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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 camerafor 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 coarsening foam, where the dynamics can be treated as stationary and hence can be benchmarked by traditional methods. However, our speckle-visibility method is particularly appropriate for samples in which the dynamics vary with time, either slowly or rapidly, limited only by the exposure time fidelity of the camera. Potential applications range from soft-glassy materials, to granular avalanches, to flowmetry of living tissue. 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 coarsening foam, where the dynamics can be treated as stationary and hence can be benchmarked by traditional methods. However, our speckle-visibility method is particularly appropriate for samples in which the dynamics vary with time, either slowly or rapidly, limited only by the exposure time fidelity of the camera. Potential applications range from soft-glassy materials, to granular avalanches, to flowmetry of living tissue.

We report an elastic-light-scattering experiment for a semidilute polymer solution under a uniform laminar shear flow. The concentration fluctuations are greatly enhanced by shear flow and their structure is highly anisotropic in both the weak (yr 1) shear regimes, where y is the shear rate and Xd is the longest relaxation time of the system. The anisotropy is qualitatively different from binary liquid mixtures under a shear flow as a result of the strong coupling between the concentration fluctuations and the hydrodynamic flow.PACS numbers: 82.70. Kj, 66.90.+r, 82.70.Dd It is well known that entangled polymer solutions near phase separation undergo significant changes when subjected to shear flow. The most notable effect is a dramatic increase in turbidity which, since its discovery nearly twenty years ago, has been interpreted as evidence of a shear-induced phase transition. 1,2 There are welldocumented cases of shear-induced changes in the phase behavior of other systems: simple binary liquid mixtures, 3,4 binary polymer melts, 5 and binary polymer solutions. 6 However, in these systems, shear flow suppresses concentration fluctuations, turbidity, and phase separation. Furthermore, the shear-induced effects are much smaller than in polymer solutions. In binary liquid mixtures, for example, the turbidity is reduced by shear only within -0.1 °C of the critical temperature and only for shear rates exceeding y-1000 s -1 . 3 By contrast, in polymer solutions, the turbidity can be enhanced for temperatures of more than 50 °C above the phaseseparation temperature at shear rates of less than 10Recently, the existence of a shear-induced change of the phase transition in semidilute polymer solutions has been called into question by a number of investigators. 7 "" 9 Part of the controversy centers around questions of how to treat the thermodynamics of these nonequilibrium systems. An even more pressing issue is the question of the mechanism for the shear-induced enhancement of the concentration fluctuations which leads to the increased turbidity. Several conflicting mechanisms have been proposed. 2,7,8 However, progress on these problems has been hampered by a lack of detailed experimental information about the structure of sheared polymer solutions.In this paper, we present light-scattering measurements of the nonequilibrium steady-state structure factor S(q,y).Our results support a mechanism proposed by Helfand and Fredrickson 7 in which concentration fluctuations are enhanced by a coupling between the poly-mer concentration and shear flow through the concentration-dependent viscosity and normal stress coefficients. 7,9~n In contrast to previous experiments, we do not find evidence of a shear-induced shift in the phase boundary at low rates of shear. At higher rates of shear, a shear-induced transition remains an intriguing possibility.Our samples consisted of a volume fraction 0=0.04 of polystyrene (PS) dissolved in dioctylphthalate (DOP). The polystyrene has a molecular weight of M W = \....

Characterization of the microscopic fluctuations in systems that are far from equilibrium is crucial for understanding the macroscopic response. One approach is to use an 'effective temperature'--such a quantity has been invoked for chaotic fluids, spin glasses, glasses and colloids, as well as non-thermal systems such as flowing granular materials and foams. We therefore ask to what extent the concept of effective temperature is valid. Here we investigate this question experimentally in a simple system consisting of a sphere placed on a fine screen in an upward flow of gas; the sphere rolls because of the turbulence it generates in the gas stream. In contrast to many-particle systems, in which it is difficult to measure and predict fluctuations, our system has no particle-particle interactions and its dynamics can be captured fully by video imaging. Surprisingly, we find that the sphere behaves exactly like a harmonically bound brownian particle. The random driving force and frequency-dependent drag satisfy the fluctuation-dissipation relation, a cornerstone of statistical mechanics. The statistical mechanics of near-equilibrium systems is therefore unexpectedly useful for studying at least some classes of systems that are driven far from equilibrium.