Predicting surface anchoring: molecular organization across a thin film of 5CB liquid crystal on silicon

Pizzirusso, Antonio; Berardi, Roberto; Muccioli, Luca; Ricci, Matteo; Zannoni, Claudio

doi:10.1039/c1sc00696g

Cited by 72 publications

(100 citation statements)

References 51 publications

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“…It is typical of a sizable collective ordering of the molecules induced by the interaction with the wall (interface anchoring). Silica surfaces favor a planar anchoring in the case of the family of cyanobiphenyl liquid crystals (CBs), as documented in experiments [33,34] and atomistic computer simulations [18]. The magnitude and the positive sign of the changes of the optical birefringence for the lowest filling fraction (f =0.12, corresponding to a partially filled, first adsorbed monolayer) indicates indeed such an alignment.…”

Section: Resultsmentioning

confidence: 89%

“…Typically they were performed upon complete filling of the confining geometry [2,4,[7][8][9]12] and thus revealed integral quantities on the phase behavior. Phenomenological considerations [1] and computer simulations on liquid crystals in thin film and nanopore confinement [13][14][15][16][17][18] indicate, however, pronounced heterogeneities, in particular interface-induced molecular layering and orientational immobility in the pore wall proximity. Despite recent experimental advancements in optical techniques directly probing orientational order parameter profiles in the proximity of planar, solid walls [19][20][21], achieving the spatial resolution necessary to rigorously explore such inhomogeneities in nanometer-sized capillaries remains still experimentally extremely demanding.…”

Section: Introductionmentioning

confidence: 99%

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Thermotropic nematic order upon nanocapillary filling

et al. 2013

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Optical birefringence and light absorption measurements reveal four regimes for the thermotropic behavior of a nematogen liquid (7CB) upon sequential filling of parallel-aligned capillaries of 12 nm diameter in a monolithic, mesoporous silica membrane. No molecular reorientation is observed for the first adsorbed monolayer. In the film-condensed state (up to 1 nm thickness) a weak, continuous paranematic-to-nematic (P-N) transition is found, which is shifted by 10 K below the discontinuous bulk transition at TIN =305 K. The capillary-condensed state exhibits a more pronounced, albeit still continuous P-N reordering, located 4 K below TIN. This shift vanishes abruptly on complete filling of the capillaries. It could originate in competing anchoring conditions at the free inner surfaces and at the pore walls or result from the 10 MPa tensile pressure release associated with the disappearance of concave menisci in the confined liquid upon complete filling. The study documents that the thermo-optical properties of nanoporous systems (or single nano-capillaries) can be tailored over a surprisingly wide range simply by variation of the the filling fraction with liquid crystals.

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Section: Resultsmentioning

confidence: 89%

Section: Introductionmentioning

confidence: 99%

Thermotropic nematic order upon nanocapillary filling

et al. 2013

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“…Computer simulations on LCs in thin film and pore geometry can give here important complementary insights [13,24,[41][42][43][44][45][46][47][48][49][50]. These studies indicate pronounced spatial heterogeneities, in particular interface-induced molecular layering and radial gradients both in the orientational order and reorientational dynamics in cylindrical pore geometry [51].…”

Section: Fig 1: (Color Online)mentioning

confidence: 99%

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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“…This experimental design is similar to methods for measuring the azimuthal anchoring energy of liquid crystals on treated substrates [19]. The mechanism of liquid crystal alignment induced by a rubbed polymer layer itself is the subject of much study but is thought to include physical grooves, aligned polymer chains, and the van der Waals torque for surfaces in contact [20][21][22][23]. The last of these is equivalent to the short-ranged Casimir torque of our proposed study, though distinguishing the van der Waals effect from other alignment effects is difficult.…”

Section: Introductionmentioning

confidence: 99%

Rotation of a liquid crystal by the Casimir torque

Somers

Munday

2015

Phys. Rev. A

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We present a calculation of the Casimir torque acting on a liquid crystal near a birefringent crystal. In this system, a liquid crystal bulk is uniformly aligned at one surface and is twisted at the other surface by a birefringent crystal, e.g. barium titanate. The liquid crystal is separated from the solid crystal by an isotropic, transparent material such as SiO2. By varying the thickness of the deposited layer, we can observe the effect of retardation on the torque (which differentiates it from the close-range van der Waals torque). We find that a barium titanate slab would cause 5CB (4-cyano-4 -pentylbiphenyl) liquid crystal to rotate by 10 • through its bulk when separated by 35 nm of SiO2. The optical technique for measuring this twist is also outlined.

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Predicting surface anchoring: molecular organization across a thin film of 5CB liquid crystal on silicon

Cited by 72 publications

References 51 publications

Thermotropic nematic order upon nanocapillary filling

Thermotropic nematic order upon nanocapillary filling

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

Rotation of a liquid crystal by the Casimir torque

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