First-passage times to anisotropic partially reactive targets

TL;DR

This paper develops an explicit approximation for the principal eigenvalue in restricted diffusion to anisotropic targets, linking geometric properties to reaction times and survival probabilities in higher dimensions.

Contribution

It introduces a simple explicit approximation involving harmonic capacity and surface area for anisotropic targets, enhancing understanding of reaction kinetics in complex geometries.

Findings

Approximation accurately predicts principal eigenvalue and mean first-reaction time.

Target anisotropy significantly affects trapping capacity and reaction rates.

Harmonic capacity varies with target shape and dimension, influencing reaction dynamics.

Abstract

We investigate restricted diffusion in a bounded domain towards a small partially reactive target in three- and higher-dimensional spaces. We propose a simple explicit approximation for the principal eigenvalue of the Laplace operator with mixed Robin-Neumann boundary conditions. This approximation involves the harmonic capacity and the surface area of the target, the volume of the confining domain, the diffusion coefficient and the reactivity. The accuracy of the approximation is checked by using a finite-elements method. The proposed approximation determines also the mean first-reaction time, the long-time decay of the survival probability, and the overall reaction rate on that target. We identify the relevant length scale of the target, which determines its trapping capacity, and investigate its relation to the target shape. In particular, we study the effect of target anisotropy on the principal eigenvalue by computing the harmonic capacity of prolate and oblate spheroids in various space dimensions. Some implications of these results in chemical physics and biophysics are briefly discussed.