Capillary flow of dense colloidal suspensions
The purpose of this thesis is to study the flow of dense colloidal suspensions into micronsized capillaries at the particle level. Understanding the flow of complex fluids in terms of their constituents (colloids, polymers, or surfactants) poses deep fundamental challenges, and has wide applications in many industrial processes. Through the use of a novel experimental procedure we find results contrasting with the predicted bulk rheological behaviour of dense colloidal systems and propose an alternative approach based on the analogy with granular systems. Quantitative predictions which successfully explain the data are obtained. In order to obtain quantitative information on the dynamics of the system, we image the flow using a fast confocal microscope and identify the trajectories of each particle. Due to the nature of the flow, conventional techniques for locating and tracking the particles fail to yield satisfactory results. To overcome this limitation, we have developed a novel technique which allows us to successfully track the particles in strongly non-uniform flow fields (published, 2006). We focus our attention on three main aspects of the flow: concentration gradients, velocity profiles and time behaviour. We initially discuss the occurrence of concentration gradients along the flow direction and relate them to the local flow profiles. We observe high density regions where the velocity is uniform across the channel (complete plugs) and lower density regions where shear is present. The observed concentration profiles can be qualitatively explained by considering the relative ow between the solvent and the suspended particles. The flow profiles in the presence of shear consist of a plug in the centre while shear occurs localized adjacent to the channel walls, reminiscent of yield-stress fluid behaviour. However, the observed scaling of the velocity profiles with the flow rate strongly contrasts yield-stress fluid predictions. Instead, the velocity profiles can be captured by a theory of stress fluctuations originally developed for chute flow of dry granular media (published ,2007). We extend the model to our case and discuss it as a function of a series of parameters (boundary conditions, volume fraction, channel size, etc.) highlighting differences and similarities with granular media. Finally we discuss the time behaviour of complete plug flows relating it to the microscopic dynamics of the particles. At variance with dilute systems, dense systems exhibit velocity fluctuations when driven into channels by a constant pressure difference. We find that there exists a threshold value of the flow rate below which oscillations in the velocity are absent and above which their frequency scales as a power law of the flow rate. Despite quantitative predictions on this issue that are still missing, we present a microscopic description of the phenomenon highlighting the interplay between the particles and the solvent.