At the Centre for Industrial Rheology, one of our most popular services is providing rheological “snapshots” for our customers’ materials. With the advent of easy-to-use, entry-level rheometers, the benefits of comprehensive rheological profiling are becoming more accessible. If you’re currently using a viscometer to characterise your materials, you may be wondering if you should be using a rheometer.
Both viscometers and rheometers are used to characterise materials; however, they differ significantly in the depth of information they provide. To demonstrate these differences practically, we selected three everyday paste-like samples to test.
- Smooth Peanut Butter
- American Mustard
- Chocolate Spread
This study aimed to explore whether a rheometer truly offers greater material insights over a viscometer when analysing complex, real-world materials.
Viscometer Testing Results and Practical Limitations
Viscometers are commonly used to measure viscosity. They work by rotating a spindle or probe and measuring the resistance offered by the material through measurement of torque. This is then converted into a viscosity value. Viscometers are inherently limited in terms of the information they can provide. Whilst measurements can be made at multiple speeds, most users only take a single viscosity reading at a defined rpm.
While viscometers appear simple and are low-cost instruments, this simplicity belies the complexity encountered when measuring non-Newtonian and structured products.
In our study, the use of a viscometer encountered practical challenges due to the structured nature of the samples. When the torque limit of the instrument was exceeded, error values would appear. This required adaptation of the RPM and spindle used, particularly when changing between samples.
Spindle attachments proved unsuitable as they left a hole within the sample, requiring T-bar accessories to be employed to carve a helical path through the sample. This process can be time-consuming and is heavily dependent on the operator’s experience with the instrument and the material being tested.
As viscometers are known for their erratic results, multiple measurements had to be taken and averaged. Following these practical challenges, averaged viscosity values were obtained as shown in Figure 5.
This single viscosity value can be valuable for quality control purposes. For example, if you are manufacturing a paste-like product, it’s important to know that its viscosity consistently falls within a predefined specification range. However, for a deeper understanding of a material’s behaviour, such as how it flows during mixing or pumping, or how it behaves once it is spread, viscometry alone is not enough.
Rheometer Testing Results and Deeper Material Insights
Where a viscometer gives a single number, a rheometer builds a complete picture. It doesn’t just measure a single viscosity point; it shows how a material behaves under differing conditions. Unlike the viscometer, which required trial and error to work, the rheometer was simpler to use. For our study, each sample was loaded on the same geometry, and the instrument ran without error messages, torque issues, or configuration changes.
As seen in Figure 6, rheometers require a significantly smaller sample volume, from as little as 1mL. This small sample size is highly conducive to excellent temperature control, allowing the sample to reach a uniform temperature in seconds. In contrast, the larger sample volumes required by viscometers pose significant challenges for temperature control. Due to the lack of convection, it is difficult to eliminate hot spots and achieve a uniform temperature throughout the entire sample before testing.
The difference in the operational torque range is a key differentiator. While a Brookfield DV2RV viscometer is limited to a narrow window between 0.07187 mN⋅m and 0.7187 mN⋅m, an entry-level rheometer such as the TA Instruments Discovery Core Rheometer offers a much wider torque range. The minimum torque offered is 0.0005 mN⋅m, over 140 times more sensitive than the viscometer’s lower limit, allowing the characterisation of delicate structures. The maximum torque of 125 mN⋅m is a 170-fold increase, allowing the rheometer to achieve significantly higher deformation rates.
We ran a series of tests to demonstrate the rheometer’s capabilities:
Shear Rate Sweep – Viscosity Across a Range of Shear Rates
Viscosity can change based on the shear rate applied This is the result of non-Newtonian behaviour, with many materials exhibiting such behaviour. A shear rate sweep, or flow curve, characterises how a material behaves across a range of shear conditions. This includes low shear rates, which reflect the material at rest, to higher shear rates that simulate shear conditions experienced during spreading or pumping. The data we obtained clearly highlighted this range.
Compared to the single values from the viscometer, these curves reveal the “full story” of how a product flows under differing shear conditions.
Thixotropy Testing
Thixotropy refers to time-dependent viscosity changes following either the application or cessation of shearing. Paste-like samples often display thixotropic behaviour as their viscosity drops during spreading, mixing, or pumping, and then slowly recovers at rest. Measuring this thixotropic response provides insights into how these samples respond to breakdown and how long they take to recover.
Oscillatory Rheological Tests
Many materials, such as the pastes in this study along with suspensions, emulsions, and gels, are better described as soft solids rather than liquids when they are at rest or under low stress. This is due to these products having a delicate internal structure that provides rigidity. Viscosity is a liquid property, and viscometers must actively break down this structure to obtain a measurement. The rigidity and strength of this structure, however, are critical properties that deliver important processing, handling and sensory attributes.
Oscillatory rheological methods are used to probe this delicate structure. These methods involve applying small oscillatory deformations to a sample. This can be thought of as gently wobbling the sample to observe how its structure responds to increasing oscillatory stress.
A key variable derived from this method is the complex modulus, which describes a sample’s overall resistance to deformation. This is essentially the rigidity of the sample. Figure 9 shows the typical output of an oscillatory rheological test.
Smooth peanut butter exhibited the highest complex modulus across the stress range tested, indicating a higher resistance to deformation. All samples exhibit a decrease in modulus at higher stresses, approaching a yield point. This is the stress required to make a material flow.
Summary
Our results make the distinction clear. A viscometer can be an excellent tool for quality control, where the goal is to verify if the same product falls within a set specification. For routine checks to ensure batch-to-batch consistency, a single viscosity value can be both quick and effective.
However, when it comes to formulation development or gaining a deeper understanding of how a product behaves, a rheometer is the tool of choice. From shear-dependent behaviour to structural insights, a rheometer provides a more in-depth picture that is simply not accessible from a viscometer alone.
For those only familiar with viscometry, rheology can initially seem complex and potentially daunting, but it doesn’t have to be. We make rheology simple, accessible and practical.
Whether you want to submit samples for advanced testing, develop methods tailored to your formulations, or explore training and support, we can help you unlock the depth of insights a rheometer can offer.