Light-induced dynamics of liquid-crystalline droplets on the surface of iron-doped lithium niobate crystals

We investigated the effect of a photovoltaic field generated on the surface of iron-doped lithium niobate crystals on droplets of a ferroelectric nematic liquid crystalline and a standard nematic liquid crystalline material deposited on this surface. When such assembly is illuminated with a laser beam, a wide range of dynamic phenomena are initiated. Droplets located outside the laser spot are dragged in the direction of the illuminated area, while droplets located inside the illuminated region tend to bridge each other and rearrange into tendril-like structures. In the ferroelectric nematic phase (NF) these processes take place via the formation of conical spikes evolving into jet streams, similar to the behavior of droplets of conventional dielectric liquids exposed to overcritical electric fields. However, in contrast to conventional liquids, the jet streams of the NF phase exhibit profound branching. In the nematic phase (N) of both the ferroelectric nematic and the standard nematic material, dynamic processes occur via smooth-edged continuous features typical for conventional liquids subjected to under-critical fields. The difference in dynamic behavior is attributed to the large increase of dielectric permittivity in the ferroelectric nematic phase with respect to the dielectric permittivity of the nematic phase.Comment: 11 pages, 9 figure

Splay nematic phase

Different liquid crystalline phases with long-range orientational but not positional order, so-called nematic phases, are scarce. It rarely occurs that a new nematic phase is discovered and such event is inevitably accompanied by a great interest. Here, we describe a transition from uniaxial to novel nematic phase characterized by a periodic splay modulation of the director. In this new nematic phase, defect structures not present in the uniaxial nematic are observed, which indicates that the new phase has lower symmetry than the ordinary nematic phase. The phase transition is weakly first order with a significant pretransitional behavior, which manifests as strong splay fluctuations. When approaching the phase transition, the splay nematic constant is unusually low and goes towards zero. Analogously to the transition from the uniaxial nematic to the twist-bend nematic phase, this transition is driven by instability towards splay orientational deformation, resulting in a periodically splayed structure. And, similarly, a Landau-de Gennes type of phenomenological theory can be used to describe the phase transition. The modulated splay phase is biaxial and antiferroelectric

Magnetic polydomain liquid crystal elastomers – synthesis and characterisation

Although liquid crystal elastomers (LCE) are a fascinating class of materials with interesting thermomechanical properties of their own, the aim is to enhance their performance or add new features, e.g. response to external stimuli. The generally weak response of organic materials can be significantly intensified by mixing nano-sized magnetic particles into the host polymer matrix. An alternative approach is chemically coupling the nanoparticles to the elastomer. We achieved this by bonding functionalised magnetic nanoplatelets to the backbone of a main-chain LCE and obtained polydomain magnetic liquid crystal elastomers. We measured the magnetisation curves in samples, which were exposed to either small or large magnetic fields – their response being a consequence of partial particle reorientation or magnetic moment flipping. In contrast to the samples, which were exposed to small magnetic field and in which the remanent magnetisation can be reset to zero by heating the sample, the samples with flipped magnetisation within the platelets cannot be reversed into the original state. Coupling of magnetic and mechanical properties shows a slight magneto-elastic response at elevated temperatures and a significant inverse magneto-elastic effect: the magnetisation caused by mechanical stretching is almost equal to the magnetisation caused by an external magnetic field

