations by considering a stochastic diffusive
transport model on a one-dimensional domain containing an arbitrary number of
layers with different jump rates. We derive closed form solutions for the th
moment of particle lifetime, carefully explaining how to deal with the internal
interfaces between layers. These general tools allow us to derive simple
formulae for the effective transport coefficients, leading to significant
generalisations of previous homogenization approaches. Here, we find that
different jump rates in the layers gives rise to a net bias, leading to a
non-zero advection, for the entire homogenized system. Example calculations
show that our generalized approach can lead to very different outcomes than
traditional approaches, thereby having the potential to significantly affect
simulation studies that use homogenization approximations.Comment: 9 pages, 2 figures, accepted version of paper published in The
Journal of Chemical Physic
Mean exit time for diffusion on irregular domains
Many problems in physics, biology, and economics depend upon the duration of
time required for a diffusing particle to cross a boundary. As such,
calculations of the distribution of first passage time, and in particular the
mean first passage time, is an active area of research relevant to many
disciplines. Exact results for the mean first passage time for diffusion on
simple geometries, such as lines, discs and spheres, are well--known. In
contrast, computational methods are often used to study the first passage time
for diffusion on more realistic geometries where closed--form solutions of the
governing elliptic boundary value problem are not available. Here, we develop a
perturbation solution to calculate the mean first passage time on irregular
domains formed by perturbing the boundary of a disc or an ellipse. Classical
perturbation expansion solutions are then constructed using the exact solutions
available on a disc and an ellipse. We apply the perturbation solutions to
compute the mean first exit time on two naturally--occurring irregular domains:
a map of Tasmania, an island state of Australia, and a map of Taiwan. Comparing
the perturbation solutions with numerical solutions of the elliptic boundary
value problem on these irregular domains confirms that we obtain a very
accurate solution with a few terms in the series only. Matlab software to
implement all calculations is available on GitHub.Comment: 31pages, 12 figure
Simplified models of diffusion in radially-symmetric geometries
We consider diffusion-controlled release of particles from -dimensional
radially-symmetric geometries. A quantity commonly used to characterise such
diffusive processes is the proportion of particles remaining within the
geometry over time, denoted as . The stochastic approach for computing
is time-consuming and lacks analytical insight into key parameters while
the continuum approach yields complicated expressions for that obscure
the influence of key parameters and complicate the process of fitting
experimental release data. In this work, to address these issues, we develop
several simple surrogate models to approximate by matching moments with
the continuum analogue of the stochastic diffusion model. Surrogate models are
developed for homogeneous slab, circular, annular, spherical and spherical
shell geometries with a constant particle movement probability and
heterogeneous slab, circular, annular and spherical geometries, comprised of
two concentric layers with different particle movement probabilities. Each
model is easy to evaluate, agrees well with both stochastic and continuum
calculations of and provides analytical insight into the key parameters
of the diffusive transport system: dimension, diffusivity, geometry and
boundary conditions.Comment: 22 pages, 3 figures, submitte
Simulation of stochastic diffusion via first exit times
In molecular biology it is of interest to simulate diffusion stochastically. In the mesoscopic model we partition a biological cell into unstructured subvolumes. In each subvolume the number of molecules is recorded at each time step and molecules can jump between neighboring subvolumes to model diffusion. The jump rates can be computed by discretizing the diffusion equation on that unstructured mesh. If the mesh is of poor quality, due to a complicated cell geometry, standard discretization methods can generate negative jump coefficients, which no longer allows the interpretation as the probability to jump between the subvolumes. We propose a method based on the mean first exit time of a molecule from a subvolume, which guarantees positive jump coefficients. Two approaches to exit times, a global and a local one, are presented and tested in simulations on meshes of different quality in two and three dimensions