  Viscoelastic phase separation of colloidal suspensions can be interrupted to
form gels either by glass transition or by crystallization. With a new confocal
microscopy protocol, we follow the entire kinetics of phase separation, from
homogeneous phase to different arrested states. For the first time in
experiments, our results unveil a novel crystallization pathway to sponge-like
porous crystal structures. In the early stages, we show that nucleation
requires a structural reorganization of the liquid phase, called stress-driven
ageing. Once nucleation starts, we observe that crystallization follows three
different routes: direct crystallization of the liquid phase, Bergeron process,
and Ostwald ripening. Nucleation starts inside the reorganised network, but
crystals grow past it by direct condensation of the gas phase on their surface,
driving liquid evaporation, and producing a network structure different from
the original phase separation pattern. We argue that similar crystal-gel states
can be formed in monoatomic and molecular systems if the liquid phase is slow
enough to induce viscoelastic phase separation, but fast enough to prevent
immediate vitrification. This provides a novel pathway to form nano-porous
crystals of metals and semiconductors without dealloying, which may be
important for catalytic, optical, sensing, and filtration applications.
