Nalini Easwar

Phase Separation in Binary Nearly-Hard-Sphere Colloids: Evidence for the Depletion Force

A binary aqueous suspension of large (L) and small (S) nearly-hard-sphere colloidal polystyrene spheres is shown to segregate spontaneously into L-rich and S-rich regions for suitable choices of volume fraction and size ratio. This is the first observation of such purely entropic phase separation of chemically identical species in which at least one component remains fluid. Simple theoretical arguments are presented to make this effect plausible.

Large force fluctuations in a flowing granular medium

We study fluctuations in the force at the boundary of a 2D granular flow. The forces are mainly impulsive at all flow rates. The probability distribution of impulses decays exponentially at large impulses, as do the forces in a static granular medium. At small impulses, the distribution evolves continuously with flow rate with no indication of the transition from collisional flow to intermittently jamming flows. However, the distribution of the time interval between collisions tends to a power law, P(tau) - tau(-3/2), showing a clear dynamical signature of the approach to jamming.

Soft matter under osmotic stress

Abstract In this article, we will show that the osmotic stress method can be successfully applied to study the thermodynamics of self-assembly phenomena in soft matter systems, such as biopolymer liquid crystals, surfactant and lipid mesophases, and polyelectrolyte–surfactant complexes. We will give two examples to that effect. Firstly, we will present intercolumnar force measurements between cylindrical surfactant micelles in solid-state polyelectrolyte–surfactant complexes as a function of ionic strength. Secondly, we will present measurements of the DNA cholesteric spherulite structure and pitch as a function of osmotic pressure in an effort to evaluate the chiral contribution to the interaction between DNA molecules.

Dynamical fluctuations in dense granular flows

We have made measurements of force and velocity fluctuations in a variety of dense, gravity-driven granular flows under flow conditions close to the threshold of jamming. The measurements reveal a microscopic state that evolves rapidly from entirely collisional to largely frictional, as the system is taken close to jamming. On coarse-grained time scales, some descriptors of the dynamics—such as the probability distribution of force fluctuations, or the mean friction angle—do not reflect this profound change in the micromechanics of the flow. Other quantities, such as the frequency spectrum of force fluctuations, change significantly, developing low-frequency structure in the fluctuations as jamming is approached. We also show evidence of spatial structure, with force fluctuations being organized into local collision chains. These local structures propagate rapidly in the flow, with consequences far away from their origin, leading to long-range correlations in velocity fluctuations.

A demonstration of phonons that implements the linear theory

Beads on a vibrating wire are used to simulate the discrete structure of a solid-state material. The novel idea of the experiment is to use very small oscillation amplitudes of the wire to avoid nonlinearities in the interaction. We achieve a good signal-to-noise ratio using a lock-in technique. We find quantitative agreement between theory and experiment for not only a mono- and a diatomic chain, but also for the bare wire. The latter agreement is the crucial aspect that distinguishes our experiment from previous ones. This agreement assures that the fundamental assumption of the theory (Hooke’s law) is satisfied. We show that the properties of phonon dispersion curves are not special, and that the same band structures occur when the wavelength of any wave becomes comparable to the length scale of a discrete periodicity.

Observation of the relaxation of composition fluctuations in a binary liquid mixture

Light scattering was used to study the decay of concentration fluctuations in a critical mixture of isobutyric acid and water. The aim of the experiment was to study the dynamic interaction between composition and velocity in the mixture. At large wavenumber, the structure factor S(k, t) is observed to decay exponentially, with a relaxation rate somewhat smaller than the equilibrium value. At small k, however, there is a marked departure from the exponential relaxation one expects in the absence of coupling between composition and velocity. Also measured was the spinodal ring diameter, km, as a function of the total scattered intensity, Ctot(t) = (2π)−2 ∫ S(k, t)k2dk. A calculation of Ruiz, based on the above coupling effect, predicts a linear variation of Inkm(t) with Ctot(t) and such a linear variation is observed.

Search for an instability on a quenched-liquid interface

We searched for signs of an instability on the interface between the two phases of a binary-liquid mixture, isobutyric acid and water, after the mixture was quenched further into the two-phase region. Such an instability would be the liquid-liquid analog of the Mullins-Sekerka instability seen in quenched alloys. Never is any dramatic growth observed, but under conditions of small dimensionless quench depth (theta<1.5 x 10/sup -3/), the intensity of light scattered from the interface grows for small values of the momentum transfer k.