Complex fluids are 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 to applied  stress or  strain due to the geometrical constraints that the phase coexistence imposes.  The mechanical response includes transitions between solid-like and fluid-like behavior as well as fluctuations.  Their mechanical properties can be attributed to characteristics such as high disorder, caging, and clustering on multiple length scales.