References of "Liquid Crystals"
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See detailInfluence of head group and chain length of surfactants used for stabilising liquid crystal shells
Sharma, Anjali UL; Lagerwall, Jan UL

in Liquid Crystals (2018), 45(13-15), 2319-2328

We investigate the stability and textural development in nematic liquid crystal (LC) shells, with aqueous interior and exterior, as a function of the type and concentration of surfactant stabiliser of the ... [more ▼]

We investigate the stability and textural development in nematic liquid crystal (LC) shells, with aqueous interior and exterior, as a function of the type and concentration of surfactant stabiliser of the shell interfaces. The LC is the common thermotropic nematic 5CB and the surfactants are commercial, of cat- as well as of anionic type, with varying alkyl chain length. In addition to stabilising the shell interfaces, surfactants are generally assumed to promote radial (homeotropic) LC alignment, based on prior studies where the surfactant concentration was well above the critical micelle concentration (CMC). Here, we focus on the low-concentration range, below CMC. We find that both cat- and anionic surfactants can stabilise shells, although the higher water solubility of cationics can render stabilisation more difficult. We also conclude that surfactants do not necessarily impose homeotropic alignment; if the surfactant concentration is very low, the director may adopt planar alignment at the 5CB–water interface. Interestingly, the threshold concentration, where the surfactant takes control of alignment, is different for the shell inside and outside. Shells stabilised by solutions of surfactant with concentration near the threshold may therefore adopt a hybrid configuration, with homeotropic inside and planar outside. [less ▲]

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See detailElucidating the fine details of cholesteric liquid crystal shell reflection patterns
Geng, Yong; Noh, Junghyun UL; Drevensek-Olenik, Irena et al

in Liquid Crystals (2017), 44(12-13),

Clusters of planar-aligned short-pitch cholesteric liquid crystal spheres generate dynamic colourful patterns due to multiple selective reflections from the radially oriented cholesteric helices in ... [more ▼]

Clusters of planar-aligned short-pitch cholesteric liquid crystal spheres generate dynamic colourful patterns due to multiple selective reflections from the radially oriented cholesteric helices in neighbour shells at varying distances. These photonic communication patterns were widely investigated for the cases of both droplets and shells, demonstrating not only intriguing optical phenomena but also potential for applications as new optical elements for photonics, sensing or security pattern generation. However, the optics of these clusters is truly complex and until now only the strongest and most fundamental reflections have been analysed and explained. In this report, we elucidate the origin of a number of more subtle reflections and we explain the extension in space of various spots as well as their internal colour variations. [less ▲]

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See detailNon-electronic gas sensors from electrospun mats of liquid crystal core fibers for detecting volatile organic compounds at room temperature
Reyes, Catherine UL; Sharma, Anshul UL; Lagerwall, Jan UL

in Liquid Crystals (2016)

Non-woven mats comprised of liquid crystal-functionalised fibres are coaxially electrospun to create soft gas sensors that function non-electronically, thus requiring no power supply, detect- ing organic ... [more ▼]

Non-woven mats comprised of liquid crystal-functionalised fibres are coaxially electrospun to create soft gas sensors that function non-electronically, thus requiring no power supply, detect- ing organic vapours at room temperature. The fibres consist of a poly(vinylpyrrolidone) (PVP) sheath surrounding a core of nematic 4-cyano-4ʹpentylbiphenyl (5CB) liquid crystal. Several types of mats, containing uniformly cylindrical or irregular beaded fibres, in uniform or random orientations, are exposed to toluene vapour as a representative volatile organic compound. Between crossed polarisers all mats respond with a fast (response time on the order of a second or faster) reduction in brightness during gas exposure, and they return to the original state upon removal of the gas almost as quickly. With beaded fibres, the response of the mats is visible even without polarisers. We discuss how variations in fibre spinning conditions such as humidity level and the ratio of core-sheath fluid flow rates can be used to tune fibre morphology and thereby the response. Considering future development perspectives, we argue that fibres turned respon- sive through the incorporation of a liquid crystal core show promise as a new generation of sensors with textile form factor, ideal for wearable technology applications. [less ▲]

