Defect interactions in a two-dimensional sheared lamellar mesophase.

The interaction between two edge dislocations in a sheared lyotropic lamellar liquid-crystalline medium is examined. The model is a mesoscale hydrodynamic model based on a free-energy functional that is minimised for a sinusoidal concentration modulation coupled with concentration and momentum equations. The defect dynamics are analysed as a function of the system size and the Ericksen number, the ratio of the shear stress and the characteristic free-energy density for deformation. Three different regimes are observed as the Ericksen number is increased when the edge dislocations are sheared towards each other, such that there is compression of layers between the defects: (a) defect motion that reduces the cross-stream separation till there is a steady spacing with plug flow between the defects, (b) defect attraction and cancellation resulting in a well-aligned state, and (c) defect creation due to a compressional instability between the defects resulting in an increase in the disorder. When the edge dislocations are sheared away from each other, such that there is extension of the layers between the defects, two distinct regimes are observed as the Ericksen number is increased: (a) bending of layers and a plug flow between the defects at their initial separation, and (b) buckling of the layers leading to creation of more defects and a dynamical steady state between defect creation and cancellation. These regimes are found to be robust for different values of the system size, from 32 to 128 layers, and for different values of the dimensionless groups that characterise the ratio of mass/momentum convection and diffusion.

The relationship between structure and rheology in a three-dimensional sheared lamellar mesophase.

The evolution of a lamellar mesophase from an initially disordered state under shear is examined using simulations of a mesoscale model based on a concentration field ψ that distinguishes the hydrophilic and hydrophobic components. The Landau-Ginzburg free-energy functional is augmented by a term that is minimised for sinusoidal modulations in the concentration field with wavelength λ = (2π/k), and the dynamical equations are the model H equations. The structure and rheology are determined by the relative magnitudes of the diffusion time for coarsening, (λ2/D) and the inverse of the strain rate -1, and the Ericksen number, which is the ratio of the shear stress and the layer stiffness. When the diffusion time is small compared with the inverse of the strain rate, there is a local formation of misaligned layers, which are deformed by the imposed flow. There is near-perfect ordering with isolated defects at low values of the Ericksen number, but the defects result in a significant increase in viscosity due to the high layer stiffness. At high values of the Ericksen number, the concentration field is deformed by the mean shear before layers form via diffusion. Cylindrical structures aligned along the flow direction form after about 8-10 strain units, and these evolve into layers with disorder through diffusion perpendicular to the flow. The layers are not perfectly ordered, even after hundreds of strain units, due to the creation and destruction of defects via shear. The excess viscosity is low because the layer stiffness is small compared with the applied shear at a high Ericksen number. This study provides guidance on how the material parameters and imposed flow can be tailored to achieve the desired rheological behaviour.

Structure-rheology relationship in a sheared lamellar fluid.

The structure-rheology relationship in the shear alignment of a lamellar fluid is studied using a mesoscale model which provides access to the lamellar configurations and the rheology. Based on the equations and free energy functional, the complete set of dimensionless groups that characterize the system are the Reynolds number (ργL(2)/µ), the Schmidt number (µ/ρD), the Ericksen number (µÎ³/B), the interface sharpness parameter r, the ratio of the viscosities of the hydrophilic and hydrophobic parts µ(r), and the ratio of the system size and layer spacing (L/λ). Here, ρ and µ are the fluid density and average viscosity, γ is the applied strain rate, D is the coefficient of diffusion, B is the compression modulus, µ(r) is the maximum difference in the viscosity of the hydrophilic and hydrophobic parts divided by the average viscosity, and L is the system size in the cross-stream direction. The lattice Boltzmann method is used to solve the concentration and momentum equations for a two dimensional system of moderate size (L/λ=32) and for a low Reynolds number, and the other parameters are systematically varied to examine the qualitative features of the structure and viscosity evolution in different regimes. At low Schmidt numbers where mass diffusion is faster than momentum diffusion, there is fast local formation of randomly aligned domains with "grain boundaries," which are rotated by the shear flow to align along the extensional axis as time increases. This configuration offers a high resistance to flow, and the layers do not align in the flow direction even after 1000 strain units, resulting in a viscosity higher than that for an aligned lamellar phase. At high Schmidt numbers where momentum diffusion is fast, the shear flow disrupts layers before they are fully formed by diffusion, and alignment takes place by the breakage and reformation of layers by shear, resulting in defects (edge dislocations) embedded in a background of nearly aligned layers. At high Ericksen number where the viscous forces are large compared to the restoring forces due to layer compression and bending, shear tends to homogenize the concentration field, and the viscosity decreases significantly. At very high Ericksen number, shear even disrupts the layering of the lamellar phase. At low Ericksen number, shear results in the formation of well aligned layers with edge dislocations. However, these edge dislocations take a long time to anneal; the relatively small misalignment due to the defects results in a large increase in viscosity due to high layer stiffness and due to shear localization, because the layers between defects get pinned and move as a plug with no shear. An increase in the viscosity contrast between the hydrophilic and hydrophobic parts does not alter the structural characteristics during alignment. However, there is a significant increase in the viscosity, due to pinning of the layers between defects, which results in a plug flow between defects and a localization of the shear to a part of the domain.

