ELECTRO-OPTICAL AND ELECTRO-MECHANICAL RESPONSE OF FERROELECTRIC NEMATIC FLUIDS

The recent discovery of the ferroelectric nematic (NF) phase opened a brand-new field of explorations in liquid crystal science thanks to new physical effects, resulting from the polar nature of this novel mesophase. Indeed, the electrical and optical properties of NF phase, in which molecules share a common direction other than a common orientation, vastly differ from the ones of conventional liquid and liquid-crystalline materials, making it an optimal candidate for both basic research and the realization of next generation electro-optical devices. The present thesis is mainly focused on three major topics, concerning various electro-optical and electro-mechanical properties of the NF phase of RM734, the first reported ferroelectric nematic mesogen. First, the surface alignment of RM734 has been explored upon confining it into flat cells, realized with various rubbed and unrubbed surfaces, proving the importance of the 2D confining surfaces on the 3D structuring and field response of NF materials. Second, the instability events, occurring when a sessile ferroelectric liquid droplet is deposited onto a ferroelectric crystal, has been studied. These phenomena has been interpreted as a particular case of Rayleigh instability, happening when the electrostatic repulsion, caused by the accumulation of polarization charges on the droplet surface, overcomes the surface tension of the liquid. Third, RM734 has been confined into microchannels connecting gold electrodes, demonstrating that the application of low electric fields forces the material to continuously follow the channels, even in sections designed to be directed against the applied field. This is caused by the elimination of the field component perpendicular to the channel surfaces, a phenomenon that has been called “ferroelectric superscreening”. The main effects described in this work, such as polar surface alignment, electrostatic droplet instability and ferroelectric superscreening, are indeed distinctive properties of the ferroelectric nematic phase, never observed in other liquid crystalline mesophases.