to Discover a Liquid's Viscosity

Which Liquid Has the Highest Viscosity?

In this project we will determine the viscosities of 5 separate liquids. The liquids we will test are corn syrup, honey, vegetable oil, milk, and water. We will find their viscosities by dropping a marble into each of these liquids and measuring the time it takes for it to reach the bottom.

Before we conduct the experiment, we must first understand what viscosity is. “Viscosity is the quantity that describes a fluid's resistance to flow”.1 It is essentially fluid friction and transforms kinetic energy of motion into heat energy, just as friction (“the force between surfaces in contact that resists their relative tangential motion”) does between two solid bodies. All fluids express some amount of viscosity. An ideal fluid has no internal friction between the molecules, meaning that it is not viscous. The reciprocal of viscosity is fluidity (“the physical property of a substance that enables it to flow.”)2 Thus, fluids that are high in viscosity tend to flow slower while fluids that are low in viscosity tend to flow faster. Different liquids have different forces: the larger the intermolecular force, the more viscous it is and vice versa.

We must also understand what terminal velocity and density are and how they relate to viscosity. “Terminal velocity is the constant velocity finally attained by a body moving through a fluid under gravity when there is no resultant force acting on it.” It has a direct relationship (“a relationship between two numbers or other variables where an increase or decrease in one variable causes the same change to occur in the second variable”) with viscosity. The more viscous a fluid is the slower a solid will fall through it. “Density is defined as the distribution of quantity per unit usually of space.” Viscosity and density are independent of each other. However, in this experiment the densities are needed in order to determine each liquid’s viscosity. The equation we will be using is: |Viscosity = |2(ΔP)ga2 |

| |9v |

where:

|Viscosity is in newton-seconds per meter squared (Nsec/m2). | |Delta (Δ) P is the difference in density between the sphere and the liquid, and is in kilograms per meter cubed (kg/m3). | |g is the acceleration due to gravity and equals 9.81 meters per second squared (m/s2). | |a is the radius of the sphere in meters (m). | |v is the average velocity, defined as the distance the sphere falls, divided by the time it takes to fall in meters per second (m/s).3 |

The viscosity of a fluid can be affected by certain variables one of which is temperature. When a fluid is heated it flows more easily, showing that the viscosity of it has decreased. This is due to the fact that as temperature increases, the average speed of molecules in a liquid increases, causing a decrease in the amount of time that they are in contact with neighboring molecules. This relationship can be described as an inverse relationship (“a relationship between two numbers in which an increase in the value of one number results in a decrease in the value of the other number”.)4 Viscosity is generally independent of pressure; however, liquids that are under extreme pressure often show an increase in viscosity. Although density and viscosity are not directly related, there are some examples of fluids that have a high density and are low in viscosity and vice versa. This is not always true though, but some fluids that this idea pertains to are: molten mercury, chloroform, and perchloroethylene. The people who made significant contributions to the study of viscosity were Sir Isaac Newton and Jean Louis Marie Poiseuille....

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