  Understanding particle motion in narrow channels is essential to guide
progress in numerous applications, from filtration to vascular transport.
Thermal or active fluctuations of channel walls for fluid-filled channels can
slow down or increase the dispersion of tracer particles. Entropic trapping in
the wall bulges slows dispersion, and hydrodynamic flows induced by wall
fluctuations enhance dispersion. Previous studies primarily concentrated on the
case of a single Brownian tracer either embedded in an incompressible fluid or
in the ideal case where the presence of fluid is ignored. Here we address the
question of what happens when there is a large ensemble of interacting Brownian
tracers -- a common situation in applications. Introducing repulsive
interactions between the tracer particles, while ignoring the presence of a
background fluid, leads to an effective flow field. This flow field enhances
tracer dispersion, a phenomenon strongly reminiscent of that seen in
incompressible background fluid. We characterise the dispersion by the
long-time diffusion coefficient of a single tracer, numerically and
analytically with a mean-field density functional analysis. We find a
surprising effect where an increased particle density enhances the diffusion
coefficient, challenging the notion that crowding effects tend to reduce
diffusion. Here, inter-particle interactions push particles closer to the
fluctuating channel walls. Then, interactions between the fluctuating wall and
the now-nearby particles drive particle mixing. Our mechanism is sufficiently
general that we expect it to apply to various systems. In addition, the
perturbation theory we derive quantifies dispersion in generic
advection-diffusion systems with arbitrary spatiotemporal drift.
