INTERMOLECULAR POTENTIALS IN LIQUID CRYSTALS: COMPARISON BETWEEN SIMULATIONS AND NMR EXPERIMENTS

1999 ◽  
Vol 10 (02n03) ◽  
pp. 403-413 ◽  
Author(s):  
RAYMOND T. SYVITSKI ◽  
JAMES M. POLSON ◽  
E. ELLIOTT BURNELL
Keyword(s):  
Liquid Crystals ◽  
Long Range ◽  
Short Range ◽  
Intermolecular Forces ◽  
Order Parameters ◽  
Intermolecular Potential ◽  
Nmr Data ◽  
Long Range Interactions ◽  
Orientational Ordering ◽  
Best Fit

The anisotropic intermolecular forces responsible for the orientational ordering in liquid crystals are probed by comparing Monte Carlo (MC) simulations with experimental nuclear magnetic resonance (NMR) results for solutes in nematic liquid crystals. In a special liquid crystal mixture where all long-range interactions are assumed to be minimized, the models for short-range interactions which best fit NMR experimental solute order parameters also best fit solute order parameters from MC simulations of hard ellipsoids. This is taken as an indication that in this special mixture the intermolecular potential is dominated by short-range forces. However, for liquid crystals where long-range interactions are important, simulations of hard ellipsoids with point quadrupoles cannot reproduce even the gross effects observed with experimental NMR data.

Intermolecular forces

1975 ◽  
Vol 272 (915) ◽  
pp. 5-12 ◽  
Keyword(s):  
Long Range ◽  
Molecular Interactions ◽  
Short Range ◽  
Intermolecular Forces ◽  
Electron Exchange ◽  
Inverse Power ◽  
Charge Distributions ◽  
Electrostatic Component ◽  
Electrostatic Induction ◽  
Long Range Interactions

The nature of molecular interactions is examined. Intermolecular forces are divided into long-range and short-range components; the former operate at distances where the effects of electron exchange are negligible and decrease as an inverse power of the separation. The long-range interactions may be subdivided into electrostatic, induction and dispersion contributions, where the electrostatic component is the interaction of the permanent charge distributions and the others originate in the fluctuations in the distributions. Typical magnitudes of the various contributions are given. The forces between macroscopic bodies are briefly considered, as are the effects of a medium. Some of the manifestations of molecular interactions are discussed.

Development of lateral interactions in the infant visual system

1992 ◽  
Vol 8 (1) ◽  
pp. 3-8 ◽  
Author(s):  
Samuel Sokol ◽  
Vance Zemon ◽  
Anne Moskowitz
Keyword(s):  
Visual System ◽  
Long Range ◽  
Short Range ◽  
Second Harmonic ◽  
Harmonic Component ◽  
Frequency Component ◽  
Human Infants ◽  
Long Range Interactions ◽  
Harmonic Components ◽  
Inhibitory Interactions

AbstractThe development of lateral inhibitory interactions in the infant visual system, as reflected by the visual-evoked potential (VEP), was studied using a radial, asymmetrical windmill-dartboard stimulus. This contrast-reversing stimulus generates VEP responses with a strong fundamental frequency component and an attenuated second harmonic component (relative to that obtained using a symmetrical stimulus). These two harmonic components reflect distinct phenomena, and appear to be the result of short-range (the fundamental) and long-range (attenuated second harmonic) lateral inhibitory interactions elicited by differential luminance-modulation of contiguous spatial regions. We studied the development of the short-and long-range interactions at 100% and 30% contrast in human infants using both VEP amplitude and phase measures. Attenuation of the second harmonic (long-range interactions) was adult-like by 8 weeks of age while the strength of the fundamental (short-range interactions) was adult-like by 20 weeks suggesting a differential development of long-range and short-range interactions. In contrast, corresponding phase data indicated significant immaturities at 20 weeks of age for both the short-and long-range components.

Defect Nucleation and Annihilation in Sheared Polymeric Liquid Crystals

2002 ◽  
Vol 734 ◽  
Author(s):  
Dana Grecov ◽  
Alejandro D. Rey
Keyword(s):  
Liquid Crystals ◽  
Shear Rate ◽  
Long Range ◽  
Short Range ◽  
Practical Importance ◽  
Topological Defects ◽  
Annihilation Rate ◽  
Shear Rates ◽  
Parallel Array ◽  
Defect Nucleation

ABSTRACTShear–induced nucleation and annihilation of topological defects due to hydrodynamic instability in nematic liquid crystals is a phenomenon of both scientific interest and practical importance. We use a complete generalized non-linear second order tensor Landau-de Gennes model that takes into account short range order elasticity, long-range elasticity and viscous effects, to simulate the nucleation and annihilation of twist inversion walls in flow-aligning nematic polymers subjected to shear flow. Shearing a homogeneous nematic sample perpendicular to the director results in an linear instability that maybe symmetric at low shear rates, and antisymmetric at higher shear rates. At even higher shear rates the onset of nonlinearities results in the nucleation of a parallel array of twist inversion walls, such that asymmetry prevails. By increasing the shear rate the following director symmetry transition cascade is observed: symmetric → antisymmetric → asymmetric → symmetric. The nucleation of the parallel array of twist inversion walls in the asymmetric mode is due to the degeneracy in reorientation towards the shear plane. The annihilation of twist walls is mediated by twist waves along the velocity gradient direction. Twist walls annihilate by three mechanisms: wall-wall annihilations, wall-wall coalescence, and wall-bounding surface coalescence. The annihilation rate increases with increasing shear rate and at sufficiently high rates the layered structure is replaced by a homogeneously aligned system. The role of short range and long range elasticity on defect nucleation and annihilation is characterized in terms of the Deborah and Ericksen numbers. Close form solutions to approximated equations are used to explain the numerical results of the full Landau-de Gennes equations of nematodynamics.

ON A COMBINED SHORT-RANGE AND LONG-RANGE INTERACTING EARTHQUAKE MODEL

2000 ◽  
Vol 11 (05) ◽  
pp. 913-919
Author(s):  
A. S. ELGAZZAR ◽  
E. AHMED
Keyword(s):  
Long Range ◽  
Short Range ◽  
Self Organized ◽  
Long Range Interactions ◽  
Earthquake Model

A self-organized critical earthquake model is proposed taking into account the effect of both short-range and long-range interactions. The model obeys both Gutenberg–Richter and Omori laws in addition to being more realistic than other models.

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