Complex Fluids

Conventional solids and liquids are made up of atoms and molecules. Complex fluids are also made up of atoms and molecules at the very smallest scales. However, these atoms and molecules are organized to form larger structures, which in turn make up the bulk fluid. The observed behavior of these materials cannot be explained from the chemical constituents alone, without taking into account these intermediate scales of organization. An example of a complex fluid is mayonnaise - an emulsion of vinegar, oil and egg. Each of the ingredients is relatively fluid, however, the material itself is solid-like; this is not because of a chemical change in the mixing process, but because the ingredients form tiny, micron-size structures called micelles.

The actual size of these structures can vary greatly from system to system. Here are a few examples:

Liquid Crystals This shows a droplet of a nematic liquid crystal. Liquid crystal molecules are rod-like organic molecules about 2 nm in length; in the nematic phase the rods tend to orient along a given direction. Example of uses: in LCDs	from the Liquid Crystal Group at Case Western Reserve University
Colloidal crystals This is a suspension of polystyrene spheres in a liquid. The spheres of diameter 0.6 mm form organized crystalline structures. Examples of colloidal suspensions: paints, inks	from David Grier's group at the University of Chicago
Emulsions An emulsion is a mixture of two immiscible liquids, such as oil and water. Shown here is hexadecane in water with 9mm droplets. Examples of emulsions: milk, mayonnaise	from David Weitz' group at Harvard University

These materials are typically very susceptible to external forces such as stresses and strains, electric and magnetic fields, or to thermal fluctuations; this gives them the name "soft" condensed matter.

A good introduction to the physics of complex fluids can be found in:

Structured Fluids by T. A. Witten, Physics Today, July 1990.

Insights from Soft Condensed Matter, also by T.A. Witten