Introduction: (Initial Observation)
Viscosity is one of the important physical properties of many fluid products. Consumers often care about the consistency of products that they buy. Any variation in the viscosity of a product from time to time can be an indication of unreliability of a product. For lubricants, motor oils, and cooking oils the viscosity is also sensitive to the temperature and varies in different temperatures. This can cause some other problems. For example, if you use a motor oil that becomes very viscose in cold weather, you will have trouble in starting the engine in winters. The effect of temperature on viscosity has been the subject of research by universities and manufacturers of lubricant oils for many years and has lead to the development of additives and materials that can control the viscosity of oil in different temperatures. The problem is that there are so many different oils and so many different applications. That is why research on viscosity and proposing solution is always a money making task for many consultant engineers. Such studies need to be done case by case and every case is different from others. In this project you will explore the effect of temperature on viscosity of an oil sample.
Some of the challenges in doing this science project are:
_ How can we test the viscosity?
_ What oils can be used for our tests?
How would you measure the viscosity of an oil? Can you come up with your own method of testing, comparing, or measuring viscosity?
After thinking for a while, it is time to do some research. Search the net for “Measuring viscosity”, “Viscosity unit”, “viscosity defined” and “viscosity test”.
Notice that in this project you don’t need (or you don’t have) to measure the viscosity. You just need to compare it. However it is good if you also learn about measurement of viscosity and do some experiments about it.
Read me for more info…
Viscosity is a measurement of the flow properties of a product. In order to understand what viscosity is, you need to realize that it is the ratio of the shear force applied and the amount of resulting deformation. The deformation of the fluid is expressed as the rate of shear. Therefore, viscosity is the relationship between shearing stress and rate of shear.
In the simplest cases, like water or aqueous solutions, the shearing stress is directly proportional to the rate of shear. The proportionality constant is called the viscosity coefficient or the viscosity of the liquid. Fluids where the proportion is direct are called Newtonian.
The unit of measurement is the poise, which is dyne.sec.cm-2. Normal viscosities are expressed as centipoise, where 100 centipoise=1 poise.
So what does this all mean from a product developer’s perspective? Well if it’s a food or a beverage you’re developing, it means you need to consider the impact of the viscosity on the product itself and also on the processing of it.
Fluids, including finished beverages, are either Newtonian or Non-Newtonian. The simplest are the Newtonian ones, like water, dilute suspensions, aqueous solutions, and emulsions. The following are some common products and their viscosities.
|FLUID||CP at 0 degrees C.||CP at 20 degrees C.||CP at 30 degrees C.|
Notice that viscosity is temperature dependent and typically decreases as the temperature rises.
Viscometers are of several types. The most often seen in food and beverage development are based on rotational viscometry; meaning they measure viscosity by sensing torque required to rotate a spindle at a constant speed while immersed in the fluid. The torque is proportional to the viscous drag on the spindle, and thus to the viscosity of the fluid.
The probe pictured above is used to measure viscosity. With the attachments shown to the left, this probe can be inserted into a flowing liquid while the gauge at the top, shown on the right-hand side of the probe, reads the fluid’s viscosity.
There are many ways to measure viscosity, including:
- attaching a torque wrench to a paddle and twisting it in a fluid.
- using a spring to push a rod into a fluid.
- seeing how fast a fluid pours through a hole.
- Seeing how fast a liquid pours through a funnel.
- Seeing how long it takes for a bubble to come up.
In our experiment we will use one of the oldest and easiest ways: we will simply see how fast a sphere falls through a fluid. The faster the sphere falls, the lower the viscosity. This makes sense: if the fluid has a high viscosity it strongly resists flow, so the sphere falls slowly. If the fluid has a low viscosity, it offers less resistance to flow, so the ball falls faster.
To compare the viscosity of different liquids or the viscosity of one liquid at different temperatures we can simply compare the falling time. However we can also go one step further and actually measure the viscosity.
The measurement involves determining the velocity of the falling sphere. This is accomplished by dropping each sphere through a measured distance of fluid and measuring how long it takes to traverse the distance. Thus, you know distance and time, so you also know velocity, which is distance/time.
