Brownian motion of a colloidal particle immersed in a polymeric solution near a rigid wall

Abstract

By using three-dimensional digital video microscopy (DVM-3D), we study the displacement of a Brownian particle immersed in a polymeric solution located near a rigid wall. The technique takes advantage of the diffraction pattern generated by a fluorescent particle that is found below the focal plane of an optical microscope. The particle is then tracked from the analysis of a sequence of digitized images to reconstruct its trajectory, which provides relevant information about the properties of the system. In a first stage, we obtain the mean square displacement (MSD) of a spherical probe dissolved in a viscoelastic solution. This MSD is then used to determine the elastic and viscous moduli of the suspension. Such measurements are consistent with bulk measurements performed by means of two techniques, namely, diffusing wave spectroscopy and mechanical rheology. Near the rigid wall, the motion of the probe particle can be split in two directions, i.e., parallel and perpendicular to the surface. For short times (but still in the Brownian regime), such motion can be characterized by means of two distance dependent friction coefficients. We observe deviations of the measured friction coefficients in comparison with the Newtonian behavior.