Viscosity of Fluids Lab: Bubble Time Method Objectives
Solidify the concept of viscosity through experimentation Test viscosities of different liquids using the Rising Bubble Technique
Introduction Viscosity is a fluid property that measures the resistance of a fluid to flow and can simply be thought of as the “thickness” of a fluid. Fluids that have a high viscosity, such as honey or molasses, have a high resistance to flow while fluids with a low viscosity, such as a gas, flow easily. The resistance to deformation within a fluid can be expressed as both absolute (or dynamic) viscosity, µ [N s/m2], and kinematic viscosity, υ [m υ [m2/s].
Absolute viscosity is determined determined by the ratio of the shear stress to the shear rate of the fluid. The shear stress is dependent on the fluid’s resistance force to flow over the area of the plate while the shear rate is the equivalent to the fluid’s gradient. µ= = = µ ℎ ℎ
These relationships shown in the equation above can be seen pictorially in Figure 1.
Figure 1: Friction between fluid and boundaries causes shear stress at a specific gradient.
While absolute viscosity is able to quantifiably compare various liquids and gases on the same scale, it does not account for an important characteristic of fluids – the the density (ρ). (ρ). Kinematic viscosity (υ) is (υ) is highly dependent on density and is measured by the time required for a specific volume of fluid to flow through a capillary or restriction. µ =
Applications of Viscosity Viscosity is an important concept that is taken into consideration in a variety of fields ranging from cooking to oil rigging. Understanding the applications of viscosity can help in both flow characterization and quality control. Quality Control Since raw materials must be consistent from batch to batch, flow behavior can be used as an indirect measure of product consistency and quality. As mentioned earlier, similar viscosities is indicative of similar flows. Viscosity has a direct effect on the ability to be processed. When designing pumping and piping systems, it should be known that a high viscosity liquid requires more power to pump than a low viscosity one. The Viscosity Index of a liquid measures how variations in temperature directly affect the viscosity of a fluid. Liquids whose viscosity is greatly dependent on temperature have a high viscosity index. This is an important characteristic of a good lubricant. Flow Characterization Rheology is the study of the flow of matter, primarily in the liquid state. The viscosity of a fluid helps predict whether the flow will be laminar or turbulent and it can be categorized accordingly. Viscosity helps explain the behavior of fluids; thus, once the behaviors are understood, they can be manipulated according to specific needs.
Measuring Viscosity Depending on the type of fluid and flow characteristics, there are specific devices and experiments that are used to measure viscosity. The two most common instruments are a viscometer, which measures a single flow condition, and a rheometer, which measures varying flow. One of the simplest methods for experimentally determining the kinematic viscosity of a liquid is using the Rising Bubble Technique. The American Society for Testing and Materials has issued approval of this technique and classified it accordingly as The Standard Test Method for Viscosity of Transparent Liquids by Bubble Time Method.
Guiding Principles of the Rising Bubble Technique The Rising Bubble Technique is an elegant, yet simplistic, method to compare the viscosities of several liquids in a single experiment. Research has shown that the time required for an air bubble in a liquid to rise is directly proportional to its viscosity. Therefore, by using a set of precision viscosity tubes with a tight tolerance on the internal diameter the relative bubble rise rate variations between the vials will be below the level of visual detection, allowing for a comparison test between liquids to be made. There are two different ways to determine the viscosity using the Rising Bubble Technique — the Comparison Method and the Time Method. Comparison Method This method allows for viscosities of fluids to be estimated relatively accurately. This test is conducted by measuring the bubble rise rate of liquid sample with an unknown viscosity and comparing them to tabulated standards of samples with a known viscosity. A comparison of air bubble speed of the samples to the standards permits estimation of viscosity; the faster the bubble rise, the lower the viscosity.
Time Method (ASTM D-1545) The Time Method determines the exact viscosity by using a specific set of Time-Test Tubes. The length of time, in seconds, required for an air bubble to travel 73mm through the specified inner-diameter tube is approximately equal to the viscosity of the liquid in “Stoke” units or “Bubble Seconds”.