The formula for determining the viscosity is impressive, decorated with Greek letters and a squared term, but simply amounts to multiplying some numbers and then dividing by some others:
delta p = difference in density between the sphere and the liquid in g/cm3
g = acceleration of gravity = 980 Cm/s2
a = radius of sphere (in Centimeters)
v = average velocity = d/t = (distance sphere falls in Centimeters)/(time it takes to fall in seconds) = Cm/s
The resulting Viscosity will be in g/Cm.s or Poise
This equation makes sense in that spheres that fall slowly have low velocities. This makes the denominator small, so the answer (viscosity) is large. Viscosity is measured in units of Pa s (Pascal seconds), which is a unit of pressure times a unit of time. This is not especially intuitive. How does it relate to flowing liquids? One way of looking at it is to realize that pressure is force per square area. This makes a little more sense: force applied to the fluid, acting for some length of time.
[Note: our experiment uses kilograms, meters, and seconds, rather than grams, centimeters, and seconds. Viscosity can be measured in g-cm-s, with the resulting unit called the poise; 10 poise = 1 Pa s. You may prefer those units to kg-m-s because densities are the more familiar grams per cubic centimeters.]
The measurement should be repeated many times to arrive at a good average value, and, most important, to observe the scatter in the results. This allows an assessment of the uncertainty in the measurement. Using spheres of different radii and densities and measuring the viscosities of at least two liquids gives a good idea of this unusual physical property and the power of an equation to predict behavior.
If you want to calculate the viscosity, you will need to know some densities. You can calculate the densities yourself or find them, however the following are some densities that you might need.
|Liquid||Density in kg/m3||Density in g/cm3|
|oil (most kinds)||920 kg/m3||0.98 g/cm3|
|shampoo||1000 kg/m3||1 g/cm3|
|water||1000 kg/m3||1 g/cm3|
|glass marble||2800 kg/m3||2.8 g/cm3|
|steel ball||7800 kg/m3||7.8 g/cm3|
Here are the viscosities of some other common substances:
|Substance||Viscosity (Pa s)||Viscosity (Poise)||Viscosity (Centipoise)|
|Air (at 18 oC)||1.9 x 10-5(0.000019)||1.9 x 10-4(0.00019)||1.9 x 10-2 (0.019)|
|Water (at 20 oC)||1 x 10-3 (0.001)||0.01||1|
|Canola Oil at room temp.||0.1||1||100|
|Motor Oil at room temp.||1||10||1000|
|Corn syrup at room temp.||8||80||8000|
100 Centipoise = 1 Poise
1 Centipoise = 1 mPa s (Millipascal Second)
1 Poise = 0.1 Pa s (Pascal Second)
Centipoise = Centistoke x Density
|Viscosity standard oils are used to calibrate viscosity measuring tools and equipment.|
| The first Professor of Physics at the University of Queensland, Professor Thomas Parnell, began an experiment in 1927 to illustrate that everyday materials can exhibit quite surprising properties. The experiment demonstrates the fluidity and high viscosity of pitch, a derivative of tar once used for waterproofing boats. At room temperature pitch feels solid – even brittle – and can easily be shattered with a blow from a hammer .
In 1927 Professor Parnell heated a sample of pitch and poured it into glass funnel with a sealed stem. Three years were allowed for the pitch to settle, and in 1930 the sealed stem was cut. From that date on the pitch has slowly dripped out of the funnel – so slowly that now, 72 years later, the eighth drop is only just about to fall.
The purpose of this project is to know the effects of temperature on viscosity of the oil.
The temperature is an independent variable. (the one that we set or modify)
The viscosity of the oil is the dependent variable. (It changes by changes in temperature)
In order to compare or measure the viscosity, we will drop a glass marble or a steel ball bearing in the oil and measure the time that it gets to the bottom. We will then modify the temperature of the oil and repeat the test again. We will record our observation data (falling time) in a table and analyze it to see the effect of temperature on viscosity. The following procedures include calculations for measurement of viscosity. If you just want to compare viscosities, simply skip the calculations and only include time in your tables. Also the following experiment assumes that you want to repeat each test 10 times and get the average. You may choose to do it less or more.
Your final results table may look like this:
|Temperature of oil||
Ball drop time
You may go to higher temperatures as well. Just remember heated oil gets much hotter than boiling water and it is very dangerous. I recommend not to try temperatures over 100º C. You may use a meat thermometer or other types of kitchen thermometers or laboratory thermometers for measuring the temperature.
If you do measure all viscosities as described in the next experiment, enter the results in a table like this:
|Temperature of oil||
This experiment is to measure the viscosity of one oil at one temperature. We drop a ball and measure the time. Then use the drop time to calculate the viscosity.
1.Choose the spheres and the oil to use for this activity. Enter the data for these materials into the Viscosity “Data Table.” If necessary, measure the radius of the sphere (hint: it is easier to measure the diameter and divide by two).
