Microscopic modeling of nematic elastic constants

Speaker: Davide Revignas (University of Padova, Italy)

Abstract:

The orientational elasticity of nematic liquid crystals is commonly understood in terms of the Oseen-Frank free energy density, where the cost of distortions from the ground state with a uniform director field is decomposed into contributions corresponding to three modes - splay, twist, and bend - each associated with material-specific parameters called elastic constants. Simple thermotropic systems made of small organic molecules exhibit comparable elastic constants, weakly dependent on the chemical structure and monotonically increasing with increasing nematic order. This behavior is in agreement with the predictions of classical microscopic theories of elasticity, whose general aim is to estimate the elastic constants starting from a microscopic model for the constituent particles and the interactions between them. Recently, however, various examples of unusual elasticity have been reported: these include the exceptionally low costs of bend or splay deformations in mesogens with unconventional shapes, which have been linked to the formation of novel nematic phases, and the elastic behavior of nematic semiflexible polymers, which is generally poorly understood. In addition, the spontaneous twisting of the director in achiral chromonic liquid crystals poses another puzzle for classical theoretical approaches to nematic elasticity. In this seminar we will show how introducing new elements in the microscopic theory can provide insights into the variety of experimental behaviors.

This seminar will take place in room D1.112