System size dependence of the structure and rheology in a sheared lamellar liquid crystalline medium.

The structural and rheological evolution of an initially disordered lamellar phase system under a shear flow is examined using a mesoscale model based on a free energy functional for the concentration field, which is the scaled difference in the concentration between the hydrophilic and hydrophobic components. The dimensionless numbers which affect the shear evolution are the Reynolds number (γ˙¯L2/ν), the Schmidt number (ν/D), a dimensionless parameter Σ=(Aλ2/ρν2), a parameter µr which represents the viscosity contrast between the hydrophilic and hydrophobic components, and (L/λ), the ratio of system size and layer spacing. Here, ρ, ν, and D are the density, kinematic viscosity (ratio of viscosity and density), and the mass diffusivity, and A is the energy density in the free energy functional which is proportional to the compression modulus. Two distinct modes of structural evolution are observed for moderate values of the parameter Σ depending only on the combination ScΣ and independent of system size. For ScΣ less than about 10, the layers tend to form before they are deformed by the mean shear, and layered but misaligned domains are initially formed, and these are deformed and rotated by the flow. In this case, the excess viscosity (difference between the viscosity and that for an aligned state) does not decrease to zero even after 1000 strain units, but appears to plateau to a steady state value. For ScΣ greater than about 10, layers are deformed by the mean shear before they are fully formed, and a well aligned lamellar phase with edge dislocation orders completely due to the cancellation of dislocations. The excess viscosity scales as t-1 in the long time limit. The maximum macroscopic viscosity (ratio of total stress and average strain rate over the entire sample) during the alignment process increases with the system size proportional to (L/λ)3/2. For large values of Σ, there is localisation of shear at the walls, and the bulk of the sample moves as a block. The thickness of the shearing region appears to be invariant with the system size, leading to an increase of viscosity proportional to L. The time for structural evolution is found to be the inverse of the strain rate γ˙-1. In the case of a significant viscosity contrast between the hydrophilic and hydrophobic parts, the average viscosity increases by 1-2 orders of magnitude due to the defect pinning mechanism, where the regions between defects move as a block, and shear localisation at the wall.

Isolation of ß-sitosterol diglucosyl caprate from Alpinia galanga.

BACKGROUND: The purpose of present investigation to isolate ß-sitosterol diglucosyl caprate from the rhizomes of Alpinia galanga. METHODS: The methanolic extract of the rhizomes of plant Alpinia galanga was subjected to column chromatography and was eluted with ethyl acetate-methanol (99:1) to yield compound (AG5) ß-sitosterol diglucosyl caprate. Various spectral techniques such as Ultraviolet (UV), Infrared (IR), Hydrogen Nuclear Magnetic Resonance (1HNMR), Carbon Nuclear Magnetic Resonance (13CNMR), and MASS spectrometry (MS), were employed to determine and elucidate. RESULTS: Chemical and spectral investigation of extract furnished a new steroidal glycoside. CONCLUSION: The isolated compound ß-sitosterol diglucosyl caprate could serve as a lead compound in synthesis of steroids.

Nutrition knowledge, beliefs and dietary habits among elderly people in Nizwa, Oman: implications for policy.

The nutritional needs of the ageing population require special attention. We undertook a cross-sectional, community-based, household survey in Nizwa wilayat, Oman to study nutrition-related knowledge and beliefs and self-reported dietary habits among a sample of elderly people. The response rate for the household interview was 99.3% from a total sample of 2041. About 45% of the elderly were overweight or obese. Overall we found poor knowledge of nutrition plus some nutritional imbalances and low levels of physical activity. Significant sex differences existed in elderly peoples' nutritional knowledge, consumption of fluids, milk and sweets, use of dietary regimens and experience of appetite change. The findings warrant reorientation of the existing health promotion strategy for the elderly.

Nutrition knowledge, beliefs and dietary habits among elderly people in Nizwa, Oman: implications for policy

The nutritional needs of the ageing population require special attention. We undertook a cross-sectional, community-based, household survey in Nizwa wilayat, Oman to study nutrition-related knowledge and beliefs and self-reported dietary habits among a sample of elderly people. The response rate for the household interview was 99.3% from a total sample of 2041. About 45% of the elderly were overweight or obese. Overall we found poor knowledge of nutrition plus some nutritional imbalances and low levels of physical activity. Significant sex differences existed in elderly peoples' nutritional knowledge, consumption of fluids, milk and sweets, use of dietary regimens and experience of appetite change. The findings warrant reorientation of the existing health promotion strategy for the elderly