Why measure a viscosity flow curve, rather than just give a number?
Very often chemists are asked to provide a single viscosity value for a product or formulation without much information about the required conditions of the measurement. For an oil or low viscosity liquid which is Newtonian this is pretty straightforward, so long as your viscometer is properly calibrated and the measurement temperature is provided. However, most industrially interesting or complex fluids such as polymer solutions, suspensions and emulsions are non-Newtonian, meaning that their viscosity varies with shear rate – shear rate being the rate at which a shearing force is applied – and may be time dependent (thixotropic) also. Therefore, for samples that are shear or time sensitive, a few more questions need to be asked in order to be able to generate the required data.
As could be expected, lowering the viscosity makes a liquid easier to pump and spread, while increasing it reduces dripping – potentially an advantage for products such as paint and ink. High viscosity can also provide the structure needed to suspend particles in medicines, personal care products and drinks. Making sure that the viscosity of a product is closely matched to end-use requirements is a valuable strategy for building product value and meeting consumer expectations.
Most complex fluids are shear thinning i.e. their viscosity decreases with increasing shear rate, although certain systems such as very concentrated suspensions can exhibit shear thickening, an increase in viscosity at high shear rates. This means we can only successfully match the viscosity of a formulation to performance requirements if we measure it under the shear conditions applied during product use.
Defining a measurement range
The key here is to provide relevant results rather than just an arbitrary number… For instance at 20°C, mayonnaise has a viscosity of 500,000 mPas or cP at a shear rate of 0.1 s-1 whereas at 100 s-1 the viscosity falls to around 2,000 mPas, so the viscosity is very dependent upon the shear rate that it experiences. In many processes a material goes through a range of shear rates. For example when pumping, the material is first drawn into the pump at moderately low shear, it then undergoes high shear as it passes the pump’s blade, and then lower shear again when exiting. Therefore, to simulate the whole pumping process, one would need to measure across a fairly wide shear rate range (i.e. 1 to 1,000 s-1).
The term “Shear Rate” describes the shearing flow rate that a sample experiences per unit volume, whereas the term “Shear Stress” describes the shearing force that a sample experiences per unit volume.
It is important to note that the shear rate is not only dependent on the fluid velocity but on the dimensions of the fluid being sheared so in the case of pipe flow both the flow rate and pipe diameter are important and this can be calculated . Also it is worth thinking about the origin of the viscosity request; is it just as a quality control parameter being requested, or is the information required to solve a process problem for instance?? If we’re considering a problem initiating flow in a pipeline, for example, the low shear rate viscosity or yield stress may be the most relevant value to quote, whereas if we’re considering steady flow rates in narrow pipes then a higher shear would be better to use.
|Process||Minimum Shear Rate (1/s)||Maximum Shear Rate (1/s)|
As you can see from the above table, different processes have a range of different shear rates associated with them rather than just a single shear rate, and a single product may be exposed to many of these different processes during its life cycle.
Although a single value of viscosity may be sufficient in some cases (made at the correct shear rate), the majority of products require more viscosity information as they experience a range of shear rates, creating the need to generate an equilibrium flow curve. Similarly, the temperature of the process needs be considered when making a viscosity measurement a this a critical factor. As a rule of thumb water based systems reduce by around 2%/°C, whereas oil based systems reduce in viscosity by around 10%/°C increase in temperature, so in the latter case good temperature control can be critical.
Simulating a shear process on a rotational rheometer
By shearing a material in a geometric configuration of known dimensions (e.g. cone and plate, parallel plates or a cup and bob) system on a rotational rheometer, it is possible to directly simulate the shear rates and stresses of more complex flow regimes such as those experienced in a mixer or a coating process. This gives us the ability to directly simulate processes related to production, storage and end-use conditions in a very controlled environment using small sample quantities, and allows us to compare different products and formulations easily and quickly.
Furthermore, if we initially define an acceptable viscosity range by measuring a flow curve for a product that is known to perform well, e.g. has good stability, pumps acceptably etc. then this can provide a target viscosity value or range for formulators or process engineers to aim for. The viscosity of the product is useful to know in many parts of formulation, production and end use as illustrated below.
By using a rheometer with a wide speed and torque range such as Kinexus it is possible not only to measure viscosities over a wide range of shear rates but to potentially achieve this in a single measurement, providing information on storage, processing and end-use simultaneously!
In my next blog we will address how to best measure a flow curve, including which measuring geometry to use and how to spot and prevent measurement artifacts.
- Malvern Instruments Application Note – Processing non-Newtonian products: Determining the pressure drop for a power law fluid along a straight circular pipe