Photosensitive azocinnamoyl liquid crystals with a bent core

V prispevku predstavljamo sintezo in karakterizacijo serije tekočih kristalov z upognjeno sredico. Upognjeno sredico sestavljajo 3 benzenski obroči povezani z azocinamoilno skupino in še 2 povezana z fotoobčutljivim AZO mostom. Dolžini stranskih repov so bili v vseh primerih enake (C12H25–), medtem ko smo na centralnem in zunanjih benzenskih obročih z substitucijo funkcionalnih skupin spreminjali električne lastnosti spojin in njihov način spontanega urejanja. V nasprotju z spojino, ki je vsebovala “gole” benzenske obroče smo v primerih z funkcionalnimi skupinami dosegli večjo prostorsko porazdelitev naboja ter nove - drugačne makroskopsko urejene faze. Pri karakterizaciji smo se osredotočili predvsen na efekte fotoizomerizacije. Znano je, da v azocinamoilnih funkcionalnih skupinah prihaja do več fotokemičnih procesov, poleg cis-trans prehoda še do ciklizacije in foto-Fries reorganizacije. Preučili smo fazni diagram tekoče kristalnih faz, ki jih tvorijo te spojine, odkrili da ena izmed spojin tvori zanimivo smektično B7 fazo. UV osvetlitev vzorcev povzroči spremembe v ureditvi, opazili smo, da se v planarno urejenih celicah te spremembe hitre in ponovljive, ter da znižajo temperaturo faznega prehoda iz smektične v nematsko fazo. V poizkusih, kjer je bil material samo nanšen na stekleno povšino, smo opazili tipično Schliren teksturo, katera se je ob UV osvetlitivi stabilizirala in je ostala stabilna tudi po več mesecih. Razlog za to smo pripisali dimerizaciji cinamoilnih funkcionalih grup, ki so preprečile cis-konformaciji da se relaksira nazaj v trans-konformacijo, predvidevamo, da zaradi omejene geometrije takšen proces ni možen v planarnih celicah

Thiol click chemistry on gold-decorated MoS2: Elastomer composites and structural phase transitions

We show that gold decorated MoS2flakes are amenable to thiol chemistry by blending them with a cross-linkable thiolated polysiloxane (PMMS). PMMS prevents restacking of dispersed MoS2when transforming the metallic to the semiconducting phase. Cross-linking PMMS yields an elastomer of good optical quality, containing individual, mostly single-layer MoS2flakes

Optical switching of bent-core liquid crystals with azocinnamoyl units

We report on observations of optical switching in bent-core liquid crystals possessing azocinnamoyl groups in both elongated side arms. UV-activated switching was observed in two of the synthesised materials, both of which were studied optically and with dielectric spectroscopy. Polarising microscopy was used to measure the changes in optical transmittance, and while no illumination effect was seen in the nematic phase, visible changes were observed in the smectic phase, most pronounced in the close vicinity of the smectic to nematic-phase transition. Switching dynamics was characterised, and we found that the average switching time varies strongly with temperature. The relaxation process is faster (below (Formula presented.)) and exhibits no significant temperature dependence. Dielectric measurements have shown that the observed smectic C phases were not polar with the molecules possessing a rather small electric dipole moment. Upon illumination, small differences in the dielectric permittivity were observed only in the smectic phase, suggesting that the illumination effect can be attributed to changes in the molecular conformational and orientational order

Photoinduced Displacement of Ferroelectric Nematic Liquid Crystal Droplets on the Surface of Lithium Niobate

International audienc

Running streams of a ferroelectric nematic liquid crystal on a lithium niobate surface

International audienceSessile droplets of a ferroelectric nematic liquid crystalline material were exposed to surface electric fields produced by pyroelectric and photogalvanic (photovoltaic) effects in X-cut irondoped lithium niobate crystals. The resulting dynamic processes were monitored by polarisation optical (video)microscopy (POM). During heating/cooling cycles, at first, the droplets change their shape from spherical to extended ellipsoidal. Then, they start to move rapidly along the surface electric field, i.e. along the crystal's polar axis (c-axis). During this motion, several droplets merge into running streams (tendrils) extending towards the edges of the top surface area. Finally, practically all liquid crystalline material is transported from the top surface to the side surfaces of the crystal. At stabilised temperatures, laser illumination of the assembly causes dynamic processes that are localised to the illuminated area. Also, in this case, the LC droplets merge into several tendril-like formations that are preferentially oriented along the c-axis of the crystal. The pattern of tendrils fluctuates with time, but it persists as long as the illumination is present. In this case, the LC material is transported between the central and the edge regions of the illuminated area