Viscometer vs Rheometer: Discover Why a Rheometer Offers More Than Just Viscosity Measurement

Philippa Cranwell | Sarah Cotts
September 30, 2024

If you’re working with materials that can be flowed, for example within pharmaceutical, food, cosmetics, or petrochemical industries, it’s likely that there is a viscometer in the laboratory that is the go-to instrument for formulation testing or QA purposes. However, does this viscometer give you the whole picture and fully meet all your needs? Like an iceberg only shows 10% of its volume above the waterline, a viscometer only gives a small snapshot of a material’s properties. Sometimes this is enough, but there are instances where a rheometer will provide significant advantage, saving time, money and effort in the long run.

This short blog will outline key advantages that can be gained through use of a rheometer over a viscometer, with the differences between each instrument outlined and examples showcasing where a rheometer provided crucial data that a viscometer could not.

Viscometer vs rheometer: what’s the difference?

Before considering the operational differences between a viscometer and a rheometer, it is crucial to understand what each measures. A viscometer characterizes the viscosity of a fluid at a single shear rate, and, as such, reports one viscosity value. This is a key drawback: often the shear rate used to report a particular viscosity is not reported and when working with non-Newtonian materials (such as creams or emulsions), where the viscosity is dependent upon shear rate, the values provided may not be accurate or reproducible.

A rheometer, by contrast, measures the way that a liquid flows in response to an applied stress and can reliably achieve a wide range of shear rates, usually between 0.001 and 1000 s^–1. In fact, the low shear rates accessible allow us to understand how materials behave when they are at rest, a property that can significantly impact upon the stability of heterogenous materials, the perception of consumer products (for example in food and personal care), and, most importantly, provide insights into a material’s microstructure, which impacts upon both performance and behavior.

The rheometer can show how viscosity changes across a range of different situations, rather than at just one point.

Using the analogy above, a rheometer can give information about the whole iceberg, whereas a viscometer can only give information about the 10% above water. While sometimes this is enough, the additional data from thorough rheological evaluation can reveal important differences between samples and enable modeling of a whole host of process that a product might undergo, either during manufacture or through use by the end-user, shown in figure 1.

Figure 1: Schematic showing the range of shear rates over which process occur, which can all be modelled using a rheometer. The narrow range of shear rates accessible to a viscometer is indicated.

Rheometer: precision engineering for precision measurement

Where a viscometer can provide simple measurements on Newtonian materials (viscosity is independent of shear rate), a rheometer can give in-depth characterization of flow and deformation for both Newtonian and non-Newtonian materials. The key reason for the marked difference in performance between viscometers and rheometers is due to the instrument mechanic. Operationally, a viscometer uses a mechanical bearing, which limits the speed and torque capabilities of the instrument so material is only measurable within a very narrow range. In contrast, a rheometer uses an advanced motor and a low friction bearing, offering unparalleled control of deformation and torque sensitivity. For example, the Discovery Hybrid Rheometer uses a unique magnetically levitated bearing, further reducing friction and enhancing sensitivity to low torque measurements. This enables accurate flow behavior measurements even under very low-shear conditions, such as a paint leveling after application or sedimentation in a suspension.

A rheometer measures more than just viscosity!

Although rheometers measure viscosity (how a material flows), they can also measure viscoelasticity, which is a critical attribute of complex fluids like emulsions, dispersions, polymers, gels, pastes, that is often overlooked. Viscoelasticity data can provide information about how much a material can deform, flex or spring before the structure is destroyed and irrecoverable damage is caused.

With what types of material would a rheometer give me an advantage?

When measuring a liquid, a key consideration is whether it is Newtonian or non-Newtonian. Newtonian liquids have constant viscosity regardless of the shear rate applied, e.g. water and honey. Measuring viscosity at several shear rates allows one to extrapolate the viscosity at high or low shear rates and in these cases, a viscometer would likely deliver the required results.

A non-Newtonian fluid changes viscosity depending upon the shear rate applied: it is not possible to accurately describe the viscosity of a non-Newtonian fluid without specifying the shear rate. Most manufactured materials are non-Newtonian, including cosmetic creams, pharmaceutical formulations, coatings, lubricants and food stuffs. In fact, their non-Newtonian behavior is a desirable property. For example, lotions and creams decrease in viscosity as they are moved faster (shear-thinning behavior), for example while being dispensed or applied. This means that they flow well and can be processed. However, their high viscosity at low flow rates helps with their stability. Shear-thinning behavior is a good thing, as it means that when at rest the materials stay in place, but when we want or need them to move, they do so readily.

Showcasing a rheometer’s value: Cosmetic creams

Cosmetic products such as face cream, sunscreen, mascara or shampoo, are stored on a shelf in a pot and are only used once or twice per day. In rheological terms, these materials spend the majority of time ‘at rest’ as no external force, or shear, has been applied, Figure 2. As shear is applied, either through agitation during transportation, processing in the factory, or application by the consumer, the sample starts to move and the viscosity changes. Different viscosity values due to different shear rates can have a large impact upon a product:

• If zero-shear viscosity is too low, sedimentation or separation of components may occur;
• Low zero-shear viscosity may lead to consumer perception that the product is ‘thin’ or ‘less luxurious’ when compared with a competitor;
• Higher or lower viscosity while spreading on the skin (where shear is applied by the consumer) may lead to either perception of a thin, poor-quality product, or a thick paste that feels draggy.