  We present a hydrodynamic study of a monolayer of squirmer model
microswimmers confined to a boundary by strong gravity using the simulation
method of multi-particle collision dynamics. The squirmers interact with each
other via their self-generated hydrodynamic flow fields and thereby form a
variety of fascinating dynamic states when density and squirmer type are
varied. Weak pushers, neutral squirmers, and pullers have an upright
orientation. With their flow fields they push neighbors away and thereby form a
hydrodynamic Wigner fluid at lower densities. Furthermore, states of
fluctuating chains and trimers, of kissing, and at large densities a global
cluster exist. Finally, pushers at all densities can tilt against the wall
normal and their in-plane velocities align to show swarming. It turns into
chaotic swarming for strong pushers at high densities. We characterize all
these states quantitatively.
