Complex fluids

Complex fluids comprise a class of systems exhibiting unusual mechanical responses to applied stress or strain that are not well understood.

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Examples

Examples of complex fluids include binary mixtures having a coexistence between two phases: solid-liquid (suspensions), solid-gas (granular), liquid-gas (foams) and liquid-liquid (emulsions). Complex fluids are a material class under extensive new interest that have important characteristics such as phase separation, caging and clustering on multiple length scales. Such materials are often highly disordered. Complex fluids are particularly interesting because of the variety of subtle architectures that the components can create. These structures include multiple length scales which attribute to the global mechanical properties. Their flow properties are influenced by the geometrical constraints that the disorder and phase coexistence imposes.

Dynamics

The dynamics of the particles in complex fluids are equally difficult to understand. The property of restitution or dissipation of particles rubbing past each other has long been a difficult problem to understand. Energy lost due to friction may be a nonlinear function of the velocity and normal forces. The topological inhibition to flow by the crowding of constituent particles is common in these systems. Under certain conditions including high densities and low temperatures, when externally driven to induce flow, complex fluids are characterized by irregular intervals of solid-like behavior followed by stress relaxations due to particle rearrangements. The dynamics of these systems are highly nonlinear in nature. The increase in stress by an infinitesimal amount or a small displacement of a single particle can result in the difference between an arrested state and fluid-like behavior.

Although many materials found in nature can fit into to the class of complex fluids, very little is well understood about them. 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.