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Energy landscapes of soft matter: From super-cooled liquids to liquid crystals

Final Activity Report Summary - LIQUIDXTAL (Energy landscapes of soft matter: From super-cooled liquids to liquid crystals)

Soft matter includes seemingly diverse condensed matter systems. The concepts of broken symmetry and order parameter play an important unifying role to link homogeneous isotropic liquids with short-range order at one end of the spectrum, and liquid crystals with long-range order in their anisotropic mesophases, at the other. Anisotropy in the molecular shape plays a crucial role in the rich phase behaviour that thermotropic liquid crystals exhibit upon temperature variation. Calamitic liquid crystals consist of rod-like molecules; discotic liquid crystals comprise disc-like molecules. In the absence of sufficient anisotropy in shape, an isotropic liquid can be supercooled below its freezing temperature by cooling fast enough to avoid crystallisation. The project undertaken was aimed at understanding the structure, thermodynamics, and dynamics of thermotropic liquid crystals and supercooled liquids, in a computational approach employing coarse-grained models, with special emphasis on the study of the underlying potential energy surface.

We have presented the energy landscape view of phase transitions and slow dynamics in thermotropic liquid crystals. A striking similarity between thermotropic liquid crystals and supercooled liquids in the exploration of the energy landscape has been revealed. The correspondence between the onset of temperature-dependent exploration of the energy landscape and the breakdown of Arrhenius behaviour for relaxation times may suggest a common landscape mechanism for slow dynamics in soft matter systems. Our study on the phase behaviour of discotic liquid crystals has provided insight into the molecular origin of apparently counter-intuitive spatial organisation of the discogens in the columnar phase. Such understanding has important implications for the rational design of materials with useful optoelectronic applications. To further our understanding, we have explored routes to helicity for assembly of achiral building blocks.

The study suggests that the existence of two competing length scales for intermolecular interactions is the key to the emergence of helicity. We have developed a general, efficient, and robust framework of geometry optimisation for rigid bodies. This computational framework has opened up a plethora of opportunities for studying diverse applications.