Effects of microscopic surface roughness on particle-plane and particle-particle interactions in a viscous fluid at low Reynolds number

The hydrodynamic interactions between noncolloidal particles and surfaces in a viscous fluid at low Reynolds number have been an area of intense investigation during the past decades due to their importance in both industry and academia.; The theoretical and experimental investigations on a particle moving down an inclined planar surface were made to examine the effects of microscopic surface roughness on the interactions between the sphere and the plane. By assuming the microscopic surface roughness on either particle or plane can be characterized as two scales, the large asperities of low coverage and the small asperities of high coverage, a physical model was proposed in which the sphere experiences four successive stages: (i) descending toward the plane while in contact with a large asperity, (ii) descending toward the plane without contact, (iii) contacting the plane on small asperities, and (iv) ascending from the plane while in contact with a second large asperity. As the angle of inclination increases, the time-averaged nominal separation between the sphere and the plane was found to increase, which is consistent with the theoretical predication that at a steep angle of inclination the apparent hydrodynamic roughness is governed by the large asperities, whereas on a horizontal plane, the roughness is governed by the small asperities.; To provide further understanding of the sphere-plane interactions in a viscous fluid at low Reynolds numbers, the translational and rotational velocities of the sphere and the hydrodynamic roughness between the sphere and plane were examined in the experiments where the plane was artificially roughened with multiple roughness scales.; The hydrodynamic interactions between suspended particles were studied through a sedimentation process in which a heavy sphere falls past a light sphere in a viscous fluid at low Reynolds number.; A further analytical study on the hydrodynamic interaction between suspended particles was conducted based on the assumption of two-scale asperities in which the microscopic surface roughness on the spheres can be characterized as the large asperities of low coverage well separated by intervening small asperities of high coverage. (Abstract shortened by UMI.)...