Collective and Molecular Dielectric Relaxation in Confined Liquid Crystals: Effect of Different Layer Thicknesses and Boundary Conditions

2003 ◽  
Vol 790 ◽  
Author(s):  
Fouad M. Aliev ◽  
Manuel Rivera Bengoechea
Keyword(s):  
Liquid Crystal ◽  
Boundary Conditions ◽  
Layer Thickness ◽  
High Frequency ◽  
Low Frequency ◽  
Pore Wall ◽  
Relaxation Processes ◽  
Short Axis ◽  
Librational Mode ◽  
Frequency Relaxation

ABSTRACTBroadband dielectric spectroscopy was used to study the influence of boundary conditions and layer thickness of liquid crystal (LC) confined to cylindrical pores on low frequency and high frequency relaxation processes. Low frequency measurements provided information on the relaxation of surface polarization that arose at LC – pore wall interface. The dynamics of molecular reorientations were investigated in high frequency experiments. In samples with axial orientation of molecules, the dielectric mode due to reorientation of molecules around their short axis was investigated. The homeotropic alignment of molecules facilitated the investigation of the librational mode. The behavior of this mode was different from the behavior observed in investigations of relaxation due to reorientation of molecules around their short axis. Broadening of the dielectric spectra was observed in confined LC. The broadening increases with decreasing liquid crystal layer thickness.

The Influence of Boundary Conditions and Surface Layer Thickness on Dielectric Relaxation of Liquid Crystals Confined in Cylindrical Pores

2000 ◽  
Vol 651 ◽  
Author(s):  
Z. Nazario ◽  
G. P. Sinha ◽  
F.M. Aliev
Keyword(s):  
Liquid Crystal ◽  
Boundary Conditions ◽  
Temperature Dependence ◽  
Layer Thickness ◽  
Dielectric Spectroscopy ◽  
Relaxation Times ◽  
Thin Layers ◽  
Short Axis ◽  
Librational Mode ◽  
Cylindrical Pores

AbstractDielectric spectroscopy was applied to investigate the dynamic properties of liquid crystal octylcyanobiphenyl (8CB) confined in 2000 Å cylindrical pores of Anopore membranes with homeotropic and axial (planar) boundary conditions on the pore walls. Homeotropic boundary conditions allow the investigation of the librational mode in 8CB by dielectric spectroscopy. We found that the dynamics of the librational mode is totally different from the behavior observed in investigations of relaxation due to reorientation of molecules around their short axis. The interpretation of the temperature dependence of relaxation times and of the dielectric strength of the librational mode needs the involvement of the temperature dependence of orientational order parameter. For samples with axial boundary conditions, layers of LCs with different thickness were obtained on the pore walls as a result of controlled impregnation of porous matrices with 8CB from solutions of different liquid crystal concentration. The process due to rotation of molecules around their short axis with single relaxation time observed for bulk 8CB is replaced by a process with a distribution of relaxation times in thin layers. This relaxation process broadens with decreasing layer thickness.

Nanocomposite Material, Liquid Crystal-Aerosil Particles: Dielectric and Photon Correlation Spectroscopy Investigations

1999 ◽  
Vol 581 ◽  
Author(s):  
F.M. Aliev ◽  
G.P. Sinha
Keyword(s):  
Liquid Crystal ◽  
Photon Correlation Spectroscopy ◽  
Low Frequency ◽  
Correlation Spectroscopy ◽  
Relaxation Processes ◽  
Photon Correlation ◽  
Optoelectronic Application ◽  
Broadband Dielectric Spectroscopy ◽  
Tumbling Motion ◽  
Frequency Relaxation

ABSTRACTNematic liquid crystal filled with Aerosil particles prospective inorganic-organic nanocom-posite material for optoelectronic application has been investigated by broadband dielectric spectroscopy (BDS) and photon correlation spectroscopy (PCS). The aerosil particles of diameter ≈ 10 nm in filled nematic liquid crystals form a network structure with linear size of LC domains about 250 nm and with random distribution of the director orientation of each domain. This material has a very developed liquid crystal-solid particle interface that makes the role of the surface layers of LC important in the determination of the properties of the material. BDS provides information on reorientational motion of polar molecules of liquid crystal while PCS probes dynamics of collective modes associated with director fluctuations. We found that the properties of 5CB are considerably affected by the network. Two bulk-like dielectric modes due to the rotation of molecules around short axes and the tumbling motion were observed in filled 5CB. Additionally, a low frequency relaxation process and dispersion of dielectric permittivity due to conductivity were observed. The treatment of the surface of filling particles has strongest influence on the properties of the slow process and it is less important for molecular modes. PCS experiment shows that two new relaxation processes appear in filled 5CB in addition to the director fluctuations process in bulk.

Effect of Different Layer Thicknesses and Boundary Conditions of Confined Liquid Crystal on Dynamics of Director Relaxation: Dynamic Light Scattering Investigations

2003 ◽  
Vol 790 ◽  
Author(s):  
Sarmistha Basu ◽  
Fouad M. Aliev
Keyword(s):  
Liquid Crystal ◽  
Light Scattering ◽  
Dynamic Light Scattering ◽  
Boundary Conditions ◽  
Nematic Phase ◽  
Pore Wall ◽  
Surface Relaxation ◽  
Relaxation Processes ◽  
Slow Process ◽  
Relaxation Dynamic

ABSTRACTDynamic light scattering (DLS) has been used to study the influence of boundary conditions (planar-axial and homeotropic-radial) and layer thickness of liquid crystal (8CB) confined to cylindrical pores on phase transitions and relaxation of director orientational fluctuations. For confined 8CB in the nematic phase two well-defined relaxation processes were observed for both axial and radial orientations of the liquid crystal. The first relaxation is qualitatively associated with bulk-like nematic director fluctuations. The second relaxation (with relaxation time slower than the first one) is most likely due to the fluctuations in the layers nearest the wall surface. In samples with radial boundary conditions, we observed the onset of smectic-A phase order formation on the pore wall even though the rest of the liquid crystal could be in the nematic phase. The influence of pore wall-liquid crystal interactions on the properties of the confined 8CB was stronger for radial boundary conditions of confined liquid crystal, than in the case of axial orientation‥ The separation between the first and the second (slow) process was clearer for thinner layers and the amplitude of the slow process was greater for thinner layers. This suggests that the slow process was related to surface relaxation of director fluctuations.

