Biomedical subjects

Satyendra Kumar

Publications and source records attributed to Satyendra Kumar.

13 recordsLinked to original sources

PubMed2006

Dynamic light scattering study of biaxial ordering in a thermotropic liquid crystal.

Dynamic light scattering from orientational order fluctuations in a liquid crystalline tetrapode reveals successive, weakly first-order isotropic to uniaxial and uniaxial to biaxial nematic phase transitions. The order parameter relaxation rates exhibit temperature dependences consistent with Landau-de Gennes mean field theory. Combined with previous evidence of a second-order uniaxial-biaxial transition in a closely related tetrapode, the present study supports the existence of a nematic-nematic tricritical point in thermotropic liquid crystals.

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PubMed2006

Solute crystallization in frozen systems-use of synchrotron radiation to improve sensitivity.

PURPOSE: To demonstrate the sensitivity of low temperature synchrotron X-ray diffractometry (SXRD) for detecting solute crystallization in frozen sodium phosphate buffer solutions. To determine the effect of annealing on solute crystallization in frozen solutions. MATERIALS AND METHODS: Sodium phosphate buffer solutions, at initial buffer concentrations ranging from 1 to 100 mM (pH 7.4) were cooled to -50 degrees C. The crystallization of disodium hydrogen phosphate dodecahydrate (Na(2)HPO(4) *12H(2)O) was monitored using a laboratory as well as a synchrotron source. At selected concentrations, the effect of annealing (at -20 degrees C) was investigated. RESULTS: With the laboratory source, solute crystallization, based on the appearance of one diagnostic peak with a d-spacing of 5.4 A, was evident only when the initial buffer concentration was at least 50 mM. In contrast, using SXRD, crystallization was detected at initial buffer concentrations down to 1 mM. In addition, the use of a high-resolution 2D detector enabled the visualization of numerous diffraction rings of the crystalline solute. At both 10 and 100 mM buffer concentration, there was no increase in solute crystallization due to annealing. CONCLUSION: By using synchrotron radiation, solute crystallization was detected with substantially increased sensitivity, making the technique useful for freeze-drying cycles of practical and commercial importance. Since numerous peaks of the crystalline solute appeared, the technique has potential utility in complex, multi-component systems.

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PubMed2006

Enhanced thermostability of silica-immobilized lipase from Bacillus coagulans BTS-3 and synthesis of ethyl propionate.

A lipase from the thermophilic isolate Bacillus coagulans BTS-3 was produced and purified. The enzyme was purified 40-fold to homogeneity by ammonium sulfate precipitation and DEAE-Sepharose column chromatography. Its molecular weight was 31 kDa on SDS-PAGE. The purified lipase was immobilized on silica and its binding efficiency was found to be 60%. The enzyme took 60 min to bind maximally onto the support. The pH and temperature optima of immobilized lipase were same as those of the free enzyme, i.e. 8.5 and 55 degrees C, respectively. The immobilized enzyme had shown marked thermostability on the elevated temperatures of 55, 60, 65 and 70 degrees C. The immobilized enzyme was reused for eigth cycles as it retained almost 80% of its activity. The catalytic activity of immobilized enzyme was enhanced in n-hexane and ethanol. The immobilized enzyme when used for esterification of ethanol and propionic acid showed 96% conversion in n-hexane in 12 h at 55 degrees C.

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PubMed2005

Thermotropic uniaxial and biaxial nematic and smectic phases in bent-core mesogens.

Two azo substituted achiral bent-core mesogens have been synthesized. Optical polarizing microscopy and synchrotron X-ray scattering studies of both compounds reveal the existence of the thermotropic uniaxial and biaxial nematic and three smectic phases at different temperatures in these single component small molecule systems. The transition from the uniaxial to biaxial nematic phase is confirmed to be second order. The transitions from the biaxial nematic to the underlying smectic phase and between the smectic phases have barely discernible heat capacity signatures and thus are also second order.

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PubMed2005

What aligns liquid crystals on solid substrates? The role of surface roughness anisotropy.

