Creaming, sedimentation, flocculation and coalescence: four clear signs that spell failure for a formulation. Stability is one of the fundamental properties of an emulsion, if the product doesn’t remain emulsified then no claims of its youth enhancing or taste bud tantalizing properties (emulsions are used in so many ways) will matter.
Predicting long term stability using one quick test is the Holy Grail for stability studies; sadly rheology cannot provide this but it can offer an indication of the result. The most useful information regarding stability can be gathered using very gentle tests to analyse emulsions under very low stress conditions.
The concept of non-Newtonian flow is often the first thing people learn when first delving into the minefield of rheology that lies beyond the basic Brookfield viscometer measurement. For shear thinning fluids the viscosity depends on the shear conditions that the sample is currently experiencing. As the applied shear rate is increased the viscosity drops, and conversely the viscosity increases as the shear rate is decreased. But how far does this increase continue?
There is of course a maximum as can be seen from the plots below gathered using our research rheometers here at the centre; the plateaus at very low stresses are referred to as the zero shear viscosity plateaus. Simply put this is the viscosity of the sample when under creeping conditions, such as would be experienced during storage.
How to interpret this information
Stokes’ law is an equation used to calculate the terminal velocity of a hard sphere falling through a continuous medium makes use of a dynamic viscosity, but as we’ve seen emulsions are often non-Newtonian and show shear thinning behaviour with increasing shear rates. So what viscosity value should be used?
This is where the zero shear viscosity comes in; when it comes to stability the conditions we are interested in are when the sample is under incredibly low stresses. The zero shear viscosity is therefore the most relevant value when using the Stokes’ law calculation in this way.
Stokes’ Law – terminal velocity of a sphere falling through a liquid
(vertically downwards if ρp > ρf, upwards if ρp < ρf ), where:
- g is the gravitational acceleration (m/s2)
- ρp is the mass density of the particles (kg/m3)
- ρf is the mass density of the fluid (kg/m3)
- η is the dynamic viscosity (Pa.s).
As seen in fig.1 there can be a vast difference (orders of magnitude) in zero shear viscosity for seemingly similar products. Viscosity is not the only factor that affects the stability of an emulsion, but it does have an effect none the less. If for two products the only difference lay in the zero shear viscosity and all other factors were the same, then the product with the higher zero shear viscosity would be the most stable long term.
In simple terms: if the zero shear viscosity is high it is more difficult for the droplets to coalesce as movement through the continuous phase is hindered. If the zero shear viscosity is low the opposite is true, the low viscosity means droplets move around and come together more easily.