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See detailThe peculiar optic, dielectric and x-ray diffraction properties of a fluorinated de vries asymmetric-diffuse-cone-model ferroelectric liquid crystal
Lagerwall, Jan UL; Coleman, D.; Körblova, E. et al

in Liquid Crystals (2006), 33(1), 17-24

A new semi-fluorinated chiral smectic liquid crystal, W504, is investigated by electro-optic, dielectric and X-ray scattering experiments. It exhibits a huge dielectric soft mode response, strong ... [more ▼]

A new semi-fluorinated chiral smectic liquid crystal, W504, is investigated by electro-optic, dielectric and X-ray scattering experiments. It exhibits a huge dielectric soft mode response, strong electroclinic effect and a birefringence which increases considerably with the director tilt angle theta; typical characteristics of a SmA - SmC transition following the de Vries asymmetric diffuse cone (ADC) model in which the non-zero director tilt in SmC arises through an ordering of tilting directions rather than an actual increase in average molecule tilt <theta(mol)>. In W504 a small increase in <theta(mol)> of about 4 degrees is however detected in the SmC phase. Although the increase in molecule inclination is much less than the increase in director tilt h, saturating close to 30 degrees, it leads to a shrinkage of the smectic layers by about 1 angstrom, a result of the large initial molecule tilt in the SmA phase, <theta(mol)>(SmA) approximate to 30 degrees. The tilting transition in W504 is thus mainly an ADC model disorder - order transition, but it also has a component of a structural transition. The semi- fluorinated molecular structure of W504 leads to a very weak electron density modulation along the layer normal, giving a vanishing form factor in bulk samples which exhibit no (001) X-ray scattering peak. In thin films the (001) peak is however observed, indicating that the electron density modulation is enhanced by the breaking of the head - tail symmetry of the liquid crystal phase at the LC - air interface. [less ▲]

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See detailOn the change in helix handedness at transitions between the sm-c* and sm-ca* phases in chiral smectic liquid crystals
Lagerwall, Jan UL; Giesselmann, Frank; Osipov, Mikhail A.

in Liquid Crystals (2006), 33(6), 625-633

Using a discrete model for the synclinic SmC* and the anticlinic SmC􏰀a phases we give a theoretical explanation for the fact that the helix twisting sense reverses at a transition between these phases ... [more ▼]

Using a discrete model for the synclinic SmC* and the anticlinic SmC􏰀a phases we give a theoretical explanation for the fact that the helix twisting sense reverses at a transition between these phases (direct transition or via the so-called chiral smectic C ‘subphases’) and we derive an explicit expression for the helical pitch in the SmC􏰀a phase. As the theory shows and as we also demonstrate experimentally, the reversal is of a different nature from helix inversions within a single phase, where the inversion is always coupled to a pitch divergence. At a clinicity change the common behaviour is instead pitch-shortening on approaching the phase transition and the associated helix twisting sense reversal. The phenomenon may be put to use in smart mixing in order to control the helix pitch, either for achieving long pitch for surface-stabilized ferroelectric and antiferroelectric liquid crystal displays; or a very short pitch, in the case of devices utilizing the deformed helix mode. [less ▲]

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See detailDifferences between smectic homo- and copolysiloxanes as a consequence of microphase separation
Rössle, Martin; Braun, L.; Schollmeyer, D. et al

in Liquid Crystals (2005), 32(5), 533-538

This paper compares smectic phases formed from LC-homo- and LC-co-polysiloxanes. In the homopolysiloxane, each repeating unit of the polymer chain is substituted with a mesogen, whereas in the ... [more ▼]

This paper compares smectic phases formed from LC-homo- and LC-co-polysiloxanes. In the homopolysiloxane, each repeating unit of the polymer chain is substituted with a mesogen, whereas in the copolysiloxanes mesogenic repeating units are separated by dimethylsiloxane units. Despite a rather similiar phase sequence of the homo- and co-polysiloxanes—higher ordered smectic, smectic C* (SmC*), smectic A (SmA) and isotropic—the nature of their phases differs strongly. For the copolymers the phase transition SmC* to SmA is second order and of the ‘de Vries’ type with a very small thickness change of the smectic layers. Inside the SmA phase, however, the smectic thickness decreases strongly on approaching the isotropic phase. For the homopolymer the phase transition SmC* to SmA is first order with a significant thickness change, indicating that this phase is not of the ‘de Vries’ type. This difference in the nature of the smectic phases is probably a consequence of microphase separation in the copolymer, which facilitates a loss of the tilt angle correlation between different smectic layers. This has consequences for the mechanical properties of LC- elastomers formed from homo- and co-polymers. For the elastomers from homopolymers the smectic layer compression seems to be rather high, while it seems to be rather small for the copolymers. [less ▲]