2.Determine the density of a sphere by measuring its mass and calculating its volume [remember that volume = (4/3) pr3]. Enter the value in the data table.
3.Enter the density of the liquid you are using (about 920 kg/m3 for oils, 1000 for shampoos) in the data table as “Fluid density.”
4.Fill a cylinder with a liquid, up to about 5 cm from the top.
5.Mark with tape a convenient starting point about 2 cm below the surface of the liquid (which will allow the sphere to reach terminal velocity before you begin making measurements). You can use either the top or the bottom of the tape, but use the same points for each measurement you make when you drop the spheres (step 8).
6.Mark an ending point about 5 cm from the bottom.
7.Measure the distance between the starting and ending points, and enter the answer in the data table as “Fall distance.”
8.You need an assistant to hold a sphere just touching the liquid while you get ready to measure the time of fall with a stop watch. The timer says “Go,” and his or her assistant drops the ball. The timer begins timing when the ball crosses the start line and ends it when it crosses the end line. You can use either the top or the bottom of the tape, but use the same points you used for the distance measurement.
9.Enter the data into the data table.
10.When you have made all 10 measurements, calculate the velocity at which the ball fell from this equation: velocity = distance/time. Enter the velocity values into the data table.
11.Now calculate the viscosity from this equation:
delta p = difference in density between the sphere and the liquid
g = acceleration of gravity
a = radius of sphere
v = velocity
12.Average your results for each experiment.
Viscosity Data Table
|Type of oil|
|oil density (p)|
|Density of sphere (p)|
|Density Contrast (delta P)|
|Radius of sphere (a)|
|gravity (g)||10 meters per second second|
|Fall distance (d)|
|Measurement number||Time (t), (seconds)||Velocity (v), (meters/seconds)||Viscosity (Pa s)|
delta p = difference in density between the sphere and the liquid
g = acceleration of gravity
a = radius of sphere
v = velocity = d/t = (distance sphere falls)/(time of it takes to fall)
Materials and Equipment:
- Oil for test (How about olive oil?)
- Spheres of different densities
- graduated cylinders or other long tubes
- meter stick
- stop watch
- Viscosity “Data Table”
- Viscosity “Histogram”
Results of Experiment (Observation):
Experiments are often done in series. A series of experiments can be done by changing one variable a different amount each time. A series of experiments is made up of separate experimental “runs.” During each run you make a measurement of how much the variable affected the system under study. For each run, a different amount of change in the variable is used. This produces a different amount of response in the system. You measure this response, or record data, in a table for this purpose. This is considered “raw data” since it has not been processed or interpreted yet. When raw data gets processed mathematically, for example, it becomes results.
If you do any calculations, write your calculations in this section of your report.
Summary of Results:
Summarize what happened. This can be in the form of a table of processed numerical data, or graphs. It could also be a written statement of what occurred during experiments.
It is from calculations using recorded data that tables and graphs are made. Studying tables and graphs, we can see trends that tell us how different variables cause our observations. Based on these trends, we can draw conclusions about the system under study. These conclusions help us confirm or deny our original hypothesis. Often, mathematical equations can be made from graphs. These equations allow us to predict how a change will affect the system without the need to do additional experiments. Advanced levels of experimental science rely heavily on graphical and mathematical analysis of data. At this level, science becomes even more interesting and powerful.
Using the trends in your experimental data and your experimental observations, try to answer your original questions. Is your hypothesis correct? Now is the time to pull together what happened, and assess the experiments you did.
Related Questions & Answers:
What you have learned may allow you to answer other questions. Many questions are related. Several new questions may have occurred to you while doing experiments. You may now be able to understand or verify things that you discovered when gathering information for the project. Questions lead to more questions, which lead to additional hypothesis that need to be tested.
If you did not observe anything different than what happened with your control, the variable you changed may not affect the system you are investigating. If you did not observe a consistent, reproducible trend in your series of experimental runs there may be experimental errors affecting your results. The first thing to check is how you are making your measurements. Is the measurement method questionable or unreliable? Maybe you are reading a scale incorrectly, or maybe the measuring instrument is working erratically.
If you determine that experimental errors are influencing your results, carefully rethink the design of your experiments. Review each step of the procedure to find sources of potential errors. If possible, have a scientist review the procedure with you. Sometimes the designer of an experiment can miss the obvious.
Sample list of references/ bibliography.
Massey, B S (1983) Mechanics of Fluids, fifth edition, ISBN 0442305524
Download free Viscosity- und Rheology E-book in English and German: