  Microswimmers are active particles of microscopic size that self-propel by
setting the surrounding fluid into motion. According to the kind of far-field
fluid flow that they induce, they are classified into pushers and pullers. Many
studies have explored similarities and differences between suspensions of
either pushers or pullers, but the behavior of mixtures of the two is still to
be investigated. Here, we rely on a minimal discrete microswimmer model,
particle-resolved, including hydrodynamic interactions, to examine the
orientational ordering in such binary pusher-puller mixtures. In agreement with
existing literature, we find that our monodisperse suspensions of pushers do
not show alignment, whereas those of solely pullers spontaneously develop
ordered collective motion. By continuously varying the composition of the
binary mixtures, starting from pure puller systems, we find that ordered
collective motion is largely maintained up to pusher-puller composition ratios
of about 1:2. Surprisingly, pushers when surrounded by a majority of pullers
are more tightly aligned than indicated by the average overall orientational
order in the system. Our study outlines how orientational order can be tuned in
active microswimmer suspensions to a requested degree by doping with other
species.
