Ranjini Bandyopadhyay

Speckle-Visibility Spectroscopy: A Tool to Study Time-Varying Dynamics

We describe a multispeckle dynamic light scattering technique capable of resolving the motion of scattering sites in cases that this motion changes systematically with time. The method is based on the visibility of the speckle pattern formed by the scattered light as detected by a single exposure of a digital camera. Whereas previous multispeckle methods rely on correlations between images, here the connection with scattering site dynamics is made more simply in terms of the variance of intensity among the pixels of the camera for the specified exposure duration. The essence is that the speckle pattern is more visible, i.e., the variance of detected intensity levels is greater, when the dynamics of the scattering site motion is slow compared to the exposure time of the camera. The theory for analyzing the moments of the spatial intensity distribution in terms of the electric-field autocorrelation is presented. It is tested for two well-understood samples, a colloidal suspension of Brownian particles and a...

Evolution of particle-scale dynamics in an aging clay suspension.

Multispeckle x-ray photon correlation spectroscopy was employed to characterize the slow dynamics of a suspension of highly charged, nanometer-sized disks. At wave vectors q corresponding to interparticle length scales, the dynamic structure factor follows a form f(q,t) approximately exp([-(t/tau)(beta)], where beta approximately 1.5. The relaxation time tau increases with the sample age t(a) approximately as tau approximately t(1.8)(a) and decreases with q as tau approximately q(-1). Such behavior is consistent with models that describe the dynamics in disordered elastic media in terms of strain from random, local structural rearrangements. The measured amplitude of f(q,t) varies with q in a manner that implies caged particle motion. The decrease in the range of this motion and an increase in suspension conductivity with increasing t(a) indicate a growth in interparticle repulsion as the mechanism for internal stress development implied by these models.

Optical limiting in single-walled carbon nanotube suspensions

Optical limiting behaviour of suspensions of single-walled carbon nanotubes in water, ethanol and ethylene glycol is reported. Experiments with 532 nm, 15 ns duration laser pulses show that optical limiting occurs mainly due to nonlinear scattering. The observed host liquid dependence of the optical limiting effect in different suspensions suggests that the scattering originates from microbubbles formed due to absorption-induced heating.

Encapsulation of Hydrophobic Drugs in Pluronic F127 Micelles: Effects of Drug Hydrophobicity, Solution Temperature, and pH

Three drugs, ibuprofen, aspirin, and erythromycin, are encapsulated in spherical Pluronic F127 micelles. The shapes and the size distributions of the micelles in dilute, aqueous solutions, with and without drugs, are ascertained using cryo-scanning electron microscopy and dynamic light scattering (DLS) experiments, respectively. Uptake of drugs above a threshold concentration is seen to reduce the critical micellization temperature of the solution. The mean hydrodynamic radii and polydispersities of the micelles are found to increase with decrease in temperature and in the presence of drug molecules. The hydration of the micellar core at lower temperatures is verified using fluorescence measurements. Increasing solution pH leads to the ionization of the drugs incorporated in the micellar cores. This causes rupture of the micelles and release of the drugs into the solution at the highest solution pH value of 11.36 investigated here and is studied using DLS and fluorescence spectrocopy.

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.

Chaotic dynamics in shear-thickening surfactant solutions

We report the observation of dynamical behaviour in dilute, aqueous solutions of a surfactant CTAT (cetyl trimethylammonium p-toluenesulphonate), below the overlap concentration

Slow dynamics, aging, and glassy rheology in soft and living matter

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Stress Relaxation in Aging Soft Colloidal Glasses

. At these concentrations, CTAT forms cylindrical micelles and shows a pronounced shear-thickening transition above a concentration-dependent critical shear rate

Dynamic Light Scattering Study and DLVO Analysis of Physicochemical Interactions in Colloidal Suspensions of Charged Disks

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Use of ultrasound attenuation spectroscopy to determine the size distribution of clay tactoids in aqueous suspensions.

. An analysis of the time-series of the stress relaxations at controlled shear rates in the shear-thickening regime shows the existence of correlation dimensions greater than two and positive Lyapunov exponents. This indicates the existence of deterministic chaos in the dynamics of stress relaxation at these concentrations and shear rates. The observed chaotic behaviour may be attributed to the stick-slip between the shear-induced structure (SIS) formed in the sheared surfactant solution and the coexisting dilute phase. At even higher shear rates, when the SIS spans the Couette, there is a transition to higher-dimensional dynamics arising out of the breakage and recombination of the SIS.