  Non-reciprocal (NR) effective interactions violating Newton's third law occur
in many biological systems, but can also be engineered in synthetic, colloidal
systems. Recent research has shown that such NR interactions can have
tremendous effects on the overall collective behaviour and pattern formation,
but can also influence aggregation processes on the particle scale. Here we
focus on the impact of non-reciprocity on the self-assembly of an (originally
passive) colloidal system with anisotropic interactions whose character is
tunable by external fields. In the absence of non-reciprocity, that is, under
equilibrium conditions, the colloids form aggregates with extremely long life
times [Kogler et al., Soft Matter 11, 7356 (2015)], indicating kinetic
trapping. Here we study, based on Brownian Dynamics (BD) simulations in 2D, a
NR version of this model consisting of two species with reciprocal isotropic,
but NR anisotropic interactions. We find that NR induces an effective
propulsion of particle pairs and small aggregates forming at initial stages of
self-assembly, an indication of the NR-induced non-equilibrium. The shape and
stability of these initial clusters strongly depends on the degree of
anisotropy. At longer times we find, for weak NR interactions, large (even
system-spanning) clusters where single particles can escape and enter at the
boundaries, in stark contrast to the small rigid aggregates appearing at the
same time in the passive case. In this sense, weak NR shortcuts the
aggregation. Increasing the degree of NR (and thus, propulsion), we even
observe large-scale phase separation if the interactions are weakly
anisotropic. In contrast, system with strong NR and anisotropy remain
essentially disordered. Overall, NR interactions are shown to destabilize the
rigid aggregates interrupting self-assembly in the passive case, helping the
system to overcome kinetic barriers.
