Emulsion is a part of a general class of two-phase systems of matter called colloids. This system has a dispersed phase of liquid droplets and a liquid continuous phase.
There are two common types of emulsions - water-in-oil emulsion and oil-in-water emulsion (Figures 1 and 2). In the former case, the dispersed phase is water and the continuous phase is oil, whereas in the latter case, the dispersed phase is oil and the continuous phase is water.
Figure 1. Water in oil emulsion (figure 1A) and oil in water emulsion (figure 1B). Emulsifier surfactant with hydrophilic head and hydrophobic tail.
The conversion of a water-in-oil emulsion into an oil-in-water emulsion is based on the volume fraction of the emulsifier as well as the phases.
An emulsifier is a type of material which stabilizes an emulsion by adsorbing at the water or oil interface. The most common forms of emulsifier are surfactants, but particulate and polymeric materials can also act as emulsifiers.
Emulsion rheology strongly relies on the droplet size and the volume fraction of the dispersed phase. Normal stress, yield stress, viscosity and viscoelasticity are the rheological parameters of major interest. The relative viscosity of a dilute emulsion with a low capillary number is expressed as follows (Taylor): (1)
and η s is the viscosity of the suspending fluid and ηd is the dispersed phase viscosity.
In this case, it is considered that the emulsion is not shear thinning and therefore the viscosity will remain the same at each shear rate.
In case of higher droplet concentrations (φ>0.6), the system turns into shear thinning and the relative zero shear viscosity is expressed as follows: (1, 2)
φm is the maximum packing fraction.
As volume fraction of the droplets raises, the shear thinning becomes more prominent. In reference 2, this is taken into account by altering φ m to obtain an optimal fit at each shear rate.
When there is an additional increase in volume fraction, it would lead to a situation where droplets get stuck and prevent particles that are moving relative to one another. In such a scenario, the system is assumed to exhibit a yield stress.
It should also be noted that this related theory considers a simple and uncomplicated emulsion and does not account for the presence of cross-linked micro-gels and other rheology modifiers which tend to exhibit significant phase volume and will affect the solvent and thus the emulsion rheology.
In order to substantiate this theory for a specified emulsion system, the zero shear viscosity of an emulsion at different droplet concentrations needs to be determined. Subsequently, the zero shear relative viscosity for individual concentration should be measured using the viscosity of the suspending media.
At length, the plotted relationship between the concentration and the zero shear relative viscosity should specify whether or not the above mentioned theory approximates the behavior of the emulsion system under analysis. The resulting data can be further examined to explore the exact fit with the above models. The effect of changing the droplet size on the viscosity can also be analyzed using the same sequence.
- The experimental test exists as a pre-configured sequence in the rSpace software. This software has been specifically designed to operate on a Kinexus rotational rheometer.
- After running a table of shear stresses, the sequence fits an Ellis Model to the data to ascertain η0 and ηr,0
- The test is again carried out for several concentrations and a plot of ηr,0 versus concentration is achieved, which can then be exported and examined without the software.
A cylindrical geometry or a parallel plate geometry can also be employed for the test. Where the material appears to display wall slip effects, a sand blasted geometry must be considered.
For measurements at low torques, larger geometries prove useful. Low torques could possibly be observed at lower frequencies. For these types of tests, a solvent trap can be used because solvent evaporation around the margins of the measuring system can nullify the test when operating at higher temperatures.
This information has been sourced, reviewed and adapted from materials provided by Malvern Panalytical.
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