Laboratory Products
Rotational Viscometry: Improve the Flow of Your Production Process Tamara Kurzmann, Anton Paar GmbH
Practically all industries rely on viscosity checks to develop, formulate, and produce a product with consistent characteristics. Many important parameters for the production control of materials and also for the development of new products are directly related to the product’s viscosity. Rotational viscometers are perfectly suited for the determination of the viscosity of samples ranging from liquid (e.g., nose drops, juices) to semi-solid (e.g., waxes, sauces). This article provides insights into the basics of viscometry, quality control parameters, and application examples.
Principles of viscosity
The physical quantity ‘viscosity’ gives information on how thick a fl uid is and how easily it fl ows. In scientifi c terms, viscosity is the measure of a fl uid’s internal fl ow resistance. If you compare a high-viscosity fl uid such as honey to a low-viscosity fl uid such as eye drops, you will fi nd that at the same temperature, the honey fl ows slower than the eye drops.
The two-plates model provides a mathematical description for viscosity (Figure 1). Think of a kind of sandwich [1]: There are two plates with fl uid placed in-between. The lower plate does not move. The upper plate drifts aside very slowly and subjects the fl uid to a stress, which is parallel to its surface: the shear stress (tau). The force applied to the upper plate divided by this plate’s area defi nes the shear stress. Force/area results in the unit N/m2
. The shear rate (gamma-dot) is the velocity of the upper plate divided by the
distance between the two plates. Its unit is reciprocal second [s-1]. According to Newton’s Law [2], shear stress is viscosity times shear rate. Therefore, the viscosity (eta) is shear stress divided by shear rate:
ƞ= τ/γ ̇
Flow behaviour Viscosity values are not constant values as they are affected by many conditions (Figure 4): • The ambient conditions: temperature and pressure.
• The substance’s inner structure: A highly viscous substance features tightly linked molecules and resists deformation.
• The shear rate or the shear stress as external force: This includes all kinds of actions like wiping a substance, or gravity. The infl uence further depends on the strength and on the duration of the external force.
A viscosity curve is used to determine the fl ow behaviour of a substance. The viscosity is plotted against the shear rate (Figure 3). Such a curve can be generated with a rotational viscometer by increasing the shear rate step-wise with a defi ned measurement point duration. The temperature and other ambient conditions are constant.
Figure 1. The virtual viscous sandwich: the two-plates model.
Viscosity measurement with a rotational viscometer
Most rotational viscometers work according to the Searle principle: A motor drives a spindle inside a fi xed cup (Figure 2). The test sequence is the following: The user attaches a spindle to the rotational viscometer and sets a speed. The spindle starts to rotate and the sample contained in the cup will follow this movement. While the driving speed is preset, the torque (better: the force) required for turning the spindle against the fl uid’s viscous forces is measured. The operator receives the dynamic viscosity and the torque (mostly in %).
For most commonly used spring-type viscometers, the rotation of the spindle causes a defl ection of a spring. Several instrument models with different spring types are available in order to measure low- viscosity to high-viscosity substances. In case of low-viscosity substances, the spring needs to be suffi ciently sensitive, whereas for samples in the high-viscosity range, a more robust spring is required.
Figure 2. Rotational viscometer – Searle principle. Motor and measuring unit (1), stand (2), user interface (3), measuring spindle (rotor) (4), sample-fi lled cup (5).
If a fl uid’s internal fl ow resistance is independent of the external force (shear rate) acting upon the fl uid, it is ideally viscous (Figure 3: curve 1). Such fl uids are named Newtonian liquids after Sir Isaac Newton. Typical materials from this group include water, mineral oil, salad oil, and solvents. Shear-thinning behaviour (or: pseudoplastic) is characterised by decreasing viscosity with increasing shear rates (Figure 3: curve 2). Typical materials are coatings, glues, shampoos, and polymer solutions. Shear-thickening (or: dilatant) means increasing viscosity with increasing shear rates (Figure 3: curve 3). Materials that typically display such behaviour include highly fi lled dispersions, such as ceramic suspensions, starch dispersions, and dental fi lling mass.
Figure 3. Viscosity curve. Different types of fl ow behaviour: Newtonian (1), shear-thinning (2), shear-thickening (3).
Yield point
The yield point (also called yield stress) is the lowest shear-stress value above which a material will behave like a fl uid and below which the material will act like a solid [3]. The yield point is the minimum force that must be applied to those samples so that they start to fl ow. The yield point is of vital importance for many practical issues and applications e.g., for quality control of fi nal products or for optimising the production process.
The yield point is not a material constant but depends on the measuring and analysis method used. There are many different methods available. On rotational viscometers, the yield point is often calculated from fl ow curves measured with a linear increase of the shear rate. The yield point is calculated using model functions (e.g., Bingham, Casson, or Herschel-Bulkley). For all these approximation models, the yield point value τ0 is determined by extrapolation of the fl ow curve towards a low shear rate value (Figure 4). Each different model function produces a different yield point because the calculation is different.
Figure 4. Flow curve. Sample without yield point (1), sample with yield point (2).
INTERNATIONAL LABMATE - FEBRUARY 2023
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