Refractive indices, density and order parameter of two liquid crystals HBT and OBT

The temperature variation of refractive indices (ne ne), birefringence (δn), density (ρ) and order parameter (S) of two liquid crystals, namely, N-(p-hexyloxybenzylidene)-p-toluidine (HBT) and N-(p-octyloxybenzylidene)-p-toluidine (OBT) are reported in the smectic (SA in OBT and SB in HBT), nematic and isotropic phases. For accurate measurement of δn, the wedge method was modified to eliminate the need to measure the wedge angle. Density measurements indicate that the smectic-nematic and nematic-isotropic phase transitions in these materials are of first order. Using refractive index values and the density data, the internal field factors (γe, γo), the ratio of principal polarizabilities f(=αeαo) and the order parameter, S, have been evaluated and their temperature dependence discussed in the light of molecular geometry. The order parameter has been determined using the isotropic internal field model (Vuks approach) and the anisotropic internal field model (Neugebauer's approach). The S values determined using these two models agree in the nematic phase but differ considerably in the smectic phase

Refractive indices, density and order parameter of some technologically important liquid crystalline mixtures

Temperature variation of the refractive indices, birefringence, density and order parameter of liquid crystalline mixtures E7, E8, N10 and PCH-1132 are reported. The birefringence of PCH-1132 is found to be abnormally low compared to other mixtures

Refractive indices, density and order parameter of some liquid crystals

The temperature variation of the refractive indices (no, ne) and density are reported for two liquid crystals. The order parameter is evaluated by both Vuks' and Neugebauer's approaches. The possible sources of error in evaluating the order parameter from refractive indices measurements have been discussed. The contradictory behavior of Δ n and ΔX during the nematic-smectic B phase transition of HBT has been explained on the basis of polydomain formation and the order parameter in the SB phase is estimated. The order parameter of two liquid crystals, HBT and OBT, is also evaluated by NMR measurements, confirming the polydomain formation in the SB phase of HBT

Study of line shape and angular variation of ESR spectra in two smectic a liquid crystals

Electron spin-resonance (ESR) measurement of the androstane nitroxide spin-probe is reported in isotropic, nematic and smectic A phases of the liquid crystals N(p-Octyloxy-benzylidine)-p-toludine [OBT] and p-nitrophenyl-p-n-octyloxy benzoate [NPOB]. Mono-domain samples in flat quartz tube were formed for the study of the angular variation of the spectra in smectic A phase. The analysis of the spectra using the theory of Polnaszek, Bruno and Freed (PBF) has resulted in the determination of the order parameter S, the rotational correlation time TR, and the anisotropic diffusion parameter N at different temperatures. The comparison between the two samples shows that the molecular geometry of the liquid crystals play important role in determining the value of the order parameter. The entire smectic A phase in OBT is characteristic of slow tumbling region (TR > 10-9 sec) and the anisotropic parameter N shows an anomalous increase in this phase. This indicates that the slowly relaxing local structure (SRLS) mechanism plays important role in the slow tumbling region. In NPOB the incipient slow tumbling region is almost at the end of smectic A phase and the SRLS mechanism does not seem to play as important role as in OBT

ESR line shape study of two nematic liquid crystals

Electron spin-resonance (ESR) measurements of the spin-probe17β-hydroxy-4,4'-dimethyl spiro[5α-androstane-3,2-oxazolidine]-3'-yloxyl in the liquid crystals 4-n-Amylacetophenon O-(4-n-heptyl benzoyl)-oxime [AAHBO] and 4-n-Butoxyphenyl hexyl benzoate [BPHB] are reported in isotropic and nematic phases. On the basis of the temperature variation of ESR line width, an extra, solid to solid, phase transition is observed in BPHB. The theory of Polnaszek, Bruno and Freed (PBF) appropriate for anisotropic viscosity diffusion is used to analyze the ESR spectra and thereby to determine the values of the order parameter, the rotational diffusion tensor and the correlation time at different temperatures for the two liquid crystals. The analysis shows that the slowly relaxing local structure mechanisms (SRLS) is active in slow tumbling region specially in AAHBO

Solid crystalline polymorphism in M-21

This paper reports on the different solid crystalline (SC) forms of 4-cyano-4'-heptyloxy biphenyl (M-21) as revealed through differential scanning calorimetric, polarizing microscopic and infrared spectroscopic investigations. There are three solid crystalline modifications, namely SCI,SCII,SCIII. The appearance and preponderance of the SC forms depend critically on the manner in which the liquid crystalline melt solidifies. Each SC phase has its CN stretching band split into two components because of Davydov or correlation splitting. The position, separation and relative intensities of the two components characterize each SC phase. The systematic study of the variations in these three features with temperature leads to interesting information about the intermolecular ordering forces and the spatial arrangement of the molecules in the unit cell. It is shown that the intermolecular interactions of the dipolar nature play a dominant role in the SC phases but contribute negligibly in stabilizing the nematic phase. The angle between the correlated molecules in the unit cell changes in the order SCI> SCII > SCIII > nematic = 0, resulting in parallel arrangement of molecules in the nematic phase

X-ray spectroscopic study of zirconium and molybdenum diselenides

The k absorption spectra of zirconium, molybdenum and selenium in ZrSe2 and MoSe2 have been recorded photographically using a Cauchois type bent crystal (mica) spectrograph. The absorption edge shifts are used along with the data for NbSe2 (Bhide and Bahl 1971J. Phys. Chem. Solids 32 1001) to propose bond schemes for these compounds

Pulsed laser treatment at Fe/C<SUB>6</SUB>H<SUB>6</SUB> interface: a Mossbauer effect study

The pulsed ruby laser induced reactive-quenching process at Fe/C6H6 Ibenzenel has been investigated using conversion electron Mossbauer spectroscopy [CEMS]. It is shown that iron carbide phases can be synthesized when an iron foil immersed in benzene is treated with ruby Laser pulses [&#955;=694 nm, pulse width ~30 ns, energy density =15 J/cm2]. The results indicate the formation of &#949;-carbide and Fe5C2 phases in the as-treated sample and its transformation to Fe3C upon thermal treatment. The result of the CEMS measurements are supported by small angle X-ray diffractometry

Onset of Nucleate Boiling and Critical Heat Flux with Boiling Water in Microchannels

This paper focuses on experimental determination of onset of nucleate boiling\ud (ONB) and critical heat flux (CHF) at the microscales, and comparison of these with\ud available correlations. The working fluid is deionised water and microchannel of four\ud different hydraulic diameters: 65, 70, 107 and 125 m, have been tested. Effect of\ud hydraulic diameter (65-125 m), mass flux (60-1410 kg/m2s) and heat flux (0-910\ud kW/m2) on ONB and CHF has been studied in detail. The heat flux for onset of nucleate\ud boiling increases with hydraulic diameter and mass flux. The critical heat flux tends to\ud increase with a decrease in hydraulic diameter and with increasing mass flux. The effect\ud of surface roughness on CHF has also been tested to a limited extent; no clear change in\ud the CHF value was observed upon changing the surface roughness by an order of\ud magnitude. The empirical correlations tested in this study predict the experimental data to\ud varying extent. These results may help better determine the lower and upper limits of heat\ud flux while designing heat sink for electronic cooling