DYNAMICS OF COMPLEX FLUIDS

Complex fluids are soft condensed materials such as emulsions, foams, polymers, colloidal suspensions, and liquid crystals, which have equilibrium microstructures that are readily distorted by an imposed flow or field. The rich dynamical behavior of complex fluids arises from the interplay of intrinsic
microstructural relaxation rates (e.g., recoiling of polymer chains, relaxation of drop-shapes, redistribution of surfactant on a fluid interface), and comparable imposed (extrinsic) rates, such as an imposed shear rate, or oscillation frequency.

Complex fluids are involved in the processing and transport of paints, inks, coatings, lubricants, detergents, polymers, polymer blends, pharmaceutical reagents, and processed foods. Enhanced oil recovery, and soil and groundwater remediation often involve complex fluids. An understanding of
complex fluid dynamcs is needed for the design of microfluidic devices, drug delivery systems, and protein separation and crystallization processes.

Engineering applications of complex fluids require the prediction or control of its viscoelastic dynamical behavior (rheology) or the stability and evolution of its microstructure. For example, the control of a coating flow may require a material that flows when subjected to a sufficient shearing stress but not under a moderate buoyancy stress. In other applications, high viscosity fluids are efficiently transported by dispersing them in a low viscosity fluid. Blood flow in the micro-circulation depends on the deformation-induced migration of red cells away from microvessel walls to form a cell-free plasma layer and a particle-dense central core which undergoes plug-like flow with lower
resistance than that corresponding to a uniform distribution of red cells. Certain diseases (e.g., diabetes) stiffen red cell membranes, which suppresses red cell migration and seriously increases the work-load on the heart.

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