Study of the wettability of ferroelectric fluid on ferroelectric crystals

The objective of the thesis is the study of the wettability of the newly discovered liquid crystalline material 4-[(4-nitrophenoxy)carbonyl]phenyl 2,4-dimethoxybenzoate (RM734), on different kinds of ferroelectric solid surfaces. RM734 is the first liquid crystal that exhibits a ferroelectric nematic phase, which makes it the first ferroelectric liquid material ever observed. The fluid nature of this new phase combined with its polarity, makes its response to electric fields stronger and intrinsically different with respect to both ferroelectric solids and dielectric fluids. When in contact with solid substrates, the RM734 polarization P is always parallel to the surface, no matter its chemistry, since any other direction would lead to an energetically costly accumulation of surface charge σ=P⋅u, u being a unit vector normal to the surface. It is thus of particular interest to investigate the behavior at the interface between RM734 and ferroelectric solids, a situation that we have here studied by depositing sessile liquid crystal droplets on different ferroelectric substrates. Specifically, we have studied the wettability of RM734 drops on different Lithium Niobate substrates with different iron doping levels and different reduction factors, extracting the contact angle as a function of temperature. Results demonstrated that this particular liquid crystal behaves differently on different types of substrates, giving rise to peculiar behaviors, from abrupt changes of the contact angle upon entering the ferroelectric nematic phase, to sudden emission of interfacial fluid jets. Our study contribute an additional piece to the collection of intriguing features characterizing the ferroelectric nematic phase and may have potential for future applications.