Problem Statement QuantumFlo is an up-and-coming pump system company working to be competitive in today’s market. They specialize in variable speed pumping technology. In order to achieve the greatest results from marketing their pumps, QuantumFlo would like to advertise as being energy efficient by requiring minimum power to run their machines. As a research and development engineer for QuantumFlo, you are tasked with testing the viscosities of five different liquids to determine which requires the least amount of power to run the machine.
Materials
Thermometer Standard Cargille Viscosity Tubes Viscosity Tube Corks Tube Holder Syringes Stopwatch Test Liquids (e.g. liquid soap, corn syrup, vegetable oil, motor oil, etc)
Procedure Complete the 2 different methods of the Rising Bubble Test to quantifiably determine which fluid would require the least amount of power to run the QuantumFlo machine. Method 1: Time Method (ASTM D-1545) 1. Fill the standard viscosity tubes with the liquid samples to be tested. Ensure that the bottom meniscus is level to the 100-mm line. 2. Measure and record the temperature of the test samples in Table 1. For every 0.1°C the temperature is above or below 25°C, account for a 1% error in the time when compared to Figure 1 standards. Table 1: Record the sample type and sample temperature.
Tube # Liquid Product Temperature (°C) Estimated Error
1
2
3
4
5
3. Insert a cork to the viscosity tubes so that the bottom of the cork is level with the 108-mm line. 4. All tubes should be in a completely vertical position. To begin the test, quickly invert the tubes. a. Start the timing device when the top of the bubble becomes tangent to the 27-mm mark. b. Stop the timing device with the top of the bubble becomes tangent with the 100-mm line. c. Alternative timing method: Record video of ball drop and import in Logger Pro for video analysis to determine time. 5. Record the time required for the 73-mm bubble rise for three trials in Table 2.
Table 2: Measure and record the bubble rise time in each sample.
Tube # Liquid Product Trial 1 Product/Tube # Bubble Rise Time (sec) Trial 2 Product/Tube # Bubble Rise Time (sec) Trial 3 Product/Tube # Bubble Rise Time (sec)
1
2
3
4
5
6. Using the collected data in Table 2 and comparing it to the standards listed in Table 3, determine the kinematic viscosity of each sample in centistokes (cST). Record values in Table 4.
Table 3: Gardner-Hodlt standards for kinematic viscosity using the rising bubble technique. A [3]
Table 4: Use the published standards in Table 3 to determine the kinematic viscosity.
Tube # Kinematic Viscosity (cST)
1
2
3
4
5
Method 2: Comparison Method Note: The viscosity of at least one product must be known in advance in order to compare the other products 1. Fill the standard viscosity tubes with the liquid samples to be tested. Ensure that the bottom meniscus is level to the 100-mm line. 2. Insert cork so the bottom of the cork is level with the 108-mm line. 3. To test, quickly invert the tubes. Rank bubble rise times between the different products to determine the relative viscosities. Record findings in Table 5. Table 5: Find the relative kinematic viscosity using known viscosities.
Slowest Rise Time ----------------------------------------- Fastest Rise Time Product/Tube # Viscosity (if known)
Questions/Deliverables 1. What characteristics are associated with a fluid that has a high-viscosity? Ones with a lowviscosity? 2. What are five occupations that have direct applications with fluid viscosity? 3. List three common fluids used every day in increasing order of viscosity. 4. In other experiments, it has been found that an increase of temperature in a liquid will decrease the viscosity. Oppositely, as the temperature of a gas increases, the viscosity also increases. Please give an explanation for these observations. 5. Provide your recommendation of which fluid sample from the experiment will require the least amount of power to run the QuantumFlo machine. Give your reasoning and any assumptions made during the experiment to come to this conclusion.
References [1] Cargille Viscosity Tubes Data Sheet [2] http://www.coleparmer.com/TechLibraryArticle/933 [3] http://enterprise.astm.org/filtrexx40.cgi?+REDLINE_PAGES/D1545.htm [4] www.quantumflo.com [5] http://www.britannica.com/EBchecked/topic/630428/viscosity
