Brownian motion under shear in a narrow gap studied by X-ray Photon Correlation Spectroscopy

Brownian motions of nanoparticles dispersed in polyethyleneimine under shear in a narrow gap were studied using X-ray photon correlation spectroscopy. In this study, sample was sandwiched between facing surfaces, and shear was applied by moving one side of the substrate at a constant velocity. The gap between the surfaces was adjusted in the range of 10^1 to 10^2 nm and the changes in Brownian motion were investigated for various shear rates. X-rays irradiated from the side to the sample, the irradiation position was varied and the dynamics at each position was investigated. The time autocorrelation function of the scattering intensity obtained from the shear parallel direction and that from the shear orthogonal direction were analyzed separately. As a result, the latter was observed to be faster relaxation. By analyzing these data, the velocity gradient at each irradiation position was calculated. When the shear rate was slow, a uniform velocity gradient was observed. On the other hand, when the shear rate was increased, the velocity gradient became nonuniform, and in the vicinity of the moving interface, the velocity gradient was smaller than in other places. This can be considered to be a slip near the interface. Details of the experiments and analysis will be presented.