Flat Wall Proximity Effect on Micro-Particle Sedimentation in Non-Newtonian Fluids.

We investigate the sedimentation of colloidal micro-spheres and red blood cells (RBCs) in non-Newtonian fluid - silicone oil with different viscosities. We use digital holographic microscopy (DHM) to obtain volumetric information of the sedimenting micro-objects. Especially, the numerical refocusing feature of DHM is used to extract the depth information of multiple particles moving inside the fluid. The effects of proximity to a flat wall and the non-Newtonian behavior on the sedimenting micro-spheres and RBCs are studied by trajectory analyzing and velocimetry. We observe that for lower viscosity values the proximity effect is more pronounced. The variation rate of the particle falling velocities versus their distance to the flat wall decreases by increasing the viscosity of the fluid. For RBCs, however, the decreasing of the velocity variations have a smoother trend. The experimental results verify the theoretical prediction that, similar to Newtonian case, a correction factor in Stokes' law suffices for describing the wall effect.