Continuous Paranematic-to-Nematic Ordering Transitions of Liquid Crystals in Tubular Silica Nanochannels

Kityk, A. V.; Wolff, Matthias; Knorr, K.; Morineau, Denis; Lefort, Ronan; Huber, Patrick

doi:10.1103/physrevlett.101.187801

Cited by 110 publications

(162 citation statements)

References 46 publications

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“…often pre-smectic behaviour is ob- * E-mail: andriy.kityk@univie.ac.at, patrick.huber@tuhh.de servable far above the bulk nematic-to-smectic A transition. [7,25] It has also been shown experimentally [12,26,27], in agreement with expectations from theory [7,28], that there is no "true" isotropic-nematic (I-N) transition for LCs spatially restricted in at least one direction to a few nanometers. The molecular anchoring at the confining walls, quantified by an interfacial field, imposes a partial orientational, that results in a partially nematic ordering of the confined LCs, even at temperatures T far above the bulk I-N transition temperature T b IN .…”

Section: Introductionsupporting

confidence: 74%

“…At higher f s pronounced changes with a characteristic kink at T P N in the C st (T ) dependences are observed. Their continuous evolution (with a precursor behaviour at high T s) contrasts with the discontinuous behaviour observed at the isotropic-tonematic transition of bulk 7CB [12].…”

Section: B Thermotropic Nematic Order: Landau-de Gennes Analysis Andmentioning

confidence: 76%

“…This model describes the external orientational field as an effective nematic ordering field σ. The KKLZ model very successfully describes the general features of the P-N transition observed in spatial confinement [12,27,[37][38][39][40], in particular the T -dependence of the effective order parameter averaged over the entire pore geometry. However, it does not give any microscopic details on the orientational behaviour with regard to spatial inhomogeneities.…”

Section: Fig 1: (Color Online)mentioning

confidence: 99%

“…A σ-value of σ c = 0.5 corresponds to a critical point in the σ−T phase diagram, which separates lines of continuous and discontinuous phase transition evolution [27]. The κ-term accounts for quenched disorder effects attributable to static variations of σ, here most prominently attributable to pore wall inhomogeneities [12,27]. It rises the free energy of the low temperature phase and thus provides, in contrast to a finite value of σ, a downward shift of the effective transition temperature.…”

Section: B Thermotropic Nematic Order: Landau-de Gennes Analysis Andmentioning

confidence: 99%

“…Both the collective orientational (isotropic-to-nematic) and the translational (smectic-toliquid or smectic-to-nematic) transitions have turned out to be significantly affected by finite size and interfacial (solid-liquid or liquid-liquid) interactions introduced by confining walls [1][2][3][4][5][6] or the geometrical constraints in nanoporous media [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24].…”

Section: Introductionmentioning

confidence: 99%

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Inhomogeneous relaxation dynamics and phase behaviour of a liquid crystal confined in a nanoporous solid

et al. 2015

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We report filling-fraction dependent dielectric spectroscopy measurements on the relaxation dynamics of the rod-like nematogen 7CB condensed in 13 nm silica nanochannels. In the film-condensed regime, a slow interface relaxation dominates the dielectric spectra, whereas from the capillarycondensed state up to complete filling an additional, fast relaxation in the core of the channels is found. The temperature-dependence of the static capacitance, representative of the averaged, collective molecular orientational ordering, indicates a continuous, paranematic-to-nematic (P-N) transition, in contrast to the discontinuous bulk behaviour. It is well described by a Landau-deGennes free energy model for a phase transition in cylindrical confinement. The large tensile pressure of 10 MPa in the capillary-condensed state, resulting from the Young-Laplace pressure at highly curved liquid menisci, quantitatively accounts for a downward-shift of the P-N transition and an increased molecular mobility in comparison to the unstretched liquid state of the complete filling. The strengths of the slow and fast relaxations provide local information on the orientational order: The thermotropic behaviour in the core region is bulk-like, i.e. it is characterized by an abrupt onset of the nematic order at the P-N transition. By contrast, the interface ordering exhibits a continuous evolution at the P-N transition. Thus, the phase behaviour of the entirely filled liquid crystal-silica nanocomposite can be quantitatively described by a linear superposition of these distinct nematic order contributions.

