Restoration of Brownian dynamics upon shear cessation in colloidal suspensions investigated by x-ray photon correlation spectroscopy

This article presents an investigation of the re-establishment of Brownian dynamics upon cessation of flow in sheared relatively dilute colloidal suspensions. The multispeckle x-ray photon correlation spectroscopy can be used to dissect the advective and diffusive components of the observed dynamics in a suspension subjected to a shear flow. Even at relatively low shear rates, the relaxation rate of intensity autocorrelation function is dominated by the Doppler shifts caused by the flow. The results show that the return to Brownian dynamics is rapid in a uniformly sheared suspension. However, in an inhomogeneous flow, the transition from advection-dominated to diffusive Brownian dynamics is relatively slow with an intermediate region characterized by nonequilibrium velocity fluctuations. The temporal decay of these velocity fluctuations appears to follow an exponential function in time. Furthermore, these nonequilibrium fluctuations are not observable in very dilute suspensions where the mean interparticle distance is larger compared to typical viscous dissipation length.