  Active matter concerns the self-organization of energy consuming elements
such as motile bacteria or self-propelled colloids. A canonical example is an
active Brownian particle (ABP) that moves at constant speed while its direction
of motion undergoes rotational diffusion. When ABPs are confined within a
channel, they tend to accumulate at the channel walls, even when inter-particle
interactions are ignored. Each particle pushes on the boundary until a tumble
event reverses its direction. The wall thus acts as a sticky boundary. In this
paper we consider a natural extension of sticky boundaries that allows for a
particle to be permanently killed (absorbed) whilst attached to a wall. In
particular, we investigate the first passage time (FPT) problem for an ABP in a
two-dimensional channel where one of the walls is partially absorbing.
Calculating the exact FPT statistics requires solving a non-trivial two-way
diffusion boundary value problem (BVP). We follow a different approach by
separating out the dynamics away from the absorbing wall from the dynamics of
absorption and escape whilst attached to the wall. Using probabilistic methods,
we derive an explicit expression for the MFPT of absorption, assuming that the
arrival statistics of particles at the wall are known. Our method also allows
us to incorporate a more general encounter-based model of absorption.
