Hybrid asymptotic-numerical approach for estimating first passage time densities of the two-dimensional narrow capture problem

TL;DR

This paper introduces a hybrid asymptotic-numerical method to accurately estimate the full probability distribution of first passage times for a random walker in a 2D domain with multiple small traps, capturing complex multimodal behaviors.

Contribution

The paper develops a novel hybrid approach combining asymptotic analysis and numerical methods to compute first passage time densities in complex 2D trapping problems, including arbitrary domains and trap shapes.

Findings

Method accurately reproduces capture time densities compared to simulations.

Capable of capturing multimodal distributions due to trap arrangements.

Variance of capture time is comparable to the mean, indicating the importance of full density analysis.

Abstract

A hybrid asymptotic-numerical method is presented for obtaining the full probability distribution of capture times of a random walker by multiple small traps located inside a bounded two-dimensional domain with reflective boundaries. As motivation for this study, we calculate the variance in the capture time of a random walker by a single interior trap and determine this quantity to be comparable in magnitude to the mean. This implies that the mean is not necessarily reflective of typical capture times and that the full density must be determined. To solve the underlying diffusion equation, the method of Laplace transforms is used to obtain an elliptic problem of modified Helmholtz type. In the limit of vanishing trap sizes, each trap is represented as a Dirac point source which permits the solution of the transform equation to be represented as a superposition of Helmholtz Green's functions. Using this solution, we construct asymptotic short time solutions of the first passage time density which captures peaks associated with rapid capture by the absorbing traps. When numerical evaluation of the Helmholtz Green's function is employed followed by numerical inversion of the Laplace transform, the method reproduces the density for larger times. We demonstrate the accuracy of our solution technique with comparison to statistics obtained from a time-dependent solution of the diffusion equation and discrete particle simulations. In particular, we demonstrate that the method is capable of capturing the multimodal behavior in the capture time density that arises when the traps are strategically arranged. The hybrid method presented can be applied to scenarios involving both arbitrary domains and trap shapes.