Colloids

Joined: 20 Jul 2008 Posts: 193

Somewhere between the sizes of an atom and a grain of sand lies the realm of small particles called colloids. As will become evident, they are everywhere. The simplest colloidal materials, also generally known as suspensions or dispersions, consist of two mixed phases. The continuous or dispersing phase may be gas, liquid, or solid (or even plasma, the fourth phase of matter). Air, water, and plastics are common examples. The colloid particles make up the dispersed or suspended phase when uniformly distributed in the second, continuous phase. The dispersed matter may also be gas, liquid, or solid, and any combination in more complex suspensions. Colloidal dispersions are considered homogeneous mixtures even though they can be heterogeneous at or below the microscale.

Colloids are typically defined as having at least one linear dimension approximately between 1 nanometer (3.94 × 10−8 inches) and 1 micrometer, or micron (3.94 × 10−5 inches). A length-scale restriction for colloids is straightforward but arbitrary and misleading; there are no absolute bounds. It is best to think of a colloid in terms of how the material behaves with respect to inertial or body forces, such as gravity and fluid flow. For example, particles that do not quickly settle out or float to the top of their dispersed phase are considered colloidal. Wood fibers used in paper-making behave as colloids in water even though they can be tens of microns in diameter and many millimeters in length; individual fibers would not be considered colloidal in air where they would settle in seconds.