Pre-submission Thesis Presentation

Suspensions are heterogeneous multiphase systems having particles dispersed in a continuous medium. Dense granular and colloidal suspensions exhibit complex flow behavior such as shear-thinning, shear-thickening and thixotropy. I will first introduce the various dense suspensions studied in this thesis, followed by the experimental techniques employed in this research work. When a less viscous fluid displaces a more viscous fluid in a confined geometry, the fluid-fluid interface becomes unstable, resulting in complex interfacial patterns. Intricate fluid displacement patterns, arising from the unstable growth of interfacial perturbations, can be driven by fluid viscoelasticity and surface tension. In my talk, I will highlight various experimental approaches that we have employed to control interfacial instabilities between a Newtonian and a viscoelastic fluid. I shall show how increasing the elasticity of the dense granular cornstarch suspension in its shear-thinning flow regime and the viscosity ratio of the fluid pair results in the suppression of interfacial instabilities. I will extend my discussion to the formation of transient reverse fingers during the displacement of dense shear-thickening cornstarch suspensions by water. I shall next discuss how the shear and time-dependent rheology of an aging colloidal suspension can be exploited to generate a wide variety of diverse interfacial pattern morphologies during the displacement of the suspension by Newtonian fluids. I will present a new parameter, the areal ratio, which we can employ to uniquely identify and segregate the observed pattern morphologies. Apart from the global features of the fully developed interfacial patterns, the temporal growths of the pattern morphologies are also studied to classify these distinct interfacial patterns. Besides being of fundamental interest, our results on the time-dependent growth of instabilities are useful in predicting and controlling the growth of interfaces in material processing. Finally, I shall discuss the effects of activity on the dynamics of passive colloids in a crowded environment.