Viscosity is a measure of a fluid's resistance to flow and it is a useful parameter in the design of efficient product processing and quality control in a wide range of industries. Viscosity describes the internal friction of a moving fluid. A fluid with a high viscosity resists flow, whereas a fluid with a low viscosity flows more easily. A glass tube viscometer is commonly used to determine viscosity, which measures the time it takes for a fluid to flow through a capillary tube. Rotational viscometers such as with a parallel plate geometry, apply shearing forces and measure the torque required to rotate the fluid. This video illustrates the principles of viscosity, demonstrates how to use a viscometer to determine the unknown concentration of a propylene glycol solution, and discusses some industrial applications.
To begin, let's review the principles of viscosity. The dynamic viscosity of a fluid describes the force per unit surface area, known as the shear stress, that is required to move fluid, in this case, parallel to a plane. The fluid in close contact with the bottom plate stays at rest, and the fluid touching the top plate is the same speed as that plate. This velocity gradient is due to internal friction between layers of fluid, and is known as the rate of shear deformation. The dynamic viscosity is defined as the shear stress divided by the rate of shear deformation. The viscosity can be influenced by temperature, fluid density, and composition. Pressure changes, however, have a negligible effect. Once the dynamic viscosity is known, the kinematic viscosity is the ratio of dynamic viscosity to the fluid's density. This is a measure of the fluid's resistance to flow due to gravity. Several instruments measure kinematic viscosity experimentally, such as the Canon-Fenske viscometer. This apparatus consists of a large temperature controlled water bath with fins to maintain constant thermo equilibrium. Different sized capillary tubes are used depending on the approximate fluid's viscosity. Using this apparatus, fluid is poured into the tube and suctioned past the etched markings. Then, the time required for the fluid to travel back through the capillary tube is measured, known as the efflux time. The efflux time is then multiplied by the viscometer calibration constant, which is specific to the capillary glass used to give the kinematic viscosity. Now that you know the fundamentals of viscosity, let's measure the kinematic viscosity of known propylene glycol solutions to determine the concentration of an unknown sample.
Start the experiment by preparing seven solutions for calibration with varying concentrations of propylene glycol and water. Label all solutions with the concentration. Obtain a sample of the unknown concentration, and set it aside. Check all liquid samples for lint, dust, or solid particles before using, and if necessary, filter the sample through a centered glass filter. Next, clean the viscometer using water and dry it with filtered air. Remember to use the appropriate size glass capillary tube for the viscometer, which is dependent on the approximate viscosity of the solution to be tested.
Now, pour a sample into the viscometer, then wipe the arm clean. Next, place the viscometer into the holder and insert it into the temperature bath. Wait 10 to 15 minutes to allow the sample to equilibrate to the bath temperature. Then apply suction to the arm, and draw the sample into the tube. Now take a stopwatch and measure the time required for the sample to flow freely down the capillary tube after leaving the marked end point. This is your efflux time. Repeat the procedure for each sample. Now that all of the viscosities have been measured, let's take a look at the results and determine the concentration of the unknown sample.
To find the concentration of the unknown sample use the data of the known samples and interpolate the viscosity of the unknown solution. First, determine the kinematic viscosities by multiplying the measured time by the viscometer constant. Then convert the kinematic viscosity to dynamic viscosity by multiplying by the mass density. Next, plot the kinematic viscosities as a function of the molarities of the known samples. The graph shows that viscosity increases with increasing propylene glycol concentration. Determine the linear relationship between kinematic viscosity and propylene concentration. Then, interpolate the concentration of the unknown sample using its measured viscosity. In this demonstration, the unknown sample contained 45 mol percent of propylene glycol. Now that you have learned how the viscosity is used to determine concentration, let's examine the many applications of viscosity in industrial settings.
Testing viscosity is important in many industrial fields ranging from petroleum refinery and solar cell production to biomedical applications and research. The precise measurement of viscosity in the petrochemical industry is of high priority because it is used not only to determine crude oil composition, but also to evaluate quality control in petrochemical products. Viscosity varies from crude oil to liquid hydrocarbons, and it must be considered for efficient extraction, handling, and refinery operational conditions. Rheological characterization of highly concentrated suspensions is also mandatory for product development of applications involving complex fluids, such as those used in the metalization of solar cells. The flow properties of conductive pastes have to be carefully adjusted to guarantee undisturbed, defect-free processing and prevent sedimentation and particle aggregation. In medicine, micro rheological techniques are diagnostic tools that investigate red blood cell deformability, and detect blood diseases such as sickle cell anemia, which affect blood viscosity. Protocols utilizing micro fluidics with adjustable shear rates and image processing are utilized to quantitatively determine red blood cell aggregation in microcirculation. These protocols are also used to characterize human tissue, such as brain matter, to better understand structure and engineer bio inspired materials.
You've just watched JOVE's introduction to viscosity. You should now know the fundamental principles of viscosity, be able to operate a viscometer, and understand the importance of viscosity in a variety of applications. Thanks for watching.