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The use of rotating vane geometries for the measurement of the flow properties of very non-Newtonian liquids has become increasingly popular over the last 20 years. Originally, these geometries were used to measure the apparent yield stresses of inorganic dispersions, but have more recently been used to measure other rheological parameters also. These include the low-strain modulus and the steady-state flow-curves of structured liquids. The particular advantages of the vane geometry are its simplicity of fabrication, ease of cleaning and more than anything else, its elimination of serious wall-slip effects. The development of the vane technique and the theory to go with it, together with its use in various areas are described, where these areas include inorganic colloidal dispersions, foods, bioengineering fermentation broths, etc.

A simple theory is presented to account for the viscometer walls effects encountered when measuring suspensions of large particles (or flocs). The often-recommended minimum ratio of gap-to-particle size ratio of 10:1 is shown to be appropriate for phase volumes up to 25%, but above this value, the ratio needs to be greater. The theory is supported by recent experimental evidence.

An account is given of the development of the idea of a yield stress for solids, soft solids and structured liquids from the beginning of this century to the present time. Originally, it was accepted that the yield stress of a solid was essentially the point at which, when the applied stress was increased, the deforming solid first began to show liquid-like behaviour, i.e. continual deformation. In the same way, the yield stress of a structured liquid was originally seen as the point at which, when decreasing the applied stress, solid-like behaviour was first noticed, i.e. no continual deformation. However as time went on, and experimental capabilities increased, it became clear, first for solids and lately for soft solids and structured liquids, that although there is usually a small range of stress over which the mechanical properties change dramatically (an apparent yield stress), these materials nevertheless show slow but continual steady deformation when stressed for a long time below this level, having shown an initial linear elastic response to the applied stress. At the lowest stresses, this creep behaviour for solids, soft solids and structured liquids can be described by a Newtonian-plateau viscosity. As the stress is increased the flow behaviour usually changes into a power-law dependence of steady-state shear rate on shear stress. For structured liquids and soft solids, this behaviour generally gives way to Newtonian behaviour at the highest stresses. For structured liquids this transition from very high (creep) viscosity (>106 Pa.s) to mobile liquid (

The ensuing mechanical response to stressing or straining a structured liquid results in various viscoelastic phenomena, either in the linear region where the microstructure responds linearly with respect to the stress and strain but does not itself change, or in the nonlinear region where the microstructure does change in response to the imposed stresses and strains, but does so reversibly. The complication of thixotropy arises because this reversible, microstructural change itself takes time to come about due to local spatial rearrangement of the components. This frequently found time-response of a microstructure that is itself changing with time makes thixotropic, viscoelastic behaviour one of the greatest challenges facing rheologists today, in terms of its accurate experimental characterisation and its adequate theoretical description. Here a history of thixotropy is given, together with a description of how it is understood today in various parts of the scientific community. Then a mechanistic description of thixotropy is presented, together with a series of applications where thixotropy is important. A list of different examples of thixotropic systems is then given. Finally the various kinds of theories that have been put forward to describe the phenomenon mathematically are listed.