Multiscale simulation of the collective behavior of rodlike self-propelled particles in viscoelastic fluids

Abstract

The key assumption behind all previous studies on collective formation of SPPs is that the suspending fluid is a Newtonian fluid. However, biological fluids like mucus and saliva are viscoelastic. In this thesis, I will study the impacts of viscoelasticity, induced by adding polymers to the Newtonian suspending fluid, on the collective dynamics of SPPs driven by only hydrodynamic interactions. We will use two simulation methods that complement each other.

The results of this study will improve our knowledge about the role of viscoelasticity in active suspensions. The impacts of this study may be interesting for designing biomedical techniques since many biological fluids are at least partially viscoelastic.

We also examined the rheology of dilute active suspensions under a large amplitude oscillatory shear. The kinemtic approach taken for this part of the thesis shows interesting results like a negative contribution to the suspension viscosity for some types of active particles.

The continuum method is based on a mean field theory for representing the constituent particles. Before numerically solving the system for long times, it is instructive to analytically solve the equations in the linear region near an isotropic state and at the onset of hydrodynamic instability where nonlinear terms are still insignificant. This linear stability analysis provides a foundation to understand the formation of collective behavior at later times. The second method uses a discrete simulation scheme on the colony of SPPs and polymers. Each swimmer, as well as each molecule of polymer, is treated individually. Both continuum and discrete techniques take a dynamics approach to understand how rheology affects the collective dynamics of SPPs. The emphasis in this thesis is on continuum scheme because of some computational limitations in the discrete simulation.

Bacteria often use interactions within the group to produce a group behavior. The collective behavior of bacteria or self propelled particles (SPPs) at high enough concentrations has opened a new area of research in fluid mechanics. This is because different types of swimmers commonly use hydrodynamic interactions (HI) as a mechanism for collective behavior although they may simultaneously use other mechanisms of interactions.

The self-driven collective structures in viscoelastic suspensions are found to be shorter and more dispersed in comparison with those in Newtonian fluids. The collective patterns in viscoelastic suspensions evolve "irregularly" rather than periodically. The competition between the relaxation time of polymers and the characteristic time in SPP's probability equation is found to be responsible for the damp in the collective structures.