**4. Conclusions and Perspectives**

Crystallization offers a variety of methods for spontaneous macroscopic chiral symmetry breaking. The corresponding processes are run close to equilibrium for Deracemization Induced by a Flux of Energy Crossing the Suspension (DIFECS) (Viedma ripening, TCID and other deracemization variants where a single flux or several fluxes of energy pass through a suspension) or with a significant departure from equilibrium for the Kondepudi experiment with preferential secondary nucleation, or even very far from equilibrium in the case of Preferential Enrichment (PE). Those processes include an important amplification of local fluctuations and their mechanisms are currently the focus of many studies. Interestingly, as the system departs more from equilibrium, the mechanical stress imposed on the system has to be softened in order to observe the spontaneous chiral symmetry breaking (see Table 1). Indeed, in Viedma ripening performed close to equilibrium, all sorts of abrasions, breakages, defects induced by shear forces, etc., are beneficial to the advancement of chiral symmetry breaking. In the Kondepudi experiment, the mechanical stress must be softer to avoid primary and secondary heteronucleation in the system. In the case of Preferential Enrichment (PE), the very large departure from equilibrium has to be associated with almost stagnant conditions. Indeed, for the latter case a simple magnetic stirrer is sufficient to return the solution to normal conditions of crystallization without macroscopic chiral symmetry breaking (see Table 1 below).

**Table 1.** Spontaneous macroscopic break of symmetry by means of crystallization. In theory, the final state of the system is not predictable. There is a stochastic evolution during the first step and then amplification. In Deracemization Induced by a Flux of Energy Crossing the Suspension (DIFECS) processes, there are agonist effects between ultrasound, microwaves, attrition, TCID and damped TCID (and photons for light-sensitive molecules).


It is also possible to control a macroscopic chiral symmetry breaking such as Viedma ripening, TCID and its variants, or SOAT or preferential crystallization. Some of these elegant processes could be made productive enough for industrial applications. Thus, there are fundamental and applied interests in these crystallization processes associated with macroscopic symmetry breaking.

**Author Contributions:** Both authors contributed to this publication. All authors have read and agreed to the published version of the manuscript.

**Funding:** This review is part of a PhD funded by the University of Rouen Normandy.

**Conflicts of Interest:** The authors declare no conflict of interest.
