Introduction Viscosity is a significant property Of materials, it is a measure Of the resistance o deformation under shear stress. If one were to measure a material’s resistance to a change in motion under a specific force it can show how the material will behave in real life applications. For example in petroleum, viscosity measures the effectiveness of lubricating oil also, when making concrete, it is important to know the self-leveling and pumping behavior of the mixture. This experiment was held in an undergraduate laboratory at City University London.
This report will be presenting an experiment on viscosity measurement of glycerol – water mixtures with a Billion Viscose 88 rotational viscometer. Rotational viscometer provide ability to measure not only the viscosity of a fluid, but also shear rate and shear stress, torque, temperature and frequency of rotation. Theory These quantities were measured, recorded or calculated: Viscosity is material’s ability to resist motion or flow. It arises from collisions of molecules in a fluid that hit each other due to different velocities of molecules.
It is defined as a ratio of shear stress and shear rate. Units: Pas. Shear stress is a measure of a load on specific area, however differently from tensile stress, shear stress acts coplanar to a material cross section instead of citing perpendicular to the cross section. It is defined as force per unit area just like tensile strength and has the same units. Units: Pa. Shear rate is the rate of change of velocity at which adjacent layers of fluids pass over each other. Units: s-l. Torque in viscosity terms is a measure of resistance exerted when rotating the viscometer.
Units: Nm. Frequency is the number of full rotations per second. Units: Hazy- Dynamic viscosity can be given by the ratio of shear stress to shear rate: Experimental Arrangement Various rotational viscometers are in use for the determination of the jocosity of Newtonian liquids. These types of material exhibit different viscosities depending on the applied shear rate. An electrical rotational viscometer was used in this experiment that would measure more than just the dynamic viscosity (see above). The cylinder had a volume of 1 ml which had to be cleaned out after use.
The viscometer had various speed settings, however if the component is rotating too fast it jeopardizes the accuracy of the data it records. As a failsafe the component displays HI on its screen to indicate to the experimenter that the speed is too high to give accurate exults and should slow the component down. The results were taken in two phases, speed-up and speed-down, the experimenter would start at the lowest speed take down all readings and move up a speed, however when the display shows HI the experimenter would then slow down the viscometer and take readings again.
As the cylinder with the solution rotated the viscometer would display the results meaning the experimenters had to write down two sets of data one for the speed-up and one for the speed-down cycle. Results Displayed below are the results of the experiment. The first graphs show the lot of shear stress (Pa) against shear rate y (1 Is). The gradient of the shear stress versus shear rate graphs is the viscosity of that fluid. Discussion The experiment went as expected with no unusual events.
It would seem that the lines of best fit of the shear stress versus shear rate graphs fit to a line with a zero intercept with only a small deviation. The speed of rotation of the inner cylinder of the rotational viscometer must be taken into account when collating results, which the apparatus does by displaying HI on its interface it shows the experimenter that a higher speed of rotation would distort the exults of the experiments, in this experiment the speed never went higher than the limit set by the viscometer, this is evident in tables 1. 1 and 1. 2 as in table 1. The speed only goes up to 5 but in table 1. 2 it goes up to 6 meaning it was a less viscous fluid and offered less resistance to the movement of the cylinder. The viscosities of both fluids can be obtained by a myriad of procedures, in this report the viscosities of the fluid will be given by the gradient of the graphs of shear stress versus shear rate, this is because a single reading from the apparatus is not reliable since it is just one value. If one were to plot a graph one would need more than one value meaning that the values of this experiment could be reproduced again.