Effect of strong electric fields on phase transitions in some liquid crystals

An electric field couples to the orientational order in liquid crystals through dielectric anisotropy and is conjugate to the order. However the ionic flow produced by the field causes a dissipation of energy in the medium rendering an experimental study of the effect of the field on the order difficult. We have devised a method of measuring the local temperature of the sample, and both the dielectric constant whose value depends on the orientational order and the conductivity which is sensitive to the short range order in the medium. We report the electric field temperature phase diagrams of pentyl cyanobiphenyl (5CB) and octyloxy cyanobiphenyl (8OCB). The results include (a) the field induced enhancement of the orientational order (b) the critical field beyond which the nematic paranematic transition is continuous and (c) the effect of the field on the nematic-smectic A transition in 8OCB. The results are analysed using the appropriate Landau theories

Observation of chaotic dynamics in dilute sheared aqueous solutions of CTAT

The nonlinear flow behavior of a viscoelastic gel formed due to entangled, cylindrical micelles in aqueous solutions of the surfactant cetyl trimethylammonium tosilate (CTAT) has been studied. On subjecting the system to a step shear rate lying above a certain value, the shear and normal stresses show interesting time dependent behavior. The analysis of the measured time series shows the existence of a finite correlation dimension and a positive Lyapunov exponent, unambiguously implying that the dynamics can be described by that of a dynamical system with a strange attractor whose dimension increases with the increase in shear rate

Structure and Rheology of the Defect-gel States of Pure and Particle-dispersed Lyotropic Lamellar Phases

We present important new results from light-microscopy and rheometry on a moderately concentrated lyotropic smectic, with and without particulate additives. Shear-treatment aligns the phase rapidly, except for a striking network of oily-streak defects, which anneals out much more slowly. If spherical particles several microns in diameter are dispersed in the lamellar medium, part of the defect network persists under shear-treatment, its nodes anchored on the particles. The sample as prepared has substantial storage and loss moduli, both of which decrease steadily under shear-treatment. Adding particles enhances the moduli and retards their decay under shear. The data for the frequency-dependent storage modulus after various durations of shear-treatment can be scaled to collapse onto a single curve. The elasticity and dissipation in these samples thus arises mainly from the defect network, not directly from the smectic elasticity and hydrodynamics.Comment: 19 pages inclusive of 12 PostScript figures, uses revtex, psfrag and epsfig. Revised version, accepted for publication in Euro. Phys. J. B, with improved images of defect structure and theoretical estimates of network elasticity and scalin

Linear and nonlinear rheology of wormlike micelles

Several surfactant molecules self-assemble in solution to form long, cylindrical, flexible wormlike micelles. These micelles can be entangled with each other leading to viscoelastic phases. The rheological properties of such phases are very interesting and have been the subject of a large number of experimental and theoretical studies in recent years. We shall report on our recent work on the macrorheology, microrheology and nonlinear flow behaviour of dilute aqueous solutions of a surfactant CTAT (Cetyltrimethylammonium Tosilate). This system forms elongated micelles and exhibits strong viscoelasticity at low concentrations ($\sim$ 0.9 wt%) without the addition of electrolytes. Microrheology measurements of $G(\omega)$ have been done using diffusing wave spectroscopy which will be compared with the conventional frequency sweep measurements done using a cone and plate rheometer. The second part of the paper deals with the nonlinear rheology where the measured shear stress $\sigma$ is a nonmonotonic function of the shear rate $\dot{\gamma}$. In stress-controlled experiments, the shear stress shows a plateau for $\dot{\gamma}$ larger than some critical strain rate, similar to the earlier reports on CPyCl/NaSal system. Cates et al have proposed that the plateau is a signature of mechanical instability in the form of shear bands. We have carried out extensive experiments under controlled strain rate conditions, to study the time-dependence of shear stress. The measured time series of shear stress has been analysed in terms of correlation integrals and Lyapunov exponents to show unambiguously that the behaviour is typical of low dimensional dynamical systems.Comment: 15 pages, 10 eps figure

New type of high field electrooptic response in nematics

We have made concurrent measurements of ionic current and optical transmission between crossed polarisers on several nematics with positive dielectric anisotropy under the action of applied low frequency (< 1KHz) square wave voltages. When the field E is low, the measured current is linear in E and there is no electrooptic response. Beyond some value of the field (E(0)similar to 100 esu), the current becomes independent of the field (phenomenon of limiting current). Further an electrooptic signal is measured at twice the frequency of the applied voltage, which exhibits a peak as a function of the field. The width of the peak is 3 to 4 times the value of E-0, and the signal level at the peak decreases as the frequency is increased. These measurements have been made on three highly polar compounds with cyano end groups. Careful observations do not show any evidence of electrohydrodynamic instabilities in the sample. It is argued that the observations can be understood if at the onset of the phenomenon of the limiting current, a strong electric field gradient is established near one of the electrodes due to the sweeping of an ionic species with high mobility. The field gradient produces a flexoelectric deformation of the director field, which in turn gives rise to the electrooptic effect. At higher fields, the stabilising dielectric torque takes over to suppress this instability