The mechanism responsible for liquid crystal (LC) alignment on mechanically buffed or UV exposed polymer films is poorly understood. A comprehensive study of LC alignment on variously prepared substrates unequivocally shows that the anisotropy in the surface roughness of the substrate completely determines the direction of LC alignment. In all the cases studied, including those where an anchoring transition occurs with temperature, the LC director (re)aligns in the directions of low roughness.

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PubMed2005

Production, purification, and characterization of lipase from thermophilic and alkaliphilic Bacillus coagulans BTS-3.

A thermophilic isolate Bacillus coagulans BTS-3 produced an extracellular alkaline lipase, the production of which was substantially enhanced when the type of carbon source, nitrogen source, and the initial pH of culture medium were consecutively optimized. Lipase activity 1.16 U/ml of culture medium was obtained in 48 h at 55 degrees C and pH 8.5 with refined mustard oil as carbon source and a combination of peptone and yeast extract (1:1) as nitrogen sources. The enzyme was purified 40-fold to homogeneity by ammonium sulfate precipitation and DEAE-Sepharose column chromatography. Its molecular weight was 31 kDa on SDS-PAGE. The enzyme showed maximum activity at 55 degrees C and pH 8.5, and was stable between pH 8.0 and 10.5 and at temperatures up to 70 degrees C. The enzyme was found to be inhibited by Al3+, Co2+, Mn2+, and Zn2+ ions while K+, Fe3+, Hg2+, and Mg2+ ions enhanced the enzyme activity; Na+ ions have no effect on enzyme activity. The purified lipase showed a variable specificity/hydrolytic activity towards various 4-nitrophenyl esters.

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PubMed2004

Biaxial nematic phase in bent-core thermotropic mesogens.

A biaxial nematic phase had been predicted with D(2h) symmetry, wherein the mesogen's long and short transverse axes are simultaneously aligned along the two orthogonal, primary and secondary directors, n and m, respectively. The unique low-angle x-ray diffraction patterns in the nematic phases exhibited by three rigid bent-core mesogens clearly reveal their biaxiality. The results of x-ray diffraction can be readily reproduced by ab initio calculations that explicitly include the bent-core shape in the form factor and assume short-range positional correlations.

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PubMed2004

Three-dimensional orientational order in the bulk and on the surface of polymer films and its effect on liquid-crystal alignment.

The correlation between orientation ordering of polymer chains in the bulk of polymer film and at the polymer-liquid-crystal (LC) interface has been studied to determine it's role in LC alignment. The bulk and surface ordering of polymer were investigated by null ellipsometry and x-ray reflectivity, respectively. Two kinds of liquid-crystalline polymers were used; side-chain azopolymers with azochromophores containing hydrophobic OC4H9 alkyl chain (P1) and strongly polar NO2 group (P2) as the end substituents. The uniaxial tilt orientation of azochromophores in the films of both polymers was induced by the oblique irradiation with unpolarized UV light. The two polymers exhibit similar chain orientation but different ordering of azochromophores on the surface of the films of P1 and P2. Surface ordering of P1 films correlates very well with the order in the bulk of the film, which are essentially determined by the UV exposure. However, orientational order of polymer chains at the surface of P2 films is different from that in its bulk and is not determined by UV exposure. This is explained by strong aggregation of azochromophores during its self-assembling at the polymer-air interface. The LC alignment is determined by the surface ordering of azochromophores. The results imply that ordering tendency can be effectively transferred from polymer bulk to polymer surface and then to LC if it is not lost at the polymer-LC interface.

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PubMed2003

Electrically controllable microlens array fabricated by anisotropic phase separation from liquid-crystal and polymer composite materials.

Anisotropic phase separation has been used to fabricate an electrically switchable microlens array from nematic liquid crystals. Nematic liquid-crystal-based microlens arrays have been built with diameters of approximately 400 microm and natural focal lengths as small as 1.6 mm. The focal length of each microlens in the array can be changed in milliseconds by an applied electric field. These devices, which have no internal substructures to scatter light, offer higher efficiency and greater light throughput than polymer dispersed devices.

