The flow properties of complex fluids

"Complex fluids" are a class of systems exhibiting "unusual" mechanical responses to applied stress or strain that are not well understood. Theoretically, these systems have been considered in several different contexts such as glasses and plastics as well as other thermal and "athermal" systems. Significant agreement has been found in considering these from a variety of theoretical perspectives. However inconsistent and controversial conclusions concerning their material properties still persists.; The careful study of these systems has the exciting potential to lead to "new physics" and new states of matter. For example, it has been suggested that these systems can jam and a "jamming phase diagram" can be used to consider how these systems can jam and unjam. Is such a theoretical framework useful? Is there really a new state of matter that is jammed? This large body of theoretical work has thus far been poorly supported with experiments.; In this thesis, we will examine the rheological properties of two experimental systems: a bubble raft and a sphere raft. These systems were studied because they represent "generic" complex fluids that can be explored over a wide range of parameters. In this way, these systems can be considered a good sample study to understand this broad class of systems. This thesis considers first the characterization of these materials. We then go on to address questions related to jamming including the jamming transition and description of fluctuations through various theoretical frameworks.; This experimental work was considered with two main experimental setups: a Couette viscometer capable of measuring stress with a torsion pendulum and a parallel plate shearing apparatus. In the bubble raft we consider different ways energy is injected into the system. By measuring how energy is dissipated in the system in the form of "stress drops", we are able to probe different time scales and length scales. In the bead raft, we consider experimental methods to measure an "effective temperature". A measurement of such a quantity in a nonequilibrium system can be a very important step in applying the "machinery" of current physical theories to these poorly understood systems. These two geometries and the investigation of two similar yet distinct complex fluids provides a preliminary experimental overview in considering the characteristics and dynamics of this interesting system class.