You need to measure the viscosity of a material. Which technique should you use? The answer depends on your material and what you need to learn about its flow behavior.
This article explains how viscosity is measured, introduces the main testing techniques and relevant standards, and helps you determine when a single viscosity measurement is sufficient and when broader rheological testing may be necessary.
What Is Viscosity Testing?
Viscosity testing measures a material’s resistance to flow under specified conditions. Viscosity depends on temperature, and, for many materials, also on shear rate.
Viscosity differs from elasticity, which describes reversible deformation. Many complex materials, including polymers, gels and pastes, exhibit both viscous and elastic behavior. This combination is known as viscoelasticity. Rheology is the broader study of how materials flow and deform in response to applied forces.
Why Viscosity Testing Matters in Research and Manufacturing
Viscosity testing helps researchers develop and refine new material formulations, while helping manufacturers evaluate their suitability for production and optimize processing methods. For example, electrode slurries must flow predictably during mixing, pumping and coating in battery production; adhesives require precise dispensing in semiconductor packaging.
An adhesive that is too viscous to dispense accurately, or a coating that runs after application, can lead to manufacturing defects, material waste and costly rework. By testing materials under conditions representative of manufacturing processes, storage and use, viscosity testing helps identify potential processing problems and maintain product consistency.
What influences viscosity at the molecular level?
Why are some liquids more viscous, or ‘thicker’ than others? At the molecular level, viscosity depends on how easily molecules can move past one another. Molecular size, shape and interactions all play a role.
Consider two silicone oils with the same polymer chemistry but different molecular weights. The lower-viscosity oil contains shorter polymer chains that can move past one another more easily. The higher-viscosity oil contains longer chains that increase resistance to flow, particularly when chain entanglement becomes significant, as illustrated in Figure 1.
How Is Viscosity Measured?
Depending on the technique, viscosity measurements may be based on:
- The time a fluid takes to flow through a narrow tube or out through the calibrated opening of a flow cup.
- The torque required to rotate a spindle in a sample placed in a container.
- The relationship between shear stress and shear rate when a material is subjected to shear in a rheometer using parallel-plate, cone-and-plate or concentric-cylinder geometries.
Two main types of instruments are used: viscometers and rheometers.
Viscometers measure viscosity under specified temperature and flow or shear conditions. These measurements are useful for comparing fluids and monitoring product consistency. Although some viscometers can operate at different speeds and temperatures, rheometers offer greater control over applied stress or deformation. They can measure how viscosity changes with shear rate, temperature and time, as well as characterize the viscoelastic behavior of complex materials such as gels and pastes.
In practice, viscosity testing involves preparing the sample, selecting an appropriate instrument and measuring viscosity at a specified temperature using a defined testing procedure. The measurement conditions, including instrument settings where relevant, should be reported alongside the results to allow meaningful comparisons.
Covalent uses an Anton Paar MCR 302 rotational rheometer with temperature control to measure viscosity and characterize the rheological properties of liquids, gels, pastes and soft solids.
What are Shear Stress, Shear Rate and Shear Strain?
Understanding shear is essential to understanding how viscosity is measured. Shear involves adjacent layers within a material moving or deforming sideways relative to one another.
The following terms describe the forces involved, and how a material flows or deforms under shear:
- Shear stress (Pa): The force per unit area acting parallel to a material’s surface, tending to cause adjacent layers to move or deform relative to one another.
- Shear flow: The continuous movement of adjacent fluid layers parallel to one another at different speeds.
- Shear rate (s⁻¹): The rate at which shear strain changes over time, calculated for simple shear between parallel plates as the difference in velocity between two layers divided by their separation.
- Shear strain (dimensionless, often expressed as %): The amount of shear deformation, calculated for simple shear as the relative sideways displacement between two layers divided by their separation.
Viscosity (Pa·s) describes a material’s resistance to shear flow. Under steady shear conditions, it is calculated by dividing shear stress by shear rate. For non-Newtonian materials, this gives the apparent viscosity at the specified shear rate.
Viscosity in Newtonian and Non-Newtonian Fluids
The distinction between Newtonian and non-Newtonian behavior is important when interpreting viscosity measurements.
- Newtonian fluids have a viscosity that remains constant regardless of shear rate, at a given temperature and pressure. Shear stress increases in direct proportion to shear rate.
- Non-Newtonian fluids have more complex flow behavior. Their apparent viscosity may depend on shear rate, time or deformation history. Shear-thinning fluids become less viscous as shear rate increases, whereas shear-thickening fluids become more viscous.
