Stains, which include the wide variety of nonparticulate materials which give color even when present in very low concentration on the soiled object.

Miscellaneous soils, which primarily include sticky substances and colorless liquids which evaporate to leave a residue.

The dirt on the soiled objects is mechanically held by surface irregularities to some extent. However, a major factor in binding dirt is the attraction between surfaces that goes under the name of van der Waal's forces. This is a theoretically complicated dipole interaction which causes any extremely small uncharged particle to agglomerate with other small uncharged particles, or to stick to an uncharged surface. Obviously, if colloidal particles bear charges of opposite sign or, if one kind is charged and the other kind is not, the attraction will be intensified and the tendency to agglomerate will be greatly reinforced. Likewise, a charged particle will tend to stick to an uncharged surface and vice versa, and a charged particle will be very strongly attracted to a surface exhibiting an opposite charge. In addition, dirt particles can be held onto a soiled surface by sticky substances or by the surface tension of liquids, including liquid greases.

Greases, stains, and miscellaneous soils are usually sorbed onto the soiled surface. In most cases, these soils are taken up as liquids through capillary action. In an essentially static system, an oil cannot be replaced by water on a surface unless the interfacial tensions of the water phase are reduced by a surface-active agent.