  We study the role of hydrodynamic interactions in the collective behaviour of
collections of microscopic active particles suspended in a fluid. We introduce
a novel calculational framework that allows us to separate the different
contributions to their collective dynamics from hydrodynamic interactions on
different length scales. Hence we are able to systematically show that
lubrication forces when the particles are very close to each other play as
important a role as long-range hydrodynamic interactions in determining their
many-body behaviour. We find that motility-induced phase separation is
suppressed by near-field interactions, leading to open gel-like clusters rather
than dense clusters. Interestingly, we find a globally polar ordered phase
appears for neutral swimmers with no force dipole that is enhanced by near
field lubrication forces in which the collision process rather than long-range
interaction dominates the alignment mechanism.
