An experimental study was conducted to visualize and evaluate the flow field surrounding the spherical particles settling in viscoplastic fluids exhibiting elastic and inelastic behavior. This study was carried out to obtain a profound understanding of the individual effects of elasticity and viscous yield stress on the settling behavior of spherical particles and shape of induced yielded regions (which illustrates the influence of fluid properties on particle settling behavior) in non-Newtonian fluids. It has been reported that both of these forces affect the settling behavior of particles in non-Newtonian fluids therefore making it imperative to determine which of these two forces play a prominent role especially when transport fluids exhibit both properties simultaneously. This knowledge is essential for the optimized design of engineering fluids for different conditions. The main objectives of this study were;  i. To depict and visualize the intrinsic flow field surrounding a settling particle in such fluids, ii.) To investigate the isolated effects of viscous yield stress and elasticity on the fluid velocity profile and flow field surrounding the settling particle, iii.) To corroborate valid postulates about the negative wake phenomena and the shape of the yielded region surrounding a settling particle using visual experimental evidence.

Two sets of fluids were prepared using two distinct Carbopol polymers (ETD 2020 and 940). First set of fluids exhibited similar shear viscosity and yield stress but different elastic properties while the second set of fluids had almost identical elasticity but disparate shear viscosity and yield stress. Rheological characterization of the fluids was conducted by the using Bohlin C-VOR Rheometer and Fann Viscometer. The settling velocities of the spherical particles (Specific gravity ranging from 2.5 – 3.9; Diameters: ranging from 2.00mm - 3.00 mm) in the various Carbopol solutions were measured using Particle Image Shadowgraphy (PIS). The fluid flow field and sheared region surrounding the settling particle was determined by using the Particle Image Velocimetry (PIV) technique.

Experimental results showed that for the same shear viscosity, increasing elasticity can dampen the particle settling velocity and fluid velocity profile significantly, which is beneficial for particle suspension during fluid transport. This inhibiting effect can also be achieved with greater potency by increasing the yield stress as well. Furthermore, experimental images showed that the shape of sheared region depends on the mean surficial stress exerted on the fluid by the settling particle and the physical property of the fluid. The set of fluids with different elastic property (but identical viscous property) gave similar shapes of yielded region when sheared by the same particle however increasing the viscous yield stress reduces the shape of the yielded region and changes the shape of the yielded region. This indicates the prominence of viscous yield stress as a major deciding factor in determining the shape of the yielded region. Finally, the existence of theoretical unyielded regions adjacent to the settling particle were observed experimentally for the first time in this study.