Types of Viscosity and Measurement Units
Viscosity can be expressed in different ways depending on the material’s properties and the measurement technique. Table 1 summarizes four commonly used viscosity terms and their units.
| Term | Definition | Common units |
|---|---|---|
| Dynamic viscosity (absolute viscosity) | Shear stress divided by shear rate; for Newtonian fluids, this ratio is independent of shear rate. | Pa·s, mPa·s, cP |
| Kinematic viscosity | The ratio of a material’s dynamic viscosity to its density. | m²/s, mm²/s, cSt |
| Apparent viscosity | Shear stress divided by shear rate under specified conditions, particularly relevant for non-Newtonian materials. | Pa·s, mPa·s, cP |
| Complex viscosity | A frequency-dependent measure derived from oscillatory rheometry that accounts for both viscous and elastic responses. | Pa·s, mPa·s |
Water has a dynamic viscosity of approximately 1 centipoise (cP) at 20°C, providing a useful reference point for comparing other liquids.
Common unit conversions include:
- Dynamic viscosity: 1 cP = 1 mPa·s = 0.001 Pa·s.
- Kinematic viscosity: 1 cSt = 1 mm²/s = 10⁻⁶ m²/s.
Converting between dynamic and kinematic viscosity requires the material’s density at the measurement temperature.
Although SI units such as Pa·s and mPa·s are widely used in scientific and technical reporting, centipoise (cP) remains common in industrial specifications. Conventions also vary by industry: lubricants are frequently characterized using kinematic viscosity in centistokes (cSt), whereas coatings, inks and adhesives commonly use cP or mPa·s.
Common Viscosity Testing Techniques
Some techniques measure viscosity under specified conditions, while others provide broader information about how a material flows and deforms.
Rotational viscometry and rheometry
- Principle: A rotating spindle or plate applies shear to a sample, and the resistance to rotation is measured.
- Suitable for: Liquids, pastes, gels and many non-Newtonian materials.
- Key output: Dynamic or apparent viscosity, including changes with shear rate and temperature when using a rotational rheometer.
Rotational rheometers can also perform shear recovery tests to assess how quickly and to what extent a material’s viscosity or structure recovers after exposure to high shear.
Oscillatory rheometry and viscoelastic characterization
- Principle: Applies repeated, controlled oscillatory deformation to measure a material’s viscous and elastic responses.
- Suitable for: Polymers, gels, soft solids and other viscoelastic materials.
- Key output: Storage modulus (G′) and loss modulus (G″), both expressed in pascals (Pa), characterizing elastic and viscous behavior, respectively.
Figure 4 compares steady-shear and oscillatory rheometry, two testing modes supported by Covalent’s Anton Paar MCR 302 rheometer.
Capillary viscometry
- Principle: Measures the flow of a material through a calibrated narrow tube under gravity or applied pressure.
- Suitable for: Primarily Newtonian liquids, such as oils and solvents, using gravity-fed instruments; specialized pressure-driven systems can characterize polymer melts.
- Key output: Kinematic viscosity for conventional gravity-fed measurements; shear viscosity at different shear rates for pressure-driven capillary rheometry.
Other viscosity measurement methods
Falling-ball viscometry
- Principle: Measures the terminal speed of a ball falling through a liquid under gravity.
- Suitable for: Homogeneous Newtonian liquids, rather than gels or structured materials.
- Key output: Dynamic viscosity calculated from the ball’s motion.
Flow-cup testing
- Principle: Measures the time required for a liquid to drain through a calibrated opening.
- Suitable for: Free-flowing Newtonian or near-Newtonian liquids, including suitable paints, coatings and inks.
- Key output: Efflux time, which can be related to kinematic viscosity when an appropriate calibration is available.
How to Choose the Right Viscosity Testing Method
Choosing the right viscosity testing method requires considering the material’s properties, including whether it exhibits Newtonian or non-Newtonian behavior, the conditions it will experience, and the information needed to evaluate its performance.
When is a single viscosity measurement enough?
For Newtonian fluids, a single viscosity measurement at a specified temperature may be sufficient because viscosity does not change with shear rate.
Single-point measurements can also be useful for routine quality control of non-Newtonian materials, provided testing conditions are consistent, and the measured value is known to correlate with product performance.
When is a single viscosity measurement not enough?
A single viscosity value may be insufficient when a material’s flow behavior changes with shear rate or time, or when its elastic properties influence performance.
For example, two adhesive formulations may have identical viscosities under specified testing conditions but behave differently during application, as illustrated in Figure 5.
