  Biological and artificial microswimmers often have to propel through a
variety of environments, ranging from heterogeneous suspending media to strong
geometrical confinement. Under confinement, local flow fields generated by
microswimmers, and steric and hydrodynamic interactions with their environment
determine the locomotion. We propose a squirmer-like model to describe the
motion of microswimmers in cylindrical microchannels, where propulsion is
generated by a fixed surface slip velocity. The model is studied analytically
for cylindrical swimmer shapes, and by numerical hydrodynamics simulations for
spherical and spheroidal shapes. For the numerical simulations, we employ the
dissipative particle dynamics method for modelling fluid flow. Both the
analytical model and simulations show that the propulsion force increases with
increasing confinement. However, the swimming velocity under confinement
remains lower than the swimmer speed without confinement for all investigated
conditions. In simulations, different swimming modes (i.e. pusher, neutral,
puller) are investigated, and found to play a significant role in the
generation of propulsion force when a swimmer approaches a dead end of a
capillary. Propulsion generation in confined systems is local, such that the
generated flow field generally vanishes beyond the characteristic size of the
swimmer. These results contribute to a better understanding of microswimmer
force generation and propulsion under strong confinement, including the motion
in porous media and in narrow channels.