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See detailElectrolyte effects on the nematic-isotropic phase transition in lyotropic liquid crystals
Mukherjee, Prabir K.; Lagerwall, Jan UL; Giesselmann, Frank

in Liquid Crystals (2005), 32(10), 1301-1306

A phenomenological approach to the description of the electrolyte effect on the nematic– isotropic phase transition in lyotropic liquid crystals is proposed. The influence of the electrolyte is discussed ... [more ▼]

A phenomenological approach to the description of the electrolyte effect on the nematic– isotropic phase transition in lyotropic liquid crystals is proposed. The influence of the electrolyte is discussed by varying the coupling between the concentration variables and the orientational order parameter. The analysis shows that the discontinuity in the first order nematic–isotropic phase transition as measured by TNI{T0 increases as a function of the NI weight fraction of the electrolyte. Here TNI is the first order nematic–isotropic phase transition temperature and TN0I is the extrapolated supercooling limit. The electrolyte dependence of the Cotton–Mouton coefficient and the non-linear dielectric effect in the isotropic phase above the nematic–isotropic phase transition are calculated. The theoretical predictions are found to be in good agreement with experimental results. [less ▲]

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See detailOn the origin of high optical director tilt in a partially fluorinated orthoconic antiferroelectric liquid crystal
Lagerwall, Jan UL; Giesselmann, Frank; Saipa, Alexander et al

in Liquid Crystals (2004), 31(9), 1175-1184

We have investigated the orthoconic antiferroelectric liquid crystal mixture W107 by means of optical, X-ray and calorimetry measurements in order to assess the origin of the unusally high tilt angle ... [more ▼]

We have investigated the orthoconic antiferroelectric liquid crystal mixture W107 by means of optical, X-ray and calorimetry measurements in order to assess the origin of the unusally high tilt angle between the optic axis and the smectic layer normal in this material. The optical birefringence increases strongly below the transition to the tilted phases, showing that the onset of tilt is coupled with a considerable increase in orientational order. The layer spacing in the smectic A (SmA) phase is notably smaller than the extended length of the molecules constituting the mixture, and the shrinkage in smectic C (SmC) and smectic C-a (SmCa) is much less than the optical tilt angle would predict. These observations indicate that the tilting transition in W107 to a large extent follows the asymmetric de Vries diffuse cone model. The molecules are on average considerably tilted with respect to the layer normal already in the SmA phase but the tilting directions are there randomly distributed, giving the phase its uniaxial characteristics. At the transition to the SmC phase, the distribution is biased such that the molecular tilt already present in SmA now gives a contribution to the macroscopic tilt angle. In addition, there is a certain increase of the average tilt angle, leading to a slightly smaller layer thickness in the tilted phases. Analysis of the wide angle scattering data show that the molecular tilt in SmCa is about 20degrees larger than in SmA. The large optical tilt (45degrees) in the SmCa phase thus results from a combination of an increased average molecule tilt and a biasing of tilt direction fluctuations. [less ▲]

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See detailFerroelectric polysiloxane liquid crystals with ‘de vries’-type smectic a* -smectic c* transitions
Rössle, Martin; Zentel, Rudolf; Lagerwall, Jan UL et al

in Liquid Crystals (2004), 31(6), 883-887

We report preliminary results of optical and small angle X-ray scattering (SAXS) experiments on the smectic A*2smectic C* transition in two ferroelectric liquid crystalline polysiloxanes. Although the ... [more ▼]

