Particle Wall Interactions and Near-Wall Dynamics of Colloidal Spheres in Crowded Environments

Over the last decade we developed instrumental equipment, based on evanescent wave light scattering, to investigate the static interaction of colloidal particles with an adjacent wall, as well as the near wall dynamics of these particles. In this presentation, I will give an introduction to the techniques, before discussing several experiments.Using Total internal Reflection Microscopy (TIRM), we investigated the depletion interaction between a colloidal probe sphere and a flat wall, induced by rod– and disc– shaped particles at rest [1,2] and under the influence of flow fields. There we could show that the strength of these depletion potentials can be tuned by the applied shear rate [2].Further we studied the near wall dynamics of a single probe sphere in a crowded environment of rods by TIRM. The standard approach extract dynamic information from TIRM data is extracting the probe particle’s diffusion coefficient normal to the wall from the mean square displacement (MSD) vs. time curves. However, in the course of our investigation we discovered that it is more reliable to determine the particle’s drift velocity from the mean displacement (MD) vs. time curves [3,4]. Using Evanescent Wave Dynamic Light Scattering (EWDLS) we studied the near-wall dynamics of spherical particles as a function of concentration, providing experimental prove for the long standing theoretical prediction that particle dynamics near walls are slowed down and are anisotropic [5]. Upon increasing particle concentration the slowing down is diminished, where the diminishment is again different in the directions parallel and normal to the interface [6].References[1] C. July, D. Kleshchanok, P. R. Lang Eur. Phys. J. E 35, 60 (2012). DOI 10.1140 epje/i2012-12060-7[2] S. De Sio and P. R. Lang, Z. Phys. Chem. 229, 1161 (2015).[3] S. Desio, PhD-Thesis Heinrich-Heine-Universitaet Duesseldorf, Germany[4] S. De Sio, Y. Liu, J.K.G. Dhont and P. R. Lang manuscript in preparation[5] P. Holmqvist, J. K. G. Dhont, P. R. Lang J. Chem. Phys. 126, 044707 (2007).[6] Y. Liu et al. Soft Matter 11, 7316 (2015)