ELFT HÍRADÓ

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PhD házivédésMáthé Marcell TiborHUN-REN Wigner FK SZFIInstabilities and cross 
effects in ferroelectric nematic liquid crystals2024. február 27. kedd, 
10.00, KFKI Telephely, I. épület 1. emeleti TanácsteremExperimental 
confirmation of the existence of the first three-dimensional liquid with 
ferroelectric properties was achieved a few years ago. The ferroelectric 
nematic liquid crystals exhibit huge spontaneous electric polarization while 
keeping fluidity. The topic of the presented PhD work covers the discovered 
cross-effects and electric field induced instabilities in polar nematics.The 
unusual symmetry of the phase allowed the exploration of new cross effects in 
ferroelectric liquids. I will present a thermo-hydrodynamical effect in 
sessile droplets, where a temperature gradient can induce stable flow of the 
material [1]. Furthermore, I will describe the presence of inverse 
piezoelectricity (electric field induced mechanical vibration) in polar 
nematic films [2].I will show interfacial instabilities in liquid bridges and 
in sessile droplets in the presence of external electric fields [3]. The 
primary instability called ramification induces the growth of stable branches 
from the liquid leading to the formation of fractal-like structures. A 
secondary instability resembles to the Rosensweig type labyrinthine 
instability observed in ferrofluids and in dielectrics, nonetheless in our 
case the required threshold field was orders of magnitude smaller. To 
understand these instabilities, I conducted various experiments and 
simulations using the finite element method. It has turned out that the 
interfacial instabilities can cause droplet fragmentation, resulting in the 
creation of distinct active particle-like objects called "febots". When 
subjected to a specific voltage and frequency range, febots exhibit a high 
degree of mobility, forming a new type of artificial self-propelled active 
system. I will present the characteristic properties and the statistical 
analysis of the febot motion.Finally, the filament forming ability of polar 
nematics is discussed [4]. I will present how ferroelectric fluid fibres are 
stabilised without and with the presence of an external electric field. 
Additionally, new measurement techniques for determining physical parameters 
of ferroelectric nematics, such as spontaneous electric polarization and 
viscosity, are presented.References[1] M.T. Máthé, Á. Buka, A. Jákli, P. 
Salamon, Ferroelectric nematic liquid crystal thermomotor, Physical Review E 
105, L052701 (2022) DOI: https://doi.org/10.1103/PhysRevE.105.L052701[2] M.T. 
Máthé, M.S.H. Himel, A. Adaka, J.T. Gleeson, S. Sprunt, P. Salamon, A. Jákli, 
Liquid Piezoelectric Materials: Linear Electromechanical Effect in Fluid 
Ferroelectric Nematic Liquid Crystals, Advanced Functional Materials, 2314158 
(2024) DOI: https://doi.org/10.1002/adfm.202314158[3] M.T. Máthé, B. Farkas, 
L. Péter, Á. Buka, A. Jákli, P. Salamon, Electric field-induced interfacial 
instability in a ferroelectric nematic liquid crystal, Scientific Reports 13, 
6981 (2023) DOI: https://doi.org/10.1038/s41598-023-34067-1 [4] M.T. Máthé, 
K. Perera, Á. Buka, P. Salamon, A. Jákli, Fluid Ferroelectric Filaments, 
Advanced Science 2305950 (2024) DOI: 
https://doi.org/10.1002/advs.202305950 Minden érdeklődőt szívesen 
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