Complex fluids

Complex fluids contain mesoscopic structures or interactions that produce behavior beyond that of simple liquids, often including viscoelasticity or shear-dependent flow.

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Viscoelasticity: A material property combining viscous flow with elastic deformation and recovery. Complex fluids often store and dissipate stress at the same time.
Colloid: A mixture containing particles dispersed in a continuous medium, with particle sizes typically between molecules and bulk objects. Colloidal particles provide mesoscopic structure in many complex fluids.
Polymer melt: A molten polymer composed of mobile chains, often with strongly viscoelastic flow. Its processing depends on the chain relaxation and elasticity characteristic of complex fluids.
Newtonian fluid: A fluid whose shear stress is proportional to shear rate, with constant viscosity at fixed conditions. It provides the simple-flow benchmark that complex-fluid constitutive behavior departs from.
Non-Newtonian fluid: A fluid whose shear stress is not proportional to its shear rate. Many complex fluids depart from the linear stress–flow relation of simple liquids.
Polymer: A large molecule built from repeated chemical units linked into chains or networks. Long chains entangle and stretch, giving polymer fluids elastic responses.
Paint: A liquid coating that forms a solid film after application and drying or curing. Shear-dependent viscosity helps paint spread under brushing while resisting drips afterward.
Ideal fluid: A hypothetical fluid with no viscosity, used as a simplifying model in fluid mechanics. Its lack of dissipation contrasts with the internal stresses of real complex fluids.
Shear thinning: A decrease in apparent viscosity as shear rate increases. Alignment or disruption of structure can make a complex fluid flow more easily under shear.
Surfactant: A surface-active molecule that lowers interfacial tension and can self-assemble in solution. Surfactant assemblies create structured fluids such as micellar solutions.