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Section: Introductionsupporting

confidence: 74%

Section: B Thermotropic Nematic Order: Landau-de Gennes Analysis Andmentioning

confidence: 76%

Section: Fig 1: (Color Online)mentioning

confidence: 99%

Section: B Thermotropic Nematic Order: Landau-de Gennes Analysis Andmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Inhomogeneous relaxation dynamics and phase behaviour of a liquid crystal confined in a nanoporous solid

et al. 2015

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Photocontrollable Photonic Crystals Based on Porous Silicon Filled with Photochromic Liquid Crystalline Mixture

Bobrovsky

Svyakhovskiy

Bogdanov

et al. 2020

Advanced Optical Materials

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Novel photocontrollable photonic crystals based on porous silicon prepared by electrochemical etching are elaborated and their great potential for creation of photoswitchable materials is demonstrated. For this purpose, silicon‐based photonic porous wafers are filled with photochromic liquid crystalline (LC) nematic mixture formulated by 4‐pentyl‐4′‐cyanobiphenyl and a specific azobenzene derivative. Results of electron paramagnetic resonance spectroscopy reveal that molecules of LC mixture are aligned along the etched channels in silicon plate, i.e., in normal direction. UV irradiation (375 nm) of the resulting composite causes a shift of photonic band gap to longer wavelengths by ≈10 nm. This phenomenon is explained by isothermal photoinduced nematic–isotropic phase transition associated with the bent‐shaped Z isomer formation of the azobenzene compound. During this transition, effective refractive index slightly increases as the initial homeotropic alignment of LC molecules transforms to the isotropic nonaligned state. This process is completely reversible under visible light action (428 nm) and can be repeated many times. Obtained photonic crystal photochromic LC composites can be considered as new promising material for photonic applications.

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Wafer‐Scale Fabrication of Hierarchically Porous Silicon and Silica by Active Nanoparticle‐Assisted Chemical Etching and Pseudomorphic Thermal Oxidation

Gries¹,

Brinker²,

Zeller‐Plumhoff³

et al. 2023

Small

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Many biological materials exhibit a multiscale porosity with small, mostly nanoscale pores as well as large, macroscopic capillaries to simultaneously achieve optimized mass transport capabilities and lightweight structures with large inner surfaces. Realizing such a hierarchical porosity in artificial materials necessitates often sophisticated and expensive top‐down processing that limits scalability. Here, an approach that combines self‐organized porosity based on metal‐assisted chemical etching (MACE) with photolithographically induced macroporosity for the synthesis of single‐crystalline silicon with a bimodal pore‐size distribution is presented, i.e., hexagonally arranged cylindrical macropores with 1 µm diameter separated by walls that are traversed by pores 60 nm across. The MACE process is mainly guided by a metal‐catalyzed reduction–oxidation reaction, where silver nanoparticles (AgNPs) serve as the catalyst. In this process, the AgNPs act as self‐propelled particles that are constantly removing silicon along their trajectories. High‐resolution X‐ray imaging and electron tomography reveal a resulting large open porosity and inner surface for potential applications in high‐performance energy storage, harvesting and conversion or for on‐chip sensorics and actuorics. Finally, the hierarchically porous silicon membranes can be transformed structure‐conserving by thermal oxidation into hierarchically porous amorphous silica, a material that could be of particular interest for opto‐fluidic and (bio‐)photonic applications due to its multiscale artificial vascularization.

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Continuous Paranematic-to-Nematic Ordering Transitions of Liquid Crystals in Tubular Silica Nanochannels

Cited by 110 publications

References 46 publications

Inhomogeneous relaxation dynamics and phase behaviour of a liquid crystal confined in a nanoporous solid

Inhomogeneous relaxation dynamics and phase behaviour of a liquid crystal confined in a nanoporous solid

Photocontrollable Photonic Crystals Based on Porous Silicon Filled with Photochromic Liquid Crystalline Mixture

Wafer‐Scale Fabrication of Hierarchically Porous Silicon and Silica by Active Nanoparticle‐Assisted Chemical Etching and Pseudomorphic Thermal Oxidation

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