Asymmetric response of a jammed plastic bead raft

Fluctuation-dissipation relations have received significant attention as a potential method for defining an effective temperature in nonequilibrium systems. The successful development of an effective temperature would be an important step in the application of statistical mechanics principles to systems driven far from equilibrium. Many of the systems of interest are sufficiently dense that they are close to the jamming transition, a point at which interesting correlations develop. Here we study the response function in a driven system of plastic beads as a function of the density in order to elucidate the impact of the jamming transition on the use of fluctuation-dissipation relations. The focus is on measuring the response function for applied shear stress. We find that even when the amplitude of the applied stress leads to a linear response in the strain, the time scale of the response is dependent on the direction of the applied stress

FINAL REPORT FOR DE-FG02-03ER46071 ENTITLED, "UNDERSTANDING FOAM RHEOLOGY FROM THE MICROSCOPIC TO THE MACROSCOPIC SCALE"

This research effort is focused on understanding the mechanical response of foams, and other complex fluids, from the microscopic to the macroscopic level. The research uses a model two-dimensional system: bubble rafts. Bubble rafts are a single layer of gas bubbles with liquid walls that float on a water surface. The work involves studies of the macroscopic response of foam under various conditions of external forcing, mesoscopic studies of bubble motion, and systematic variations of the microscopic details of the system. In addition to characterizing the specific properties of the bubble raft, a second aim of the research is to provide experimental tests of various general theories that have recently been developed to characterize complex fluids. Primarily, the focus is on testing the proposed jamming phase diagram paradigm. This paradigm suggests that a general âjammedâ state of matter exists and is common to a wide range of systems, including foam, colloids, granular matter, glasses, and emulsions. Therefore,we have extended our research in two directions. First, we have included studies of plastic bead rafts. These are systems of plastic beads floating on the air-water interface. The advantage of plastic beads is that they do not pop, so they can be studied for the much longer periods of time required to measure the slow dynamics associated with the jammed state. Also, they allow us to explore a different density regime than the bubbles. Second, to better understand the role of defects in jamming behavior, we have done a few experiments on the impact of defects on domain growth

Viscoelastic shear banding in foam

Shear banding is an important feature of flow in complex fluids. Essentially, shear bands refer to the coexistence of flowing and non-flowing regions in driven material. Understanding the possible sources of shear banding has important implications for a wide range of flow applications. In this regard, quasi-two dimensional flow offers a unique opportunity to study competing factors that result in shear bands. One proposal is the competition between intrinsic dissipation and an external source of dissipation. In this paper, we report on the experimental observation of the transition between different classes of shear-bands that have been predicted to exist in cylindrical geometry as the result of this competition [R. J. Clancy, E. Janiaud, D. Weaire, and S. Hutzlet, Eur. J. Phys. E, {\bf 21}, 123 (2006)]

Direct observation of twist mode in electroconvection in I52

I report on the direct observation of a uniform twist mode of the director field in electroconvection in I52. Recent theoretical work suggests that such a uniform twist mode of the director field is responsible for a number of secondary bifurcations in both electroconvection and thermal convection in nematics. I show here evidence that the proposed mechanisms are consistent with being the source of the previously reported SO2 state of electroconvection in I52. The same mechanisms also contribute to a tertiary Hopf bifurcation that I observe in electroconvection in I52. There are quantitative differences between the experiment and calculations that only include the twist mode. These differences suggest that a complete description must include effects described by the weak-electrolyte model of electroconvection

Impact of noise on domain growth in electroconvection

The growth and ordering of striped domains has recently received renewed attention due in part to experimental studies in diblock copolymers and electroconvection. One surprising result has been the relative slow dynamics associated with the growth of striped domains. One potential source of the slow dynamics is the pinning of defects in the periodic potential of the stripes. Of interest is whether or not external noise will have a significant impact on the domain ordering, perhaps by reducing the pinning and increasing the rate of ordering. In contrast, we present experiments using electroconvection in which we show that a particular type of external noise decreases the rate of domain ordering