On the free energy of nematic wetting layers

Sullivan, D. E.; Lipowsky, Reinhard

doi:10.1139/v88-094

Cited by 16 publications

(8 citation statements)

References 19 publications

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“…For systems with C in the range 1.0-1.6, however, we find that the best fit to our data is Γ ∝ ln t. This logarithmic growth, is due to the exponential decay of the order parameter profiles. Within the experimental temperature range, we observe a crossover between the logarithmic and power law behaviours [11] for the systems with the largest values of C, i.e. the complete-wetting scaling region increases with C, as expected.…”

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confidence: 82%

“…If the nematic layer is sufficiently thick (larger than l 0 , with 10 −5 cm ≤ l 0 ≤ 10 −3 cm), the director will distort over the surface layer in order to yield the equilibrium orientations at both the NI and NV interfaces. Under these conditions, the nematic coverage has been shown to diverge as Γ ∝ t − 1 2 [11]. The largest layer thickness reported by Kasten and Strobl is l ≈ 200-250 Å [4], which is too small to allow for this type of distorsion.…”

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confidence: 96%

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Wetting and interfacial order at nematic free surfaces

Río¹,

Gama²,

Miguel³

et al. 1996

Europhys. Lett.

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PACS. 61.30Cz -Theory and models of liquid crystals. PACS. 68.45Gd -Wetting. PACS. 68.10Cr -Surface energy (surface tension, interface tension, angle of contact, etc.).Abstract. -We apply a density-functional theory to the free surface of a model liquid crystal, to investigate the orientational order at the nematic free surface of systems exhibiting orientational wetting at the isotropic liquid-vapour interface. It is shown that the theory accounts for the growth of the surface ordered layer which develops at the nematic-vapour interface, as the temperature increases up to the triple point, as was recently reported for 4-cyano-4 -n-alkyl-biphenyls. We have found a correlation between the growth of this layer and the non-monotonic temperature dependence of the interfacial tension below the triple point. The theoretical results are in excellent agreement with the results for 8CB.Fluids of strongly anisotropic molecules, such as liquid crystals, exhibit a large variety of fluid phases, characterized by different types of long-range order (orientational and positional). The description of the fluid interfaces (density profiles and interfacial tension) between two (or more) of these phases is an extremely interesting problem both theoretically as well as regarding potential applications (e.g., displays). In the simplest case of planar interfaces the broken translational symmetry induces, in general, a lower rotational symmetry at the interface and thus surface-induced alignment is expected to be important when liquid crystalline phases are involved [1]. As a consequence, nematic free surfaces exhibit a wealth of behaviour characterized by a wide variety of interfacial orders and structural phase transitions (wetting, anchoring, etc.) may (and do) occur [1].The first attempt to account for the existence of a preferred molecular orientation at the nematic-isotropic liquid (NI) interface was made by de Gennes [2]. Although several theories were developed following de Gennes' pioneering work, their consideration of a single orientational order parameter implies that there is no excess order at the nematic-vapour (NV) interface [3], by contrast with recent experimental observations. Indeed, Kasten and Strobl [4] have carried out a detailed study of the free surface of the homologous series of 4-cyano-4 -n-alkyl-biphenyls (nCB), with n = 5, 6, 7, 8, by using reflection ellipsometry. They found that the isotropic-vapour (IV) interface is completely wet by a homeotropically aligned nematic (N) film for n = 6, 7, 8, while partial wetting ocurs for ( * ) Permanent address:

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confidence: 82%

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confidence: 96%

Wetting and interfacial order at nematic free surfaces

Río¹,

Gama²,

Miguel³

et al. 1996

Europhys. Lett.

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“…If the thickness of such a film grows arbitrarily large as T reduces to T IN then at T IN the nematic phase is said to completely wet the substrate isotropic interface. Detailed theoretical predictions are available for this behaviour [7]. Otherwise, this interface is only partially wet or non-wet by nematic.…”

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confidence: 99%

Anchoring and orientational wetting of nematic liquid crystals on semi-fluorinated self-assembled monolayer surfaces

et al. 2002

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We demonstrate that it is possible to achieve exceptionally fine control over the anchoring of liquid crystals via the use of semi-fluorinated self-assembled monolayers of varying spacer length. We use this approach to map the detailed shape of an anchoring transition surface in thermodynamic phase space and to explore the links between anchoring and orientational wetting phenomena. These results allow one to design a substrate that will place a standard liquid-crystal film arbitrarily close to an anchoring transition between homeotropic and planar anchoring.

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“…For this set of parameters the tilt angle at the NV interface is 4 = 0 . Following an argument of Sullivan and Lipowsky [38], competing orientations at the NI and NV interfaces induce an elastic long-ranged repulsive interaction between them, which favors the growth of an N layer at the IV interface. For this system and within our numerical calculations we have been unable to verify this prediction, i.e.…”

Section: E Wetting Transitionsmentioning

confidence: 98%

Surface-induced alignment at model nematic interfaces

et al. 1995

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We have applied the generalized van der Waals theory to a model liquid crystal that includes, explicitly, all of the second-order terms in the spherical harmonic expansion of the anisotropic intermolecular potential. We have investigated the orientational order induced by each one of these terms, as well as the order resulting from the competition of various terms included in the potential. It was shown that, for appropriate choices of the relative strengths of the spherical harmonic coeKcients, the theory is capable of accounting qualitatively for all the orientational e8'ects observed at nematic interfaces, including tilted orientations. In particular, different molecular alignments at the nematic-vapor and nematic-isotropic interfaces of a given nematogen were described as the result of competing terms in the anisotropic interactions. Additionally, we have shown that temperaturedriven orientational transitions may occur in systems characterized by this type of interaction. For a given choice of parameters, we have also found an orientational transition, which is related to a wetting transition at the nematic-vapor interface. Finally, it was shown that complete wetting of the isotropic liquid-vapor interface by the nematic phase may be destroyed as a result of the competition between di8'erent terms in the potential. Similarly, a reentrant wetting transition at the nematic-vapor interface by the isotropic phase was found, as a result of this competition, at the nematic-vapor interface.

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On the free energy of nematic wetting layers

Cited by 16 publications

References 19 publications

Wetting and interfacial order at nematic free surfaces

Wetting and interfacial order at nematic free surfaces

Anchoring and orientational wetting of nematic liquid crystals on semi-fluorinated self-assembled monolayer surfaces

Surface-induced alignment at model nematic interfaces

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