Non-Equilibrium Phenomena in Sheared Soft Matter and Active Granular System

Abstract

In this thesis, we have studied non-equilibrium phenomena in sheared soft matter systems and in granular active matter. The surfactant systems are driven out of thermal equilibrium by applying external forcing at the boundary. We have used rheology, in-situ x-ray scattering techniques, in-situ optical microscopy to study the flow behavior and the structural changes of surfactant gels and Langmuir monolayers. The flow of soft gel and colloidal glass below their yield stress shows large burst in the shear rate due to internal reorganization as confirmed by in-situ optical microscopy. The statistical properties of the shar rate fluctuations resembles those observed in ground acceleration in earth-quakes, like Omori law, Gutenberg-Richter law and power law distribution of inter-occurrence time. The ordered mesh phase under shear shows isomorphic twinning transition having slightly different lattice parameters, giving rise to the splitting of the Bragg peaks and the six or eight points modulation of the Bragg rings. The rheology of the surfactant’s Langmuir monolayer shows a phase transition at yield strain separating the absorbing to fluctuating steady state. Further, the Grazing Incidence X-ray Diffraction of peptide-lipid monolayer at the air-water interface has been studied to understand the effect of shear on the structural properties of 2D nano-crystallites. In the second part, we have studied the effect of dissenters on the collective behavior of particles which have a tendency to flock. The system of two-step-tapered rods undergoes the flocking phase transition at a threshold area fraction φc ∼ 0.12 having high orientational correlations between the particles. However, the one-step-tapered rods (same as the two-step-tapered rods but without the 1mm middle step) do not flock. We use these one-step-tapered rods as the motile dissenters in the flock-forming granular matter of aligners (the two-step-tapered rods). We mix and disperse them to follow the system in the steady-state and our experiments give a quantitative estimation of dissenter’s effects on the flocking. At the critical fraction of dissenters, f ∼ 0.3, the flocking order of the system gets destroyed completely. The variance of the system’s order parameter shows a maximum near the dissenter fraction f ∼ 0.05, suggesting a finite-size crossover between the ordered and disordered phases.