The ratio between shear stress and shear rate as the fluid flows is called the dynamic viscosity of the fluid. Dynamic viscosity is the measure of the internal resistance of the fluid – it reflects the response of the fluid to the irreversible change due to flow. The dynamic viscosity of the Newtonian fluids is independent of the shear rate. When small solid particles are dispersed in Newtonian fluid, such dispersions can exhibit the properties of non-Newtonian fluids. The viscosity of non-Newtonian fluids depends on the shear rate during flow and is usually expressed as a function of shear rate. The greater the polydispersity of the dispersed particles in the fluid in terms of their size, the denser their packing in the fluid can be or the higher the concentration of their suspension can be. Energy dissipation can increase or decrease with the increase of shear rate when flowing non-Newtonian fluids. The increase in energy dissipation leads to the property of dilatancy, i.e., the increase in viscosity with increasing shear rate, which is usually undesirable in practice; its decrease leads to the pseudoplasticity, i.e., the decrease in viscosity with increasing shear rate. In some non-Newtonian fluids, the occurrence of thixotropy during and after flow can be observed, i.e., time-dependent behaviour with a hysteresis loop of shear stress as a function of shear rate. Some non-Newtonian fluids exhibit both elastic and viscous properties, commonly referred to as visco-elastic properties. In our work, we studied five complex, non-Newtonian, highly concentrated calcium carbonate suspensions commonly used as pigment suspensions in the paper industry and added to coating mixtures for surface treatment of paper. They differed in composition due to the different calcium carbonate particle size distributions. We studied the rheological properties of these basic pigment suspensions and their coating colours with starch and latex.
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