What is Viscosity and Why is it Important to Measure it?
HomeAdviceWhat is Viscosity and Why is it Important to Measure it?
Ever wondered why it takes more effort to stir oil or golden syrup than water or milk? It’s all to do with viscosity — one of the most widely measured properties that’s critical to a wide range of products and industries, including adhesives, food products, lubricants and petroleum products, and paints and coatings.
Let’s learn why some materials flow with greater ease than others, and why viscosity is integral in the world of testing and quality control.
What is viscosity?
In its simplest form, viscosity is referred to as the thickness of a fluid. For example, water (low viscosity) and tomato paste (high viscosity). However, this definition doesn’t hold true when evaluating fluids with different densities.
That’s why viscosity is best defined as the measure of a fluid’s internal resistance to flow under an applied force. As long as there’s relative motion between layers of fluid, viscosity can be defined and measured.
The viscosity of a liquid can be described using three main terms:
Water-thin: This refers to liquids with free flowing characteristics such as water, perfumes, and liquid ink. They are typically easier to pump, mix, and transport.
Semi-Viscous: Slightly thicker in nature than water-thin liquids, examples of semi-viscous products include corn oil and maple syrup.
Viscous: High viscosity liquids such as shampoo and peanut butter are often thicker, more difficult to handle, and require greater exertion to move.
What does viscosity measure?
As we’ve established, viscosity is the measure of a materials’ internal flow resistance, typically a fluid. Numerous factors can affect the viscosity of a liquid, including pressure, temperature, and shear rate — the latter is a key part of the equation used to measure viscosity.
What is viscosity measured in?
Measuring viscosity is based on a fairly simple formula:
viscosity = sheer stress / shear rate
Shear stress is often referred to as tangential stress. It can be defined as the unit area amount of force that acts on a material when subjected to a certain type of movement like turning or torsion.
Shear rate of a fluid relates to the speed of movement of which layers of a fluid can move past each other.
The viscosity result is usually conveyed in centipoise (cP), but other units such as centistokes (cSt), staybolts, and mPa·s (millipascal second) can also be used. For context, centipoise is the equivalent of 1 mPa·s.
A fluid with a low centipoise will flow and move easily and swiftly. A rating of a higher centipoise is going to flow much slower.
A laboratory technician wearing blue gloves pours a thick, yellow liquid from a glass beaker into a test tube in a laboratory setting.
How does viscosity of a liquid change with temperature?
Viscosity can be altered greatly by temperature, which is why when viscosity is quoted its temperature is noted after the viscosity.
Typically if the temperature of a fluid goes up, the viscosity of a fluid drops. When the temperature of the fluid decreases, the viscosity of fluid rises. A good example of this would be honey, which would become less viscous if you heated it up.
Fluids can also alter when being pumped if not handled in the correct way. Some fluids such as water remain at a constant viscosity when agitated (providing environmental conditions remain the same). In other words, no matter how much you stir, shake, or whisk water, its viscosity remains constant. These are known as Newtonian fluids.
Newtonian fluids
Newtonian fluids get their name from Sir Issac Newton. He described the flow behaviours of fluids with a linear relationship between shear stress [mPa] and shear rate [1/s]. This relationship is known as Newton's Law of Viscosity, where the proportionality constant η is the viscosity [mPa-s] of the fluid.
Examples of Newtonian Fluids include Water, honey, alcohol, and oil. Only temperature can affect the viscosity of these liquids.
But what about liquids that don’t obey Newton’s Law of Viscosity? These are known as non-Newtonian fluids.
Non-Newtonian fluids
The viscosity of non-Newtonian fluids is dependent on the shear rate (Shear Thinning or Thickening) or the deformation history (Thixotropic fluids).
Non-Newtonian fluids display either a non-linear relation between shear stress and shear rate, have a yield stress, or viscosity that is dependent on time or deformation history or a combination of all the above.
They alter depending on the force applied — referred to as shear in one of four ways:
Dilatant: Also referred to as shear-thickening fluids, this is where the liquid thickens when agitated (single cream, milk, etc).
Psuedoplastic: Exhibit shear thinning properties, where the viscosity decreases upon an increase in shear stress. For instance, blood or tomato ketchup.
