Light Scattering in a Critical Binary Mixture under a Flow

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In this work we have investigated shear-induced effects arising from a coupling between the phase separation in a binary mixture and shear in presence of a macroscopic flow. Besides, for the purpose to get informations associated with the time-dependent correlation function in a sheared system we have measured dynamic light scattering for a dilute solution of Brownian particles with shear. We summarize here our main results :(1) It has been observed using a technique of phase-contrast optical microscopy that the phase separation of a critical mixture under shear results in the formation of a stationary domain structure, sometimes referred to a string phase, in which the highly anisotropic concentration-domains in the form of long tubes oriented parallel to the flow are present. The long tube-like string phase were broken up to small spherical domains when the shear was turned off.(2) We have measured the effective viscosity in a critical mixture involving phase-separating concentration … More -domains, which are imposed to shearing by a Couette flow in a gap between two concentric cylinders. The effective viscosity thus obtained are almost compatible with the rheological equation predicted very recently by Onuki, derived from the biccontinuous condition that the shear stresses should balance at the interface when the emerging two phases have different viscosities.(3) The time-dependent correlation function has been investigated for a sheared solution of a polystirene latex, which depends on the scattering geometry, especially the scattered wave vector component associated with the direction of the flow. The factors affecting the time-dependent correlation function include the rate of shear, the scattering volume observed, and the orientation of the scattering angle with respect to the velocity and shear in the solution. Despite these problems there may be ways to measure the critical fluctuations of a binary mixture under the flow for a given scattering geometry.The present results would provide a key to understanding the shearing process in a fluid mixture in terms of critical-point universality. Less