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PubMed2003

Observation of novel liquid-crystalline phase above the bulk-melting temperature.

In this paper, we show that a noncrystalline but ordered smectic-like phase exists above the bulk-melting temperature (T(m)) at poly(n-alkyl acrylates)-air interface. The surface ordered phase is one monolayer thick and undergoes a sharp transition from order to disorder 10 degrees C above T(m) for n=22. The presence of a surface phase that does not exist in the bulk has important implications in the design of thermally responsive adhesives.

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PubMed2002

Effect of mosaicity in x-ray studies of critical behavior at the nematic to smectic-A transition.

Previous studies of critical behavior at the nematic to smectic-A transition by high-resolution x-ray scattering were performed using low magnetic fields of 0.1-0.8 T. In those studies, the transverse resolution was limited by the sample mosaicity which complicated data analysis. In order to understand the effect of sample mosaicity on the measured values of critical exponents, the divergence of the smectic order correlation lengths xi(||, perpendicular ) and susceptibility sigma(o) was studied in a magnetic field ranging from 0.25 to 5 T. The use of high (5 T) field reduced the sample mosaicity and improved the effective transverse resolution by almost two orders of magnitude. Three liquid crystals, two mixtures of 6th and 7th homologs of 4,4(')-dialkylazoxybenzene (DnAOB) and 4-n-octylcyanobiphenyl (8CB) were studied. 15 wt% (D6.15AOB) and 40 wt% (D6.4AOB) mixtures of D7AOB in D6AOB have a wide nematic range, while 8CB has a narrow nematic range. Analysis of the data at different fields revealed a different and proper way to apply the mosaicity correction. The Gaussian mosaicity correction was found to be temperature independent but significantly ( approximately 3.5 times) smaller than the width of the sharpest q( perpendicular )-scan, which has traditionally been used for mosaicity correction in all previous studies. The values of the critical exponents measured over almost four decades of reduced temperature were: nu(||)=0.79+/-0.02, nu( perpendicular )=0.69+/-0.02, gamma=1.46+/-0.04 for D6.15AOB; nu(||)=0.79+/-0.02, nu( perpendicular )=0.67+/-0.02, gamma=1.44+/-0.04 for D6.4AOB; and nu(||)=0.70+/-0.02, nu( perpendicular )=0.52+/-0.02, gamma=1.24+/-0.04 for 8CB. The results for the two mixtures suggest that in wide temperature range nematics, far from the tricritical point, the exponents may be material independent. No significant effects of mosaicity on the values of the coefficient c of the fourth-order term in the structure factor were observed.

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PubMed2002

X-ray diffraction study of Anodisc filters.

X-ray diffraction from Anodisc membrane filters is measured and analyzed. The patterns are consistent with pores of constant diameter and a Gaussian distribution of pore-pore spacing. The mean distance between pores, measured using x rays is 0.37 microm compared to 0.32 microm calculated from the nominal density of pores/cm(2). The results may be modeled both in terms of the convolution of a structure factor of the pores with the resolution function, and by modeling the source as a collection of incoherent sources with an ideal monochromator crystal. The incoherent source analysis provides an explanation for the common observation that the resolution function in many x-ray spectrometers is better fit to a sum of Lorentzians, rather than a single Lorentzian.

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PubMed2002

Critical behavior at the nematic-to-smectic-A transition in a strong magnetic field.

The critical divergence of the smectic-order correlation lengths parallel and perpendicular to the director, xi( parallel, perpendicular), and the susceptibility, sigma(0), near the nematic-to-smectic-A phase transition has been measured in a 5 T magnetic field. The high magnetic field reduced the mosaicity and improved the effective transverse resolution by almost 2 orders of magnitude and permitted measurements nearly over four decades of reduced temperature. The results provide important new insights into mosaicity correction. The values of the critical exponents of xi( parallel, perpendicular) and sigma(0) were nu( parallel) = 0.79+/-0.02, nu( perpendicular) = 0.68+/-0.02, gamma = 1.45+/-0.04, respectively. They become smaller for a narrower nematic range.

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