Studies of instabilities in particulate suspensions

Abstract

The focus of this project is to study the dynamics of particulate suspensions. This study is divided into two parts. Part one focuses on the dynamics of self-propelled particles, such as microorganisms. The dynamics of such active particles shows temporal and spatial pattern formation. Bioconvection is one such phenomenon. The understanding of these behaviours can be utilized in cell biological applications. There is a lot of literature available which discusses the individual and collective dynamics of microorganisms. The most convincing theory for collective dynamics is given by Pedley and Kessler (1992). Hopkins and Fauci (2002) proposed a discrete model for bioconvection and got some encouraging results from their two-dimensional simulations. We propose a more realistic model by extending the Stokesian Dynamics method for self-propelled particles. In this model, active particles are modelled as rigid dipolar spheres. Each sphere is associated with a stresslet and strain rate, which governs its motion, apart from interparticle hydrodynamic interactions. We are able to simulate up to 80 active particles in an unbounded domain. Our simulations are quasi-two-dimensional, restricting movements of the particles to a plane. The video images of the simulations clearly show the streaming motion of active particles. Similar streaming motion is also observed for bacteria in a freely suspended soap film (Wu and Libchaber, 2000). The velocity distribution is highly non-Gaussian, in contrast to that of sheared passive particles. Part two covers experimental work on instabilities in coating flows of suspensions. The experiment is for a flat belt rising from a pool of neutrally buoyant suspension. The experiment is done to verify the recently proposed theory by Govindarajan et al. (2001). They proposed that the instability arises from the coupled dynamics of the film thickness and the particle concentration and is caused by shear-induced diffusion of the particles. Our experiments show that the instabilities previously observed in the partially filled horizontally rotating cylinders (Tirumkudulu et al., 2000; Tirumkudulu and Acrivos, 2001; Botte and Thomas, 1999) are also observed in the coating of flat plates.