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Industrial and engineering aspects of LC applications
Published in L. Vicari, Optical Applications of Liquid Crystals, 2016
In the historical view of LC displays, scattering effects appeared earlier than TN. The first was the dynamic scattering mode, which is based on dynamic disorder caused by ions’ drift in an LC layer. Unfortunately it was used for just a few years due to poor reliability. Then the TN mode, based on field effect, was invented, and took the major place in display applications.
Electroconvection characterization of guest-host nematic liquid crystals for dynamic scattering mode applications
Published in Liquid Crystals, 2019
Mansoureh Shasti, J. T. Gleeson, Paul Luchette
Electro-hydrodynamic convection (EHC), and its related phenomenon dynamic scattering are well-known effects in nematic liquid crystals [1–7]. Indeed, the dynamic scattering mode was the basis of one of the first liquid crystal display technologies. However, this effect ultimately proved to be not competitive with more familiar display modes such as twisted nematic (TN) [8]. One of the factors limiting the usefulness of dynamic scattering mode within display devices was its relatively large (compared to TN) operating voltage. The minimum operating (or threshold) voltage required to induce EHC in a nematic depends critically on the frequency (of the applied electric field), as well as various physical properties of the material, most notably the elements of the dielectric and conductivity tensors.
Topologically non-equivalent textures generated by the nematic electrohydrodynamics
Published in Liquid Crystals, 2019
G. Pucci, F. Carbone, G. Lombardo, C. Versace, R. Barberi
By increasing the voltage, the system undergoes a series of bifurcations and the light is increasingly scattered [10,11], until a regime called dynamic scattering mode (DSM) is achieved [12]. In contrast to isotropic fluids, a turbulent nematic experiences a transition to a stochastic regime (DSM2), which is characterised by high density of topological defects. This transition was called DSM1-DSM2 transition and occurs by nucleation [13]. At the threshold voltage amplitude V, DSM2 nuclei expand into DSM1 until they fill the whole sample (Figure 1(d)). DSM2 nuclei appear darker because they scatter light more than DSM1. Topological defects can be observed by switching-off the field and taking a snapshot before their annihilation [11] (Figure 2(a)). The DSM1-DSM2 transition has been historically referred to as a transition between turbulent regimes [13–15] and several of its features have been thoroughly investigated, such as spatial ordering of plume-like fluctuations [16], universal fluctuations of the growing interfaces [17], clustering of elastic energy [18], spatiotemporal decorrelation properties [19] and intermittency [20]. As this transition is not found in turbulent isotropic fluids (see, for example, [21]), it could be ascribed to the intrinsic anisotropy of nematic fluids. However, its basic mechanism remains unclear.
Time-dependent electrical properties of liquid crystal cells: unravelling the origin of ion generation
Published in Liquid Crystals, 2018
While the generation of ions in the bulk of liquid crystals has received its due attention in the literature, the generation of ions on the surface remains poorly explored and understood. During early years of the liquid crystal display era, electrochemical reactions on the electrodes were considered a major factor of ion generation/re-generation cycle in liquid crystals operating in the dynamic scattering mode [27–29]. With the paradigm change and transition to the use of electric field effects in liquid crystal devices, this source of ion generation (electrochemical reactions on the electrodes) became insignificant. Indeed, the use of alignment layers and AC electric fields along with highly purified liquid crystals practically eliminated the possibility of this type of ion generation in liquid crystals [8,10,12]. Nowadays, it is commonly accepted that substrates of the liquid crystal cell should be considered as a possible source of ions in liquid crystals because of their uncontrolled contamination with ions. Still, quantitative models describing substrates of the liquid crystal cell as a source of ion generation in liquid crystals are practically missing. This source of ions in liquid crystals can become critical to a broad variety of emerging applications of liquid crystals utilising very thin cells [33].