New type of high-field electro-optic response in nematics

We have made concurrent measurements of ionic current and optical transmission between crossed polarizers on several nematics with positive dielectric anisotropy under the action of applied low frequency (&lt; 1 KHz) square wave voltages. When the field E is low, the measured current is linear in E and there is no electrooptic response. Beyond some value of the field (E<SUB>0</SUB> approximately 100 esu), the current becomes independent of the field (phenomenon of limiting current). Further an electrooptic signal is measured at twice the frequency of the applied voltage, which exhibits a peak as a function of the field. The width of the peak is 3 to 4 times the value of E<SUB>0</SUB>, and the signal level at the peak decreases as the frequency is increased. These measurements have been made on three highly polar compounds with cyano end groups. Careful observations do not show any evidence of electrohydrodynamic instabilities in the sample. It is argued that the observations can be understood if at the onset of the phenomenon of the limiting current, a strong electric field gradient is established near one of the electrodes due to the sweeping of an ionic species with high mobility. The field gradient produces a flexoelectric deformation of the director field, which in turn gives rise to the electrooptic effect. At higher fields, the stabilizing dielectric torque takes over to suppress this instability

Observation of Chaotic Dynamics in Dilute Sheared Aqueous Solutions of CTAT

The nonlinear flow behavior of a viscoelastic gel formed due to entangled, cylindrical micelles in aqueous solutions of the surfactant cetyl trimethylammonium tosilate (CTAT) has been studied. On subjecting the system to a step shear rate lying above a certain value, the shear and normal stresses show interesting time dependent behavior. The analysis of the measured time series shows the existence of a finite correlation dimension and a positive Lyapunov exponent, unambiguously implying that the dynamics can be described by that of a dynamical system with a strange attractor whose dimension increases with the increase in shear rate

Effect of a strong electric field on the reentrant nematic to smectic A<SUB>d</SUB> phase transition

We report experimental studies on the effect of a strong electric field on the nematic-smectic Ad (N-Ad ) and A -reentrant nematic (Ad -NR ) phase transitions in a binary mixture. Both the transition temperatures increase with field. The A -N transition point changes much more rapidly than that of Ad -N transition, indicating that the Ad phase will get bounded at a sufficiently high field. We have included the effect of the electric field in our molecular model of reentrant phases in highly polar compounds, in which the mutual orientation of near neighbour molecules changes from an antiparallel to a parallel configuration as the temperature is lowered. It is argued that the cross-over temperature increases with field which accounts for the observed trends. For suitable parameters, it is also predicted that either a nematic-nematic transition line can branch off from the Ad -NR line or a smectic A-smectic A transition line can meet the Ad -NR line at an appropriate field. In the latter case, the Ad -NR line has an associated slope change which is consistent with the experimental data

Formulation of stomach-specific floating microparticles of nizatidine and their radiographic evaluation

Nizatidine is an anti-secretogogue and a gastroprotective drug with a half-life of 1-2 h and is well absorbed in the stomach. This study aimed to optimize the process and develop floating microparticles of nizatidine that are based on low methoxyl pectin. Oil-in-oil dispersion method and Taguchi orthogonal array design were employed, and the prolonged residence time of the microparticles in the stomach was demonstrated. The constraints for independent variables, viz. A-polymer, B-internal solvent volume, C-surfactant, D-stirring rate and E-stirring time were set to generate the experimental runs. Particle size, percentage yield, micromeritic properties, entrapment efficiency,&nbsp;in vitro&nbsp;buoyancy and&nbsp;in vitro&nbsp;release were characterized. Surface morphology, zeta potential,&nbsp;in vitro&nbsp;release kinetics and&nbsp;in vivo&nbsp;floating performance of the optimized formulation was examined. The microparticles were free-flowing, irregular in shape and had a mean particle size distribution of 73-187 μ. Low methoxyl pectin played a predominant role in achieving buoyancy and optimum gastric retention for the modified release of the drug, suggesting Korsmeyer-Peppas model as the possible release mechanism.&nbsp;In vivo&nbsp;radiographic study in rabbits revealed that the drug was retained in the stomach for a period of 6 h. These results indicate that nizatidine floating microparticulate system provides modified drug release for the effective treatment of gastric ulcer