  Cluster growth in a coagulating system of active particles (such as
microswimmers in a solvent) is studied by theory and simulation. In contrast to
passive systems, the net velocity of a cluster can have various scalings
dependent on the propulsion mechanism and alignment of individual particles.
Additionally, the persistence length of the cluster trajectory typically
increases with size. As a consequence, a growing cluster collects neighbouring
particles in a very efficient way and thus amplifies its growth further. This
results in unusual large growth exponents for the scaling of the cluster size
with time and, for certain conditions, even leads to "explosive" cluster growth
where the cluster becomes macroscopic in a finite amount of time.
