| 1. |
- Barbero, Giovanni, et al.
(author)
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Continuum description of the interfacial layer of nematic liquid crystals in contact with solid surfaces
- 2009
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In: J. Chem. Phys.. - : AIP Publishing. - 0021-9606. ; 130
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Journal article (peer-reviewed)abstract
- We investigate when it is possible to introduce surface physical parameters characterizing the nematic/substrate interface. The analysis is performed by solving the problem assuming that the presence of the surface introduces a spatial variation, mainly localized close to the limiting surfaces, of the bulk properties of the nematic (delocalized model). The results of the calculation are compared to the prediction of a model in which the presence of the surface is taken into account by means of new physical parameters, localized to the surface (localized model). We show that if the viscous dissipative effects or the surface alignment effects are considered, the two models predict the same relaxation times and the same threshold for the Freedericksz transition is obtained. From these results we deduce that the localized models are equivalent to the delocalized ones. A continuum description of the interfacial layer of nematic liquid crystals in contact with solid surface in terms of surface properties is then correct, which makes the solution of this kind of problems simpler. Also a softening of the elastic constants near the surfaces can be represented by a localized surface energy term.
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| 2. |
- Barbero, Giovanni, et al.
(author)
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Nonmonotonic behavior of the nematic tilt angle in a temperature-induced surface transition
- 2008
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In: PHYSICAL REVIEW E. ; 77
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Journal article (peer-reviewed)abstract
- We report on a nonmonotonic temperature behavior of the pretilt in cells containing polymers as alignment layers. The latter are very promising since they enable control of the anchoring of the liquid crystal. We show that such a behavior cannot be interpreted by the standard Landau or mean-field theory. We propose a generalization of the mean-field model, including the temperature dependence of the anchoring due to the polymer layer that enables a description of the nonlinear behavior. The agreement between the predictions of the model and the experimental data is good.
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| 3. |
- Dahl, Ingolf, 1950, et al.
(author)
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Divide Triangles
- 2008
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In: The Wolfram Demonstration Project.
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Journal article (peer-reviewed)abstract
- This Demonstration illustrates a way to divide a triangle or the convex hull of a set of points into a mesh of subtriangles, with maximum side length less than a selected value. Delaunay triangulation is applied to the convex hull before splitting the triangles. Then the subtriangles are colored by methods that involve both random numbers and the orientation of the triangles. The aim is to obtain some "instant art", which also is fun to manipulate.
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| 4. |
- Dahl, Ingolf, 1950
(author)
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Ferroelectricity, SSFLC, bistability and all that
- 2002
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In: arXiv.org.
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Review (other academic/artistic)abstract
- In the book “Ferroelectric and Antiferroelectric Liquid Crystals” by S. T. Lagerwall, the concept “polar liquid crystals” is proposed for the concept earlier known as “ferroelectric liquid crystals”, reserving the word “ferroelectric liquid crystals” for the case of “surface stabilization”. Thus Lagerwall in this way, by redefinition, becomes the coinventor of “ferroelectric liquid crystals”. The trouble is that a closer look on the invention reveals a state of bad logic and a total confusion. The concepts “polar”, “ferroelectric”, “hysteresis”, “SSFLC” and “bistability” are essential in the writing of Lagerwall, but these words are not used in a rigorous way. Also the pictorial evidence used by Lagerwall to illustrate the discovery of surface stabilized liquid crystals raises several questions. An alternative view of the physics of SSFLC cells is presented.
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| 5. |
- Dahl, Ingolf, 1950
(author)
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How to measure the Mueller matrix of liquid-crystal cells
- 2001
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In: Meas. Sci. Technol.. - 0957-0233. ; 12, s. 1938-1948
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Journal article (peer-reviewed)abstract
- The Mueller matrix is the transfer matrix in the Stokes algebra that describes the polarization of natural light. This matrix is very versatile for the task of characterizing the optical properties of liquid-crystal cells, since it can be used for comparison with theoretical calculations, the determination of material parameters and the modelling of the cell as an optical building block for technological use. We have constructed a Mueller-matrix spectrometer, with the ability to perform fast, dynamic measurements of the Mueller matrix of small areas of liquid-crystal cells throughout the visible range. To illustrate the potential of the instrument, dynamic measurements on a ferroelectric-liquid-crystal cell are presented and analysed. The optical measurements indicate that there is an asymmetry between the up and the down state, tilted smectic layers and polarization reversal initiated at the boundaries.
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| 6. |
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| 7. |
- Dahl, Ingolf, 1950
(author)
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Obtuse-Angle Shadowing Networks and Distance-Based Interpolation
- 2009
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In: The Wolfram Demonstrations Project.
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Journal article (peer-reviewed)abstract
- Bad data is much more common than good data. Suppose that you have a set of scattered points in any or an unknown number of dimensions and that although you may not know their coordinates, you know how to calculate the distances between them. You can use this information to define an obtuse-angle shadowing network connecting nearby points in the set. If you have a function defined on this point set, you might use this network to define a distance-based interpolation function to estimate the function value for a new point of the same kind. Such networks and this kind of interpolation should have applications in various fields, for example, image processing and machine learning. This Demonstration applies obtuse-angle shadowing networks to small 2D point sets and also illustrates distance-based interpolation, assuming random function values at the points. The interpolated function is shown as a contour plot. You can change the function value at the green point.
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| 8. |
- Komitov, Lachezar, 1944, et al.
(author)
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Controllable alignment of nematics by nanostructured polymeric layers
- 2009
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In: Liquid Crystals. - 1366-5855 .- 0267-8292. ; 36:6 & 7, s. 747-753
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Journal article (peer-reviewed)abstract
- A method for a continuous control of the pretilt angle of the easy axis in the range 0-90° degrees and of the anchoring strength by using nanostructured polymers as alignment layers is described. The nanostructured polymers are blends of two different side-chain polymers each of them promoting planar and homeotropic alignment, respectively. A model to interpret the alignment of a nematic liquid crystal induced by such polymer layers is proposed. We show that in this case the anisotropic part of the surface tension can be approximated by a simple extension of the Rapini-Papoular expression. The predicted trend of the pretilt of the easy axis versus the concentration of the side-chain polymer promoting the planar alignment, for instance, is in good agreement with the experimental data. We also show that the effective anchoring strength of the system depends on the concentration of the side-chain polymer promoting planar alignment, and exhibits a minimum for a well-defined value of this quantity. The results obtained in this work seems to be of importance for liquid crystal displays technology since the control of the pretilt and the anchoring strength strongly affect the performance of liquid crystal displays.
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| 9. |
- Komitov, Lachezar, 1944, et al.
(author)
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Nano-Structured Polymers for Alignment Layers in LCDs
- 2008
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In: SID Symposium Digest of Technical Papers. - : Wiley. - 0097-966X. ; 39:1, s. 598-599
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Journal article (peer-reviewed)abstract
- By employing a nano-engineering approach, nano-structered polymers were designed, synthesised and used as materials for alignment layers in LCDs. Such materials make possible to control the molecular tilt angle in a broad range, from 0° to 90°, and they seems to enable the control of the anchoring strength as well.
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| 10. |
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