  We experimentally study the dynamics of active particles (APs) in a
viscoelastic fluid under various geometrical constraints such as flat walls,
spherical obstacles and cylindrical cavities. We observe that the main effect
of the confined viscoelastic fluid is to induce an effective repulsion on the
APs when moving close to a rigid surface, which depends on the incident angle,
the surface curvature and the particle activity. Additionally, the geometrical
confinement imposes an asymmetry to their movement, which leads to strong
hydrodynamic torques, thus resulting in detention times on the wall surface
orders of magnitude shorter than suggested by thermal diffusion. We show that
such viscoelasticity-mediated interactions have striking consequences on the
behavior of multi-AP systems strongly confined in a circular pore. In
particular, these systems exhibit a transition from liquid-like behavior to a
highly ordered state upon increasing their activity. A further increase in
activity melts the order, thus leading to a re-entrant liquid-like behavior.