We report preliminary results of optical and small angle X-ray scattering (SAXS) experiments on the smectic A*2smectic C* transition in two ferroelectric liquid crystalline polysiloxanes. Although the optical tilt angle in the SmC* phases reaches values up to 30‡, temperature-dependent SAXS measurements clearly reveal that the smectic layer spacing is basically conserved during the A*–C* transition as well as in the subsequent C* phase. Connected with the A*–C* transition we further observed a significant increase in bire- fringence, hence reflecting an increase of orientational order. The practical absence of layer shrinkage and the enhanced orientational ordering are consistent with the de Vries diffuse cone model of smectic A2smectic C transitions. [less ▲]

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See detailOn the phase sequence of antiferroelectric liquid crystals and its relation to orientational and translational order
Lagerwall, Jan UL; Rudquist, Per; Lagerwall, Sven T. et al

in Liquid Crystals (2003), 30(4), 399-414

The substance MHPOBC is the oldest and still most important reference antiferroelectric liquid crystal (AFLC). There is still considerable controversy concerning the correct phase designations for this ... [more ▼]

The substance MHPOBC is the oldest and still most important reference antiferroelectric liquid crystal (AFLC). There is still considerable controversy concerning the correct phase designations for this material and, in particular, about the presence or absence of SmC* in its phase sequence. By means of dielectric spectroscopy and polarizing microscopy, we show that whereas the pure compound lacks the SmC* phase, this phase rapidly replaces the SmC*b subphase through the reduced purity resulting from temperature-induced chemical degradation which is hard to avoid under standard experimental conditions. X-ray investi- gations furthermore show that this change in phase sequence is coupled to a decrease in translational order. This explains the large variations in the reported phase sequence and electro-optic behaviour of MHPOBC, in particular concerning the SmC*b phase which has been said to exhibit ferro-, ferri- as well as antiferroelectric properties. It is likely that the sensitivity of the AFLC phase sequence to sample purity is a general property of AFLC materials. We discuss the importance of optical and chemical purity as well as tilt and spontaneous polarization for the observed phase sequence and propose that one of the key features determining the existence of the different tilted structures is the antagonism between orientational (nematic) and translational (smectic) order. The decreased smectic order (increased layer interdigitation) imposed by chemical impurities promotes the synclinic SmC* phase at the cost of the AFLC phases SmC*a , SmC*b , SmC*c and SmC*a . We also propose that the SmA* phase in FLC and AFLC materials may actually have a somewhat different character and, depending on its microstructure, some of the tilted phases can be expected to appear or not to appear in the phase sequence. AFLC materials exhibiting a direct SmA*–SmC*a transition are found to be typical ‘de Vries smectics’, with very high orientational disorder in the SmA* phase. Finally, we discuss the fact that SmC*b and SmC*c have two superposed helical superstructures and explain the observation that the handedness of the large scale helix may very well change sign, while the handedness on the unit cell level is preserved. [less ▲]

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See detailPhases, phase transitions and confinement effects in a series of antiferroelectric liquid crystals
Lagerwall, Jan UL; Parghi, Deven D.; Krüerke, Daniel et al

in Liquid Crystals (2002), 29(2), 163-178

Using a variety of optical and electrooptical techniques as well as dielectric spectroscopy, we have investigated three homologues in the chiral liquid crystal series nF1M7, where n denotes the length of ... [more ▼]

Using a variety of optical and electrooptical techniques as well as dielectric spectroscopy, we have investigated three homologues in the chiral liquid crystal series nF1M7, where n denotes the length of the unbranched side-chain. The main focus of the study is the series of smectic C subphases, i.e. SmCa*, SmC1/3* and SmC1/4*. During switching in the SmCa* phase, a peculiar redirection of the plane of biaxiality, distinguishing this phase from SmA* and SmC*, was observed. We present a simple explanation for this behaviour which correlates well with the clock model description of the SmCa* phase. We found a zero mesoscopic polarisation for the SmC1/4* phase and a non-zero mesoscopic polarisation for SmC1/3*, observations which are consistent with a distorted clock model. The dielectric spectroscopy investigations, performed at several different cell gaps, clearly show that the dielectric response in these materials is easily dominated by surface-induced structures if the cell gap is reduced, and thus reflects the bulk thermodynamic phase in very thick cells only. [less ▲]

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