Rheopectic: Fluids that thicken or solidify under constantly applied pressure (ink, body armour, etc).
Thixotropic: Viscosity progressively decreases when a stress is applied. For instance, liquid cements.
Examples of non-Newtonian fluids include cornflour and water, ketchup, adhesives, and creams. Both the speed of the spindle and temperature affect the viscosity.
Dynamic vs kinematic viscosity
Dynamic viscosity
Dynamic viscosity is the measure of a fluid’s resistance to shear flow when an external force is applied. It is beneficial for describing the behaviour of fluids under stress, and especially useful in describing non-Newtonian fluids by observing how viscosity changes as the shear velocity changes.
Kinematic viscosity
Kinematic viscosity is the ratio of dynamic viscosity to density of the liquid. It’s a measure of a fluid’s resistance to shear flow under the weight of gravity (no external forces except gravity are acting).
It is most useful in describing Newtonian fluids. Centistokes is the most common unit of measurement for kinematic viscosity. Centisokes equals 1 millimetre squared per second (mm²/s).
How do you measure viscosity?
There are several methods for measuring viscosity, but the best way to get a precise viscosity measurement is with a test instrument called a viscometer. It measures the viscosity of liquids and assesses their flow properties. There are many viscometer variations, all used to determine different viscosities and classify the rheological properties of a liquid.
Like the capillary viscometers, there are several types of rotational viscometer with many different spindle sizes and shapes suitable for just about any fluid viscosity measurement.
A digital rotational viscometer with a blue and silver design measures the viscosity of a liquid inside a glass beaker in a laboratory setting.
Cone and plate viscometers
The cone and plate viscometer principle adopts the shear method to measure viscosity. It typically uses a rotating cone or disc-shaped rotor to cut the fluid and measure the required torque. From the measured torque and geometric parameters of the rotor, the viscosity of the fluid can be calculated.
Falling rod viscometers
A falling rod viscometer is designed to measure the time it takes a precisely weighted rod to fall through a set distance within a sample of the liquid. A more viscous liquid will create greater resistance to the falling rod, resulting in a longer fall time. The easiest way to understand this principle is the higher the viscosity of the liquid, the slower it will fall.
Why do we measure viscosity?
The rheological properties of a fluid are important for many reasons:
Is the product going to have the same look and feel my customer is expecting? For example, if a shower gel is too thin then when the customer tries to apply it, it will dissipate and they will have to use more product to be able to wash. Conversely, if it's too thick, they won’t use enough and it will also be extremely difficult to get it out of the bottle.
The viscosity of the fluid is critical where it’s pumped through a pipeline or dosing system — if the viscosity is too high, the pump won’t be able to cope.
The amount of product dispensed in a process whether automated or manual is affected by its rheological properties.
In printing, ink viscosity has a huge impact on the print quality.
Viscosity measurement applications
Viscosity must be taken into consideration to fulfil a mass of products and processes across different industries, including:
Cosmetics — The shampoos and conditioners we use must strike a balance between being thick enough to coat the hair effectively while still being straightforward to rinse out.
Construction — If paint is too thin, it runs and blends together. This might be brilliant if you’re a watercolour artist, but ineffective for transforming the colour of your living room walls.
Food industry — Food manufacturers must measure viscosity to ensure the correct appearance, stability, and texture of their products. The next time you have a salad, compare the viscosity of a vinaigrette to a creamier dressing which requires more shaking to get it out the bottle.
Achieve accurate viscosity measurements with AML Instruments
In any application where precision and consistency are paramount, understanding the viscosity of a fluid and how different factors can affect its resistance to flow can ensure your product(s) meet specific performance standards.
Whether you need to control the colour and consistency of your inks, or need a viscometer to master the viscosity of your industrial adhesive formulations to ensure optimal bonding performance, we’ll be happy to help.
AML Instruments supply a wide range of tools to measure viscosity including Melt Flow Indexers (MFIs) and Flow Cups to positively impact your quality control, research and development, and final viscosity result. We also offer on-site and off-site viscometer calibration and repair services to ensure your instrument is giving you the most accurate measurement possible and eliminates the need for costly backups of your viscometers.
If you’re looking for more information on how we can help optimise your viscosity equipment and measurements, reach out to our friendly, knowledgeable team today.
