Formation of porous crystals via viscoelastic phase separation

TL;DR

This study reveals a new crystallization pathway during viscoelastic phase separation in colloidal suspensions, leading to sponge-like porous crystal structures with potential applications in catalysis and filtration.

Contribution

It introduces a novel experimental observation of crystallization pathways and porous crystal formation during viscoelastic phase separation, using a new confocal microscopy protocol.

Findings

Unveiled a new crystallization pathway involving stress-driven ageing.

Observed three distinct routes of crystallization: direct, Bergeron, and Ostwald ripening.

Demonstrated formation of nano-porous crystals without dealloying.

Abstract

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.