DEFINITION OF VISCOSITY

The definition of viscosity is “the quantity that describes a fluid’s resistance to flow. Fluids resist the relative motion of immersed objects through them as well as to the motion of layers with differing velocities within them.” Knowing how thick your material is can have huge impact on your ability to get work done. Material such as honey or glue tend to be thicker and are described as having a higher viscosity, whereas water and glycol are less viscous and flow faster.

Temperature also plays a critical role in the viscousness of a liquid. Generally speaking, the warmer a liquid is, the lower its viscosity is and the easier it flows. The colder it is, the more sludge-like it becomes and has a higher viscosity. Understanding how to calculate viscosity can be helpful when deciding if your material is ready to use or not. So how do you determine the viscosity?

Measuring the Viscosity of the Liquid

Fill your tall graduated cylinder with the liquid so it is about 2 cm from the top of the cylinder. Use your marker to make a mark 2 cm below the surface of the liquid. Mark another line 2 cm from the bottom of the cylinder.

Measure the distance between the two marks on the graduated cylinder. Suppose that the distance is 0.3 m.

Let the ball go on the surface of the liquid and use your stopwatch to time how long it takes for the ball to fall from the first mark to the second mark. Suppose it took the ball 6 seconds (s) to fall the distance.

Calculate the velocity of the falling ball by dividing the distance it fell by the time it took. In the example:

v=0.3 m/6 s=0.05 m/s

Calculate the viscosity of the liquid from the data you have collected:

viscosity=2(ball density− liquid density)ga2/9v

where g = acceleration due to gravity = 9.8 m/s2 a = radius of ball bearing v = velocity of ball bearing through liquid

Plug your measurements into the equation to calculate the viscosity of the liquid.

For the example, the calculation would look like this:

viscosity=2(23,866−2,500)(9.8)(0.01)/9(0.05) =93.1 pascal seconds

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