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: ?= K? t where K is the Instrument constant, ? 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. ?=2 Rs2(? s-? )F9 U
The Essay on Viscosities Of Liquids
This experiment focuses on measurements of different trials of various concentrations. The collected data is used to compare and contrast to the ideal binary solutions and their components. The Ostwald viscometer is a useful laboratory equipment to measure the viscosities of many binary solutions. Background Molecules have the ability to slide around each other, result in a flow. Such a flow has a ...
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 shows that glycerol has a constant viscosity. Thus, glycerol is a Newtonian liquid.
For starch, the flow begins when yield stress is reached and a sheer thinning behaviour is observed. The viscosity of starch is high at low shear rates and vice versa. This non-Newtonian flow is also known as Herschel Bulkley Flow. For ZnO glycerol, a linear plot with a yield stress is obtained. This means that ZnO glycerol has a Bingham flow with a minimum value of shear stress. Thus, it is a Newtonian liquid. For 10% Bentonite, the curve shows a thixotropic flow. This non-Newtonian flow is a time dependent viscosity.
When constant sheer stress is applied, the viscosity of the liquid decreases over time. For aqueous cream 1:1, the graph shows combination of Bingham flow and thixotropic flow. It is also a non-Newtonian liquid. Rotational sphere viscometer can be used to determine the flow behaviour for both Newtonian and non-Newtonian liquid. In conclusion, a proper understanding of rheological properties of pharmaceutical materials is essential to the preparation, development, evaluation and performance of the pharmaceutical dosage forms.
