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Time dependent fluids are fluids whose viscosity depends on the duration of flow, The well known thixotropic fluids form a part of this category, because they become runny on being stirred and gel on being left. This phenomenon is of great importance, particularly in paints and in the best drilling muds, and is widely encountered in foodstuffs such as ketchup, mayonnaise and honey.It is important that the flow behaviour of the different kinds of fluid is understood, because the demands of industry require that many different kinds of fluid, including time dependent ones, be pumped from place to place efficiently.In addition, thixotropic materials such as shaving creams, some toothpastes and artists' colours are manufactured in a continuous process so that their consistency during the production process has to be adjusted for easy piping and later for easy flow out from the customers' tubes. The manufacturers of medical and food products such as salves, penicillin and canned baby foods adjust the consistency of their products using viscosity measurements, and in the dairy, flour and starch industries viscosity determinations provide automatic control by closing down the mixing and cooking process at a predetermined 'consistency end point'.In the present article it is hoped to clear up some of the confusion caused by inconsistencies in the terminology used to label the different time dependent effects and to describe how the various phenomena can be used to advantage.Below is a resume of the four main categories of fluid flow, which are dealt with in greater detail in some of the references.() Time independent fluids: (a) Bingham plastic and plastic fluids-fluids with a yield value; (b) shear thinning fluids -viscosity decreases as shear rate increases; (c) shear thickening fluids -viscosity increases with shear rate. (2) Time dependent fluids: (a)Thixotropic fluids; (b) negative thixotropic fluids. 38(3) Viscoelastic fluids : Fluids that possess elasticity as well as viscosity.(4) Complex rheological fluids: These exhibit a combination of properties from at least any two of the above.

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Time independent or purely viscous fluids are those whose viscosity is not dependent on their duration of flow, which means that the well known thixotropic materials, such as some kinds of paints, lie outside this class. This class of fluids, however, does include the best understood sub-group, the newtonian fluids. The flow of these particular fluids is well understood but as they form only a small part of a whole gamut of flow behaviour, it is important both to industry and in everyday life that the lesser documented fluids be studied and their properties understood. Sometimes the properties of fluids are a nuisance, at other times an advantage-if there were no such thing as Bingham plastic behaviour, how could striped toothpastes exist?Before going on to describe the particular properties of the time independent fluids, it is instructive to learn what relationship they bear with respect to other types of fluids. The four groups are as follows:(1) Time independent or purely viscous fluids: (a) newtonian fluids, (b) non-newtonian fluids.( 2 ) Time dependent fluids: (a) thixotropic fluids, (b) negative thixotropic fluids.(3) Viscoelastic fluids.(4) Complex rheological fluids: these exhibit a combination of properties from at least any two of the above. It should be understood that it is far easier to invent groups of fluids than it is to fit any specific fluid into a particular group.Details of the other types of fluid may be found in some of the references given.