The Frank constants of nematic 5CB at atmospheric pressure

Bunning, John D.; Faber, T. E.; Sherrell, P. L.

doi:10.1051/jphys:019810042080117500

Cited by 112 publications

(43 citation statements)

References 20 publications

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“…where n is the LC director, and k 11 7 10 ÿ12 N, k 22 3:5 10 ÿ12 N, and k 33 1 10 ÿ11 N are, respectively, the splay, the twist, and the bend elastic constants [20]. For the surface energy, we used the commonly adopted form [4,11,21] …”

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confidence: 99%

Liquid Crystal Orientation Transition on Microtextured Substrates

et al. 2003

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A uniform alignment of liquid crystal (LC) with finite pretilt was observed on microtextured substrates that were lithographically fabricated with alternating horizontal and vertical corrugations. As the period of alternation was decreased toward 0:8 m, the nematic LC alignment on these substrates changed from inhomogeneous in plane, copying the substrate corrugations, to a uniform configuration with a large pretilt of 40 . This transition is pertinent to a frustrated boundary wherein a lowering in the LC elastic energy due to spatial variation in the LC orientation compromises an increase in the surface anchoring energy. A model based on this idea demonstrates good agreement with the experiment. This result may open up a new arena for tailoring substrate characteristics for LC alignment. DOI: 10.1103/PhysRevLett.91.215501 PACS numbers: 61.30.Hn, 61.30.Dk, 61.30.Eb Substrate conditioning for the alignment of liquid crystal (LC) molecules is essential to the operation of LC displays. Most existing techniques used to produce the alignment effect involve uniform treatment of the substrate surface. An unconventional approach to uniform LC alignment, based on inhomogeneous patterning of the substrate, was proposed some time ago [1,2]. The idea is to create a situation wherein the LC director, in coping with the inhomogeneous substrate pattern, acquires so large an elastic energy that the LC has to compromise with a uniform configuration. In general, this frustrated boundary condition can be achieved by making the pattern period small enough. A few versions of this idea, including the use of inhomogeneous surfactant treatment [1,3] and inhomogeneous rubbing [4], have been reported successful. More recently, an interesting application of these substrates in multistable display, which may potentially lead to lower energy consumption, was demonstrated [5,6]. In these demonstrations, the LC director makes zero pretilt with the substrate, and the switching between different stable LC states was achieved by varying an in-plane electric field. However, out-of-plane switching is preferred since the electric field can be more uniform. But that requires the LC pretilt to be finite [7], which has not been found in any reported embodiments [1,3,4]. In yet some designs, a large LC pretilt is crucial as it widens the range of LC elastic constants usable, which would otherwise be too narrow to be practical [8]. Here we show that on lithographically made substrates with alternating horizontal and vertical corrugations, the nematic alignment undergoes an orientational transition that results in a large pretilt angle (40 ). A simple model used to simulate the transition not only showed good agreement with the experiment but also revealed that the polar surface energies of the frustrated boundary are only 1=10 of typical values. Weak surface anchoring energies are virtuous for lowering device energy consumption and response time [9]. Figure 1 shows a subset of the microtextured substrates employed in this study. They consist of arra...

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Liquid Crystal Orientation Transition on Microtextured Substrates

et al. 2003

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“…For traditional nematics at temperatures in a wide range below the clearing temperature, the bend modulus II is on the order of 10 Ϫ12 J/m and is three-halves to two times the splay modulus I . [11][12][13][14] The values of these moduli have not yet been determined for nematic elastomers, but, because of the rubbery nature of these materials, it seems reasonable to expect that the moduli would be at least an order of magnitude greater. 15 So, we took I ϭ10 Ϫ11 J/m and II ϭ2ϫ10 Ϫ11 J/m.…”

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confidence: 99%

Biaxial disclinated states in nematic elastomers

Fried

Korchagin

Todres

2003

The Journal of Chemical Physics

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We use a continuum model to investigate the isochoric axial contraction and expansion of a right circular cylindrical specimen composed of a nematic elastomer that is cross-linked in a uniaxial state and then annealed. We build on previous work by relaxing the constraint that the molecular conformation be spherical or uniaxial, allowing instead for biaxiality. The material exhibits an energetic preference for states involving a disclination of strength ϩ1 along the cylinder axis surrounded by a region in which the conformation of the polymer chains is indeed biaxial. We show that such states represent minimizers of the total free-energy. Also, the reactive pressure necessary to enforce the constraint of material incompressibility within the disclination core is found to be reduced by an order of magnitude when the conformation is biaxial rather than uniaxial. A bifurcation analysis is used to analytically determine the thresholds of axial expansion and contraction at which the material prefers a disclinated state. These thresholds are found to be consistent with numerical predictions. Finally, the stability of the solutions for the studied parameters is also investigated.

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“…Using the method described in [2] a well-defined single domain magnetic deformation of the director pattern can be obtained. The amplitude of this deformation can be determined optically [13,14]. In the experiments the transmission of a polarized light beam through an analyser, where polarizer and analyser are mutually perpendicular and make an angle of 45° ( 45°) with the plane defined by the director and the direction of the magnetic field, is monitored (see figure 4).…”

Section: H3comentioning

confidence: 99%

“…Below the threshold field (Be) the transmitted intensity drops to zero indicating a complete restoration of the homeotropic alignment. From the position of the maxima and minima, the ratios KI /K3 and K3/Ax can be determined by fitting the experimental curve of AO as a function of B (see figure 6) with the theoretical expression [13,14] using a non-linear least squares fit. A good fit of the theoretical curve to the data points was obtained (a typical value for the sum of squares of residuals was 0-2).…”

Section: H3comentioning

confidence: 99%