  The separation of substances into different phases is ubiquitous in nature
and important scientifically and technologically. This phenomenon may become
drastically different if the species involved, whether molecules or
supramolecular assemblies, interconvert. In the presence of an external force
large enough to overcome energetic differences between the interconvertible
species (forced interconversion), the two alternative species will be present
in equal amounts, and the striking phenomenon of steady-state, restricted phase
separation into mesoscales is observed. Such microphase separation is one of
the simplest examples of dissipative structures in condensed matter. In this
work, we investigate the formation of such mesoscale steady-state structures
through Monte Carlo and Molecular Dynamics simulations of three physically
distinct microscopic models of binary mixtures that exhibit both equilibrium
(natural) interconversion and a nonequilibrium source of forced
interconversion. We show that this source can be introduced through an internal
imbalance of intermolecular forces or an external flux of energy that promotes
molecular interconversion, possible manifestations of which could include the
internal nonequilibrium environment of living cells or a flux of photons. The
main trends and observations from the simulations are well captured by a
non-equilibrium thermodynamic theory of phase transitions affected by
interconversion. We show how a nonequilibrium bicontinuous microemulsion or a
spatially modulated state may be generated depending on the interplay between
diffusion, natural interconversion, and forced interconversion.
