Rheology may be conveniently divided into bulk
rheology, in which effects due to the surface of the system can be neglected, and
surface rheology, in which such effects are predominant. It should be noted that in
surface rheology the neglect of bulk behaviour is permissible only in exceptional circumstances, such as for very thin films surrounded by a gas.
Source:
PAC, 1979, 51, 1213
(Manual of symbols and terminology for physicochemical quantities and units. Appendix II: Definitions, terminology and symbols in colloid and surface chemistry. Part 1.13. Selected definitions, terminology and symbols for rheological properties)
on page 1215