Practical technique selection
Table 2 connects common testing questions with appropriate viscosity measurement techniques and identifies where additional rheological testing may be needed.
| What do you need to know? | Potential technique or test | What it reveals |
|---|---|---|
| Is the material’s viscosity consistent between batches? | Rotational viscometry | Provides viscosity measurements under specified conditions for comparison between batches. |
| Does a free-flowing liquid meet its kinematic viscosity specification? | Capillary viscometry | Measures kinematic viscosity at a specified temperature. |
| How does viscosity change with shear rate? | Rotational rheometry (flow curve) | Reveals whether viscosity is independent of shear rate or exhibits shear-thinning or shear-thickening behavior. |
| How does viscosity change with temperature? | Rotational viscometry or rheometry at different temperatures | Shows how temperature affects resistance to flow. |
| Does the material require a minimum stress before it begins to flow? | Additional rheological testing: yield stress measurement (rotational rheometry) | Estimates the yield stress under the specified test conditions. |
| Does the material recover after shearing? | Additional rheological testing: shear recovery test | Characterizes the extent and rate at which viscosity or structure recovers following shearing. |
| How does the material respond to oscillatory deformation at different frequencies? | Oscillatory rheometry (frequency sweep) | Characterizes frequency-dependent viscous and elastic responses. |
| How do material properties change during curing? | Oscillatory rheometry (curing study), with an appropriate curing setup | Tracks changes in viscoelastic properties as curing progresses. |
Silicone oil example
Silicone oils are polymeric fluids used in damping systems, including those used in optical equipment. Where their response to repeated mechanical deformation matters, oscillatory rheometry can provide additional information beyond conventional viscosity measurements.
The frequency sweep below illustrates how the viscous (G″) and elastic (G′) responses of a silicone oil change with frequency. Their crossover indicates a change from predominantly viscous to predominantly elastic behavior over the frequencies tested.
For more details about available techniques and testing capabilities, explore Covalent’s rheology testing services.
What Standards Apply to Viscosity Testing?
Viscosity testing standards establish procedures for measuring viscosity and related flow properties under defined conditions. Developed by organizations such as ASTM International and ISO, they specify requirements for testing equipment, sample preparation, temperature and measurement conditions to help ensure consistent, comparable results.
The appropriate standard depends on the material being tested, the measurement technique and the property of interest.
| Standard | Measurement | Typical application |
|---|---|---|
| ASTM D445 | Kinematic viscosity using capillary viscometry | Petroleum products, lubricating oils and other suitable liquids |
| ASTM D2196 | Apparent viscosity, shear thinning and thixotropy using rotational viscometry | Non-Newtonian materials, including paints and coatings |
| ISO 2555 | Apparent viscosity using a single-cylinder rotational viscometer | Liquid resins, emulsions and dispersions |
| ASTM D4440 | Complex viscosity and viscoelastic properties using oscillatory rheometry | Molten polymers, including thermoplastics |
Standardized methods are particularly useful for manufacturing quality control, product qualification and comparing results between laboratories.
For research and development, customized testing procedures may be more appropriate for investigating material behavior under specific conditions. These procedures can provide controlled, repeatable measurements without necessarily following a published ASTM or ISO standard.
Applications of Viscosity Testing
Viscosity and rheological testing help researchers and manufacturers evaluate material properties that influence processing and performance across a range of applications.
Battery and Energy Storage
During battery electrode manufacturing, slurry viscosity influences pumping and coating uniformity. Flow curves and shear recovery tests characterize slurry behavior under coating-relevant shear conditions and following shearing, helping manufacturers achieve more uniform electrode thickness and material distribution.
Semiconductors
In semiconductor packaging, die-attach adhesives must flow through dispensing equipment while retaining their shape after application. Flow curves and shear recovery tests, together with yield stress measurements where relevant, help assess these properties and guide adjustments to minimize unwanted spreading that could affect component placement and assembly quality.
Photonics
For UV-curable optical coatings, oscillatory rheometry combined with an appropriate UV-exposure system can track changes in viscoelastic properties during curing and help identify gelation. This information helps manufacturers establish appropriate curing conditions and control the transition from liquid to solid.
Conclusion
Selecting the right viscosity testing method is essential for obtaining meaningful information about a material’s properties and performance. Where flow behavior depends on shear rate, time or other conditions, broader rheological testing can provide the information needed to guide processing and performance decisions.
Covalent’s scientists can help identify the most appropriate viscosity or rheological testing method for your materials. Explore Covalent’s rheology testing services to learn more or request a quote for your specific requirements.
