Rheology, Diffusion and Micro-structure of Sheared Suspensions of Deformable Particles
Open AccessThe collective behavior of deformable particles in sheared suspensions are studied in this dissertation. Using fully resolved front tracking simulations of suspensions with large number of particles and performing statistical analysis of the movement of the particles we characterize the physics of their collective behavior. The particle dynamics in the extremes of confinement-- unbounded shear flow and tightly confined flow, are distinguished and analyzed. In unbounded shear flow the motion of the deformable particles that make up a suspension, relative to the background flow, is diffusive. They exhibit both self-diffusion and gradient diffusion. A method for calculating the shear induced diffusivity of suspensions of deformable particles is developed and employed to compute gradient diffusivity for deformable particles. This is the first prediction of shear induced gradient diffusivity for deformable particles through simulations, to the best of found information. A novel approach to compute the collective diffusivity that was developed originally for statistically homogeneous rigid sphere suspensions is applied to the inhomogeneous drop and cell suspensions. In this approach the dynamic structure factor function of the suspension is calculated from the particle positions and its variation with time is analyzed to obtain the diffusivity. Though this approach was developed for homogeneous suspensions, it is shown to provide insight into non-homogeneous suspensions as well. Using these tools the gradient diffusivity has been computed for a range of parameters and data-driven models are provided that can be employed in various applications dealing with emulsions and suspensions.The existing parallel front tracking code is augmented with models for cellular mechanics that now allow us to model complex multi-component suspensions consisting of capsules, cells, vesicles etc. Effects of individual cell dynamics on the shear induced gradient diffusivity of a red blood cell (RBC) suspension are investigated through numerical simulations using this new capability. The non-spherical resting shape of RBCs gives rise to qualitatively different regimes of cell dynamics in a shear flow such as tank-treading, tumbling and swinging, depending on the elastic capillary number (i.e. cell stiffness). The transition of the cell motion from one regime to another causes significant changes to to the diffusivity. At an intermediate range of capillary numbers, RBCs cease tumbling which is accompanied by a noticeable drop in the coefficient of diffusivity. Further increase of the capillary number increases the diffusivity due to increased deformation of cells.In tightly confined drop suspensions (monolayer) subjected to shear flow, the drops self-organize leading to emergence of flow-oriented drop-chain morphology. This phenomenon is reproduced \textit{in-silico} and the underlying hydrodynamic mechanisms that give rise to this emergent behavior are identified. In pure shear flow, the confined drops act as (hydrodynamic-) quadrupoles in the far-field and get attracted to each other and align with the flow. The attraction is balanced by a short-ranged near-field hydrodynamic repulsion that results in stabilization of the drop chains. The fully resolved simulations are used to develop a simple quasi 2-dimensional pairwise interactions model for the evolution of the drop micro-structure. The predictions of this simplified model are compared with benchmark simulations. The simplified model is quite accurate and captures all qualitative aspects of the drop chain formation process. Under certain conditions its predictions even give a quantitatively equivalent results to the computationally expensive benchmark 3D front tracking simulations.
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Malipeddi_gwu_0075A_15670.pdf | 2022-03-06 | Open Access |
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