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1. Complex liquids or liquid complexes are materials intermediate between conventional liquids and solids, displaying fluid-like as well as solid-like behaviour.
Examples are polymeric melts or solutions, glasses, gels, foams and granular matter. Many of these systems are inherently disordered and strongly heterogeneous with large fluctuations on a wide range of length and time-scales. Furthermore many complex fluids, such as glasses or gels, never relax to equilibrium, which makes a theoretical analysis difficult.
2. Thus, these are binary mixtures that have a coexistence between two phases: solid-liquid (suspensions or solutions of macromolecules such as polymers), solid-gas (granular), liquid-gas (foams) or liquid-liquid (emulsions). They exhibit unusual mechanical responses that are applied stress or strain due to the geometrical constraints that the phase coexistence imposes. They mechanical response includes transitions between solid-like and fluid-like. behaviour as well as fluctuations.Their mechanical properties can be attributed to characteristics such as high disorder, caging, and clustering on multiple length scales.
3. Shaving cream is an example of a complex fluid. Without stress, the foam appears to be a solid: it does not flow and can support (very) light loads.
However, when adequate stress is applied, shaving cream flows easily like.
a fluid.
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