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Image by Pawel Czerwinski

Summary of a liquid viscosity experiment

For this experiment the liquid viscosity of castor oil was determined by dropping steel spherical balls into a tube of castor oil. By considering the forces acting on the ball whilst in the fluid, it was found that the viscosity was 1.37±0.01 Pa s with a percentage error of 71.25% to the literature value of castor oil’s viscosity. 


This experiment was to find out the dynamic liquid viscosity of castor oil using stainless steel spheres of different radii at room temperature. Viscosity is a fluid’s resistance to flow and is formally defined as the ratio of the shearing stress to the shearing velocity. Viscosity arises due to the friction between the particles in a fluid, because as the particles slide past each other there will be work done to slide over another particle which will produce heat. For this experiment, laminar flow was a valid model for the sphere falling down the castor oil tube, because the terminal velocity of the sphere is quite small and was reached quickly so there will not be any turbulence effects. Laminar flow is defined when liquids flow smoothly with layers that do not mix, however when the layers do mix then the flow is turbulent with different layers of the fluid swirling and mixing. As the sphere was travelling through the fluid there were 3 forces acting on it: drag force, weight and buoyancy.

This investigation can be furthered as viscosity is vital in the field of medicine. There are certain drugs that are directly injected into patients so by knowing the optimal path for the drug to be administered, the drug can reach the patient’s vital areas as quickly as possible.


Overall, this experiment was interesting as it led to numerous insights as to what may have affected the motion of a sphere falling down the tube of a viscous fluid, regardless of the simplicity of the experiment. Even though the uncertainties did not overlap with the literature value and the percentage differences were very high (in part due to calculations of the momentum), the experiment was not a failure due to the scientific insight it provided to make a carry out a more accurate experiment.

By Siam Sama 

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