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Abstract Part I of the paper deals with the general properties of the flow of concentrated suspensions. The shear stress/rate of shear characteristics of Newtonian, non‐Newtonian, plastic and thixotropic types of flow are briefly set out. The types of suspensions encountered in the paint industry and their flow properties are described, and it is concluded that no generalization is yet possible on the effect of particle size, shape or nature of the chemical interface. In Part II some recent research on the flow of glass spheres in simple liquids is reported: glass spheres were used because they are geometrically simple and are easily made, and the methods of preparing and fractionating them into different size ranges are described. The importance and difficulty of particle‐size analysis are discussed. Results obtained with suspensions containing different concentrations of spheres with different size‐distributions are given and it is found that the particle‐size distribution very much affects the viscosities of suspensions with more than about 30% of spheres by volume. It was found that the more nearly the particle sizes are equal to each other, the lower is the concentration of spheres required for a given viscosity. It was also found that for particle sizes less than 1/μ in diameter, the smaller the size, the greater the thixotropy.
P. Stephen Williams (Sun,) studied this question.