Dynamics of Ferroelectric and Nematic Liquid Crystals Confined in Porous Matrices

1994 ◽  
Vol 366 ◽  
Author(s):  
Fouad M. Aliev
Keyword(s):  
Liquid Crystals ◽  
Heterogeneous Media ◽  
Volume Fraction ◽  
Low Frequency ◽  
Pore Wall ◽  
Polar Molecules ◽  
Average Pore ◽  
Surface Polarization ◽  
The Difference ◽  
Frequency Relaxation

ABSTRACTWe performed dielectric spectroscopy measurements to study dynamics of collective modes of ferroelectric (FLC) and molecular motion of nematic (NLC) liquid crystals with polar molecules confined in silica macroporous and microporous glasses with average pore sizes of 1000 Å (volume fraction of pores 40%) and 100 Å (27%) respectively. For FLC the Goldstone and the soft modes are found in macropores. The rotational viscosity associated with the soft mode is about 10 times higher in pores than in the bulk. These modes are not detected in micropores although low frequency relaxation is present. The last one probably is not connected with the nature of liquid crystal but is associated with surface polarization effects typical for two component heterogeneous media. The difference between the dynamics of orientational motion of the polar molecules of NLC in confined geometries and in the bulk is qualitatively determined by the total energy Fs of the interaction between molecules and the surface of the pore wall, which is found Fs ≈ 102erg/cm2.

Low-frequency relaxation processes in single-crystal proustite, Ag3AsS3

1989 ◽  
Vol 1 (41) ◽  
pp. 7669-7679 ◽  
Author(s):  
S R Yang ◽  
K N R Taylor ◽  
F Y Zhang
Keyword(s):  
Single Crystal ◽  
Low Frequency ◽  
Relaxation Processes ◽  
Frequency Relaxation

Light scattering studies of high-frequency relaxation processes in organic glass formers

1996 ◽  
Vol 8 (47) ◽  
pp. 9593-9598 ◽  
Author(s):  
R M Pick ◽  
A Aouadi ◽  
C Dreyfus ◽  
R Torre ◽  
P Bartolini
Keyword(s):  
Light Scattering ◽  
High Frequency ◽  
Organic Glass ◽  
Relaxation Processes ◽  
Frequency Relaxation

Dielectric Behaviour of 1-(4-Isothiocyanatophenyl)-2-(4-hexyl- bicyclo[2,2,2]octane-1)ethane in Nematic and Isotropic Phases

1996 ◽  
Vol 51 (8) ◽  
pp. 933-938 ◽  
Author(s):  
J. Jadżyn ◽  
C. Legrand ◽  
P. Kędziora ◽  
B. Żywucki ◽  
G. Czechowski ◽  
...  
Keyword(s):  
Dipole Moment ◽  
Relaxation Time ◽  
Liquid Crystal ◽  
Order Parameter ◽  
Nematic Phase ◽  
Frequency Region ◽  
Relaxation Processes ◽  
Short Axis ◽  
Electric Permittivity ◽  
Orientation Axis

Abstract Static and dynamic investigations of the dielectric properties of the liquid crystal 1-(4-isothio-cyanatophenyl)-2-(4-hexyl-bicyclo[2,2,2]octane-1)ethane in the nematic and isotropic phases have been carried out in the frequency region from 1 kHz to 1 GHz. Two relaxation processes, described by the Debye functions, have been observed not only in the isotropic but also in the nematic phase, when the measurements of the electric permittivity vs. frequency have been made parallel to the orientation axis of liquid crystal. These processes are related to the rotation of the permanent dipole moment around two main molecular axes. The height of the potential barrier which hinders the rotation of the liquid crystal molecule around the short axis in the ordered nematic phase and the order parameter of the liquid crystal investigated have been estimated on the basis of the relaxation time values in the nematic and isotropic phases.

Study on Absorbing Properties of Honeycomb Absorbing Materials

2013 ◽  
Vol 815 ◽  
pp. 645-649
Author(s):  
Hui Min Sun ◽  
Zhao Zhan Gu ◽  
Ran Ran Yang
Keyword(s):  
Layer Thickness ◽  
High Frequency ◽  
Free Space ◽  
Low Frequency ◽  
Resonance Peak ◽  
Test Method ◽  
Electromagnetic Parameters ◽  
Frequency Wave ◽  
Absorbing Materials

Honeycomb absorbing materials were measured using the method of free space in this paper. The reflectance of honeycomb absorbing materials was calculated and simulated, and it was verified based on the measured results. It was demonstrated that this test method was feasible. Through studying on absorbing properties of honeycomb, the results have showed that the radar absorbing properties of honeycomb are related to electromagnetic parameters, as well as thickness of the dip-coatings. With the increase of thickness of the dipping layer, the radar absorbing capability of high frequency and low frequency wave are significantly increased. It is worth noting that the resonance peak moved to the low frequency with the increase of dipping layer thickness. These results are useful for design of honeycomb absorbing materials.

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