Distinctive features of pretransitional behaviour between nematic phases as revealed by DDM

Pretransitional behaviour, with strong softening of the bend or splay elastic constants, is characteristic of two of the nematic to nematic phase transitions reported in the last decade. Such softening is strongly reflected in the pretransitional behaviour of the thermally excited director fluctuations. Here we give a comprehensive overview of the cross-Differential Dynamic Microscopy (c-DDM) method and its application to the investigation of thermal director fluctuations and phase transitions. For this, we build on the potentialities of the method for the investigation of the standard nematic phase of E7 to compare with the pretransitional behaviour of the nematic to twist bend nematic phase transition as well as the nematic to ferroelectric nematic transition.