Glossary - S

second law of thermodynamics

see: thermodynamics.

self-avoiding

A chain or surface is said to be self-avoiding if it is not allowed to intersect itself. Most real systems are self-avoiding; polymer chains don't self intersect because it is not possible to have two molecule in the same place. However, in some theoretical models it is useful to allow self intersection as it makes calculations simpler. Such models are termed phantom. Allowing self intersection may or may not affect the properties of the model.

simulation

The examination of a problem often not subject to direct experimentation or analytical solution- most often by the use of a computer.

soap

A cleansing and emulsifying agent made usually by action of alkali on fat or fatt acids and consisting essentially of sodium or potassium salts of such acids. The formation of special structures by amphiphilic molecules is crucial for the cleaning properties of soap.

string theory

A potential formulation of quantum gravity, which assumes that the fundamental building units of nature are one-dimensional strings.

surface tension

Energy per unit area.

A force resulting from inter-molecular forces that is responsible for bubbles forming (as a consequence of being energetically favorable), water drops hanging from faucets and a meniscus forming when a column of water rises in glass tubes. It causes a reduction in the ratio of surface area to volume of a geometrical object - to a minimum in the absence of other competing effects.

Note: A sphere is the shape that a 3d object must have in order to have the smallest possible surface area for a given fixed volume. As a consequence of having the smallest surface area it also has the smallest energy associated with the surface (as Surface energy = surface tension * surface area).

surfactant

The term surfactant comes from shortening the phrase `surface active agent'. Amphiphilic molecules that have the unique property of getting absorbed at various interfaces and changing the properties of the interface are called surfactants. Surfactants are crucial for the properties of soaps and other detergents and have wide ranging applications from oil recovery to efficient delivery of drugs at a desired site in the body.