Time dependent rheology of artificial slurries

Abstract

Gels, formed by suspending cellulose acetate particles of regular shape and size in a Newtonian medium, were sheared in a rotational viscometer to study their time dependent rheological behavior. Qualitative observations of the effect of the well defined particle parameters, shape surface area and concentration on the flow parameters were made.The viscosity decay behavior fitted the model previously proposed by Pinder for slurries of natural particles.By considering the effect of the weighting salt used in the suspending medium, it was found that the theory that ionic forces are active in stabilizing the thixotropic gel structure was supported.reundlich and Juliusberger") first defined thixotro-F py as 'the reversible isothermal gel/sol/gel transformations induced by shear and subsequent rest'. The present day accepted definition is by Alves, Boucher and Pigford (2), 'The Thixotropic fluids possess a structure the breakdown of which is a function of time as well as the rate of shear. This structure can rebuild itself if not prevented from doing so by externally applied forces.'There is still considerable disagreement in the literature whether the time decreasing behavior of viscosity should be further classified as false body and thixotropic. In this work this classification has been found useful. These two terms shall therefore be used as follows:Thixotropic dispersions decay with shear to an equilibrium state which is Newtonian, independent of shear rate.False body dispersions decay to an equilibrium state which is pseudoplastic, the viscosity decreases with shear rate.Measurement of the time dependent properties of dispersions is generally carried out using a rotating concentric cylinder type viscometerca-lP'. A good review of the measurement techniques and their limitations has been made by Bauer and Collins"".Over the last half century there has been an increasing awareness of time dependent flow systems. They confronted industry initially as a problem to be overcome or avoided, but as data have been gathered, increasing use has been made of the property of time dependen~y"~).The reversible nature of the time dependent behavior has been explained by a number of authors in terns of a breaking down and subsequent rebuilding of some form of structure. Kistler(*5) postulated the existence of cybotactic complexes in liquids. A random mesh theory of gel structure was proposed by Brough-000000000000000000000000000000000000000000000000000000000000~0000000000000 1 Present addreas: Imperial Chemicals Limited, England.On a soumis A une action de cisaillement, dans un viscos i m h e rotatif, des gels form& par suspension dans un milieu newtonien de particules d'acfitate de cellulose de formes et dimensions rkgulihes; le but ktait d'ktudier leur comportement rhkologique avec le temps. On a fait des observations qualitatives sur l'effet qu'exercent des paradtres bien dkfinis des particules, B savoir la forme, la surface et la concentration, sur les parametres d'kcoulement. La diminution de viscositd concorde bicn avec les...

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