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Time dependent rheology of artificial slurries Brown, John Peter

Abstract

A method was developed for the preparation of slurries showing time dependent Theological properties, using artificial particles of constant shape and size, such that the effects of the slurry parameters, particle shape, size, concentration, and dispersing medium viscosity could be investigated separately. Two particle shapes were investigated, elliptical plate and rectangular plate. There were five sizes of elliptical plate particles with the surface area varied at nearly constant length to width ratio, and four sizes of rectangular plate particles with a length to width ratio varying from 1.92 to 4.85. Only one particle thickness was considered. The rheological state exhibited by the time dependent dispersions was found to be dependent on all the parameters investigated. The thixotropic state occurring with the smaller size particles became increasingly time dependent with the increase in particle surface area or length to width ratio. The transition to the false bodied state occurred with the largest size particles and the largest length to width ratios. Equilibrium apparent viscosity was found to increase linearly with particle surface area for thixotropic slurries with constant particle shape and dispersing medium viscosity. Also, equilibrium apparent viscosity was found to increase linearly with dispersing medium viscosity for thixotropic dispersions with constant particle shape, size, and concentration. The variation of the parameter of length to width ratio was found to have the greatest effect on the time dependency of the slurry. Ragged decay curves were obtained from the slurries made up from particles with the larger surface areas and length to width ratios. The ragged decay curves indicated a structural breakdown or that particle interference was taking place. The rate of decay of apparent viscosity of the slurries was shown to follow a curve which is approximated by the mathematical relationship for a second order-zero order reversible reaction mechanism. This theoretical model was found to fit regardless of slurry parameters or rheological state. The main determinants of the forward rate constant k₁ were found to be particle surface area, length to width ratio, concentration and dispersing medium viscosity.

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