Modelling the flow behaviour of very shear-thinning liquids

With the arrival of the latest control led-stress rheometers. we are now able to measure the steady-state flow-curves of structured liquids over a very wide range of applied shear stress. For many such liquids, these flow-curves show novel forms, which are usually very shear-thinning in character. Here, we introduce a category of empirical mathematical models-which are either simplifications, or else elaborations of the well-known and often-used Cross and Ellis models-that describe the behaviour of such intensely shear-thinning structured liquids, which had hitherto been described only at high shear rates or shear stresses using a flow model with a 'yield stress' parameter, e.g., Bingham, Casson or Herschel-Bulkley types. These new models describe the flow-curves, whether above or below any previously described 'yield stresses'. The new models are found to be applicable to a range of structured liquids of commercial interest ranging from drilling muds, through molten chocolate to non-aqueous, flocculated dispersions. (C) 2001 Elsevier Science Ltd. All rights reserved.