In this practical, viscometry and rheometry are introduced. Viscometry is the measurement of viscosity while rheometry refers to the experimental techniques used to determine the rheological properties of materials, that is the quantitative and qualitative relationships between deformations and stresses and their derivatives. The definition of viscosity was put on a quantitative basis by Newton, who was first to realize that the rate of flow was directly related to the applied stress. The constant of proportionality is the coefficient of dynamic viscosity, more usually referred to simply as the viscosity. Simple fluids which obey this relationship are referred to as Newtonian fluids and those which do not are known as non-Newtonian.
In this experiment, single point instruments such as capillary viscometer and falling sphere viscometer are used to determine the viscosity of Newtonian liquid. For capillary viscometer, the rate of flow of the fluid through the capillary is measured under the influence of gravity or an externally applied pressure. The important equation for the calculation of dynamic viscosity using a capillary viscometer is:
where K is the Instrument constant, ρ is the Density of the the Newtonian liquid and t is the Time. Based on the results, the viscosity of the Newtonian liquid A is 0.0256 Pa s. For the falling sphere viscometer, Stokes’ Law is used to determine the relationship between the rate of fall of a sphere through a liquid and the dynamic viscosity of the liquid. Ƞ=2 Rs2(ρs-ρ)F9 U
Based on the results, the dynamic viscosity of the Newtonian test liquid B 28.578 Pa s.
In the next experiment, the rheological behaviour of 5 pharmaceutical materials (glycerol, starch, ZnO glycerol, 10% Bentonite and aqueous cream 1:1) are tested using a rotational viscometer. By plotting graphs of Shear Stress versus rps (proportional to shear rate) for each material, the flow behaviour of each material can be described. From the graph, it...
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