How to measure viscoelasticity

Viscoelasticity refers to a material's ability to exhibit both elastic (recoverable and solid-like) and viscous (fluid-like flow) behaviours when subjected to stress or deformation. In simple terms, viscoelastic materials combine characteristics of both liquids and solids: they can deform like a fluid under long-term stress but also recover their shape like a solid when the stress is removed.

Purely elastic materials deform instantly under stress but return to their original form when the force is removed, like a spring. On the other hand, viscous materials deform over time and do not recover, like oil. Viscoelastic materials, however, show a combination of these behaviours: they deform elastically initially and continue to flow or stretch over time, with only the elastic part being reversible once the force is removed. The speed of deformation also influences the balance between elastic and viscous behaviours, with faster deformations shifting the material toward elasticity.

This dual behaviour makes viscoelasticity crucial in fields like food science, biotechnology, biomechanics, material science, and polymers where materials must balance resilience and flexibility to function effectively under different stresses.

Viscoelasticity is typically measured using a Texture Analyser to apply stress or strain to the material and record its response over time. Common tests used in texture analysis/materials testing include:

Stress relaxation test: This test measures how the material relaxes under constant strain. A probe deforms the material to a fixed distance/strain and holds it, while the force required to maintain the deformation decreases over time, indicating viscoelastic relaxation.

Creep test: In a creep test, a constant force/stress is applied to the material, and its deformation over time is recorded. This test is useful for understanding how materials deform under long-term loads.

Viscoelasticity measurement using a Texture Analyser allows manufacturers to assess how materials respond to stress or deformation over time. Below are key applications:

• Food product viscoelastic analysis: Assessing the viscoelastic properties of food products like dough, gels, and sauces by measuring how these materials deform and flow when stretched, indicating their elasticity. Viscoelasticity plays an important role in the texture and mouthfeel of food products. For example, bread dough must have the right balance of elasticity and viscosity for proper rising and baking.
• Biomaterial viscoelasticity measurement: Testing the viscoelastic behaviour of biomaterials such as hydrogels and cartilage by subjecting them to cyclic compression to measure their relaxation time and recovery, which is essential for tissue engineering.
• Cosmetic product viscoelastic testing: Evaluating the viscoelastic properties of cosmetic gels and lotions by measuring how these products perform when stretched or spread to assess product performance, ensuring a good consumer experience.
• Pharmaceutical viscoelasticity measurement: Testing the viscoelastic properties of pharmaceutical gels and suspensions by measuring their response to back extrusion to ensure consistency and efficacy in drug delivery systems.
• Foam material viscoelastic analysis: Evaluating foam materials used in cushions and insulation by measuring their deformation and recovery under compressive stress, which provides insights into their damping behaviour.
• Polymer assessment: Rubber, silicone, and plastics are classic examples of viscoelastic materials. These materials can stretch and return to their original shape, but also flow slowly under sustained stress. Polymers are used in a wide range of applications, from tires to medical devices.
• Medical devices and Biomechanics assessment: Understanding viscoelasticity is critical for designing implants, prosthetics, and medical devices that mimic the natural movement and deformation of biological tissues.

As a viscoelastic material shows time-dependent behaviour (it will keep stretching for as long as a force is applied), a measurement of viscoelasticity must include a holding period. In a typical viscoelasticity test, a creep test which measures deformation over time under a constant load or relaxation test which measures the decrease in force over time while maintaining constant deformation, can be used.  These tests can be performed in compression or tension.

Stress and strain are often used in the place of force and distance when the samples can easily be given a constant and measurable cross-section. However, for the majority of quality control tests in industry, it is more convenient to use force and distance (for samples with a uniform height) or strain (for samples with varying height). Strain is simply the deformation distance divided by the starting height.

A Compression test involves a sample being placed between two flat plates (usually the instrument base and a flat probe). The probe must be larger than the sample to avoid penetration or cutting. Ideally, samples for this test type will have flat, parallel top and bottom surfaces. However, if testing a finished product, that might not be possible – some samples simply may not be able to be modified to form a piece with convenient geometry.

Friction between the sample and the plates can also cause errors in the viscoelasticity measurement, although this is often a small effect. To help avoid friction, lubricating the sample or bonding it to the probe and instrument base can be useful. Additionally, very tall samples should be avoided when testing in compression as they can buckle, but samples that are too short can result in inaccuracies due to the effect of testing too close to the base.

When measuring viscoelasticity, the force vs distance/time or distance vs time graph typically shows the following types of response based on whether a creep test or relaxation test is performed.

Properties that can be measured

Initial deformation: The immediate deformation upon applying the load, representing elastic behaviour.

Creep phase/region: After the initial deformation, the material continues to deform slowly over time while the force remains constant. This slow, ongoing deformation is the viscoelastic creep behaviour.

Steady state: When deformation stabilises, showing the material's long-term response.

A full explanation of these curves and their analysis can be accessed within Exponent Connect software. Existing Exponent users can upgrade to Exponent Connect specification

Below is a video example of how we can help you understand curve analysis for an example property. 

• Temperature: Temperature significantly influences viscoelasticity measurements, as it affects the molecular mobility and relaxation times of materials. As temperature increases, materials tend to behave more like viscous liquids because their molecular motion increases. Conversely, at lower temperatures, materials behave more like elastic solids. For example, rubber behaves more like a stiff solid at low temperatures, but becomes much more elastic and viscoelastic as it heats up.
• Time: The duration of the test is crucial in viscoelasticity measurements, as time-dependent behaviours like creep and stress relaxation are key features of viscoelastic materials.
• Strain rate: The rate of strain or deformation also affects viscoelastic behaviour. At high strain rates, viscoelastic materials tend to behave more like elastic solids, as they do not have enough time to flow. At low strain rates, they behave more like viscous fluids.
• Sample geometry: The shape and dimensions of the test sample can impact viscoelasticity measurements, affecting how stress and strain are distributed throughout the material.
• Test geometry: The choice of test geometry (e.g., shear, tension, bending) influences the type of viscoelastic properties that can be measured and how the results are interpreted.

The Stable Micro Systems Texture Analyser excels in optimising viscoelasticity measurements through its comprehensive approach and specialised features. It accurately quantifies viscoelastic properties using specialised tests like creep and relaxation tests with PID control, providing a holistic view of a material's time-dependent behaviour.

The Texture Analyser captures detailed force-time-distance profiles, enabling in-depth analysis of both viscous and elastic components across a wide range of products in food, cosmetics, pharmaceutical, and materials industries. Its temperature control capabilities are crucial for studying viscoelasticity, as temperature significantly affects the viscous component of materials.

Backed by Stable Micro Systems' expertise in graphical interpretation and method development, makes the Texture Analyser the ideal choice for researchers and manufacturers seeking to refine product formulations and maintain quality standards in viscoelasticity testing across various industries.