  Suspensions of spherical active particles often show microphase separation.
At a continuum level, coupling their scalar density to fluid flow, there are
two distinct explanations. Each involves an effective interfacial tension: the
first mechanical (causing flow) and the second diffusive (causing Ostwald
ripening). Here we show how the negative mechanical tension of contractile
swimmers creates, via a self-shearing instability, a steady-state life cycle of
droplet growth interrupted by division whose scaling behavior we predict. When
the diffusive tension is also negative, this is replaced by an arrested regime
(mechanistically distinct, but with similar scaling) where division of small
droplets is prevented by reverse Ostwald ripening.
