Áramlási jelenségek folyadékkristályokban = Flux phenomena in liquid crystals

Standard homeotróp nematikus folyadékkristályokban kimértük az elektrokonvekciós mintázatok jellemzőit. Feltérképeztük a szuperponált mágneses tér ill. nyíróáramlás hatását a mintázat jellemzőire. Kidolgoztuk a mintázat lebomlásának precíz elméleti leírását, amivel a relaxációs idő hullámszámfüggését diffrakcióval megmérve kíváló egyezést találtunk. Meghatároztuk a szmektikus fázissal is rendelkező nematikusokban megfigyelt standard és nemstandard elektrokonvekciós mintázatok jellemzőit. Megmutattuk, hogy banán nematikus folyadékkristályban is fellép elektrokonvekció. Feltártuk a nematikus anizotrópiái, orientációja és az elektrokonvekciós mintázatok kialakulása és morfológiája közötti kapcsolatot. Egy általunk kidolgozott új mérési módszerrel bizonyítottuk, hogy a banán nematikus flexoelektromos együtthatója kb. 1000-szerese a hagyományos nematikusokénak. | We measured the characteristics of electroconvection patterns in standard homeotropic nematic liquid crystals. We explored the effect of superposed magnetic field or shear flow on the patterns. We worked out a rigorous theoretical description for the decay of patterns which had an excellent agreement with the wavenumber dependence of the relaxation times measured by diffraction. We determined the characteristics of the standard and nonstandard electroconvection patterns observed in nematics with smectic phases. We showed that electroconvection exists in banana nematics too. We explored the relation between the anisotropies and orientation of nematics and the existence and morphology of electroconvection patterns. We worked out a new measuring method by which we proved that the flexoelectric coefficient of a banana nematic is about 1000 times bigger than in usual nematics

Flexoelectricity and pattern formation in nematic liquid crystals

We present in this paper a detailed analysis of the flexoelectric instability of a planar nematic layer in the presence of an alternating electric field (frequency $\omega$), which leads to stripe patterns (flexodomains) in the plane of the layer. This equilibrium transition is governed by the free energy of the nematic which describes the elasticity with respects to the orientational degrees of freedom supplemented by an electric part. Surprisingly the limit $\omega \to 0$ is highly singular. In distinct contrast to the dc-case, where the patterns are stationary and time-independent, they appear at finite, small $\omega$ periodically in time as sudden bursts. Flexodomains are in competition with the intensively studied electro-hydrodynamic instability in nematics, which presents a non-equilibrium dissipative transition. It will be demonstrated that $\omega$ is a very convenient control parameter to tune between flexodomains and convection patterns, which are clearly distinguished by the orientation of their stripes

Temporal response to harmonic driving in electroconvection

The temporal evolution of the spatially periodic electroconvection (EC) patterns has been studied within the period of the driving ac voltage by monitoring the light intensity diffracted from the pattern. Measurements have been carried out on a variety of nematic systems, including those with negative dielectric and positive conductivity anisotropy, exhibiting "standard EC" (s-EC), those with both anisotropies negative exhibiting "non-standard EC" (ns-EC), as well as those with the two anisotropies positive. Theoretical predictions have been confirmed for stationary s-EC and ns-EC patterns. Transitions with Hopf bifurcation have also been studied. While traveling had no effect on the temporal evolution of dielectric s-EC, traveling conductive s-EC and ns-EC patterns exhibited a substantially altered temporal behavior with a dependence on the Hopf frequency. It has also been shown that in nematics with both anisotropies positive, the pattern develops and decays within an interval much shorter than the period, even at relatively large driving frequencies.Comment: 19 pages, 5 figure

Nemkirális építőelemekből álló polárosan rendezett folyadékkristályok = Mesogens with polar ordering of non-chiral building blocks

A pályázat munkatervében körvonalazott kutatási tervnek megfelelően a munka több szálon futott. Első lépést a vegyületek kémiai szintézise jelentette, ami a vizsgálni kívánt molekulaszerkezetű vegyületek megtervezését, a szintézisút kidolgozását és magát az előállítást foglalta magában. Számos esetben a vegyületekből elegyek is készültek, a fizikai folyamatok jobb megértése céljából. A pályázatból támogatott kémiai munka igen eredményes volt, nagy számú, többségében új vegyület előállítása történt meg. Ezt követték az anyagok fizikai-kémiai és fizikai módszerekkel történő vizsgálatai, amelyek során számos új eredmény és felismerés született, amit a megjelent publikációk száma és a folyóiratok minősége is demonstrál. Új, kísérleti vizsgálati módszereket és esetenként elméleti modelleket is kidolgoztunk. A pályázat támogatásával 45 cikket jelentettünk meg, amelyek kumulatív impakt faktora 103 (ebbol 5.15 IF/MFt adódik) és 96 konferenciaelőadást tartottunk nemzetközi rendezvényeken. | The research work during the project has been conducted in several directions according to the work plan of the proposal. The first step was the chemical synthesis of the desired materials which consisted of the design of the molecules, the finding of the synthetic rout and the actual production process of the compounds. In several subprojects mixtures have also been prepared from the pure compounds in order to be able to follow and understand some specific physical processes. The chemical work carried out during the project was very fruitful, a great number of mostly new substances have been synthesized. As a next step, the physico-chemical and the physical investigations followed, which have produced several original results and achievements as demonstrated by the large number of the publications and the impact factors of the journals. New experimental methods and theoretical models have been worked out. During the project 45 publications appeared with a cumulative impact factor of 103 (5.15 IF/MFt) and 96 conference contributions have been made at international meetings

Electric field-induced interfacial instability in a ferroelectric nematic liquid crystal

Studies of sessile droplets and fluid bridges of a ferroelectric nematic liquid crystal in externally applied electric fields are presented. It is found that above a threshold a fingering instability occurs, resembling to Rayleigh-type instability observed in charged droplets in electric fields or circular drop-type instabilities observed in ferromagnetic liquids in magnetic field. The frequency dependence of the threshold voltage was determined in various geometries. The nematic director and ferroelectric polarization direction was found to point along the tip of the fingers that appear to repel each other, indicating that the ferroelectric polarization is essentially parallel to the director. The results are interpreted in analogy to the Rayleigh and circular drop-type instabilities

Suppression of spatially periodic patterns by dc voltage

The effect of superposed dc and ac applied voltages on two types of spatially periodic instabilities in nematic liquid crystals, flexoelectric domains (FD) and electroconvection (EC), was studied. The onset characteristics, threshold voltages and critical wave vectors, were determined. We found that in general the superposition of driving with different time symmetries inhibits the pattern forming mechanisms for FD and EC as well. As a consequence the onset extends to much higher voltages than the individual dc or ac thresholds. A dc bias induced reduction of the crossover frequency from the conductive to the dielectric EC regimes and a peculiar transition between two types of flexodomains with different wavelengths were detected. Direct measurements of the change of the electrical conductivity and its anisotropy, induced by the applied dc voltage component, showed that the dc bias substantially affects both parameters. Taking into account the experimentally detected variations of the conductivity in the linear stability analysis of the underlying nemato-hydrodynamic equations, a qualitative agreement with the experimental findings on the onset behaviour of spatially periodic instabilities was obtained