Why does mayonnaise act both like a liquid and a solid? What causes shaving cream to flow differently from toothpaste? These types of questions are at the heart of soft condensed matter, the study of materials with both fluid and solid properties (often called "complex fluids"). Moreover, the mechanical properties and ability to flow are in fact the defining features of soft materials, and are key to the practical utility of soft materials. The answers to these questions relate the mesoscopic structure of a complex material to its macroscopic properties (such as its viscoelastic modulus). With the wide variety of mesoscopic structures, it might be expected that the answers would depend strongly on the details of each material, and that the study of such systems would be the study of many special cases. However, recently the analogy of jamming suggests the possibility of universal behavior of complex fluids under stress, and in particular, that such systems may behave like granular media. In each case, the material behaves in many ways like a solid. The analogy of jamming depends on the microscopic behavior of such systems, yet there is little experimental evidence to support the analogy apart from macroscopic similarities between these systems.
This review introduces a variety of soft materials, describing both their microscopic structure and their macroscopic flow properties. Features of soft jammed materials are discussed, and open questions for any theory of jamming are proposed.