  Phase separation is not only ubiquitous in diverse physical systems, but also
plays an important organizational role inside biological cells. However,
experimental studies of intracellular condensates (drops with condensed
concentrations of specific collections of proteins and nucleic acids) have
challenged the standard coarsening theories of phase separation. Specifically,
the coarsening rates observed are unexpectedly slow for many intracellular
condensates. Recently, Folkmann, et al. [Science {\bf 373}, 1218 (2021)] argued
that the slow coarsening rate can be caused by the slow conversion of a
condensate constituent between the state in the dilute phase and the condensate
state. A consequence of this conversion-limited picture is that standard
theories of coarsening in phase separation (Lifshitz-Slyozov--Wagner Ostwald
ripening and drop coalescence schemes) no longer apply. Surprisingly, I show
here that the model equations of conversion-limited phase separation can
instead be mapped onto a grain growth model in a single-phase material in three
dimensions. I further elucidate the universal coarsening behavior in the late
stage using analytical and numerical methods.
