Rheological profiling for bottled products
Rheological profiling for bottled products.
Controlled stress rheometry techniques can be employed to characterise the flow attributes of topical emulsions. One such attribute of interest is “pourability”: the ability for the contents of a bottle of lotion to be poured out on inversion. Three topical lotions were investigated for pourability.
The lotions were described as follows:
Lotion 1: “Runny”
Lotion 2: “Just pourable, creamy”
Lotion 3: “Unpourable and gelled”
Using a controlled-stress rheometer, stress-sweep viscosity profiles were performed on the samples. In this method the sample is subjected to incrementing shear stresses and viscosity is monitored throughout. The following results were obtained:
Figure 1: Stress sweep viscosity profiles for 3 lotions.
At low stresses a Newtonian plateau is clearly seen, where viscosity is at a maximum and independent of shear rate. As stress is increased shear-thinning ensues and viscosity decreases markedly with increasing stress. This is the process of yielding, where sample structure, such as colloidal or dissolved-polymer interactions, is disrupted leading to a dramatic decrease in viscosity. It is clear from the relative distribution of the yields along the stress axis that a range of structure strengths exist. At high stresses viscosity levels towards another Newtonian plateau.
When a bottle is inverted, gravity imposes a stress in the direction of flow. If this stress is large in comparison to the yield stress range of that particular fluid, significant flow ensues. If the stress is low, however, or the yield stress of the fluid is high, then the fluid remains at, or close to, its zero-shear viscosity and only creep flow occurs.
Quantifying the yield stress
Although the yield occurs over a range of stresses, for practical comparative purposes we can define a single yield stress. There are a number of ways to do this but that is beyond the scope of this article. For simplicity, let’s identify the stress at which the viscosity/shear rate profile is at its greatest slope, i.e. the point of inflection of the curves. By plotting the derivate of viscosity with respect to stress we can estimate this point from the curve minima (see fig 2).
Figure 2: Slope of viscosity/shear stress profile highlights "inflection yield stresses".
Estimated “inflection yield stresses” are as follows:
Lotion 1: 5.6 Pa
Lotion 2: 16 Pa
Lotion 3: 56 Pa
These values clearly correlate with the consumer observed attribute of “pourability”.
Zero-shear viscosity contribution to stability.
Another desirable attribute in an emulsion is stability against creaming and separation. One key rheological contribution to emulsion stability is a high zero-shear viscosity. The zero-shear viscosities for the three lotions approximate:
Lotion 1: 870 Pa.s
Lotion 2: 10040 Pa.s
Lotion 3: 5175 Pa.s
From this information we can see that, all other things being equal (which is obviously not necessarily the case in a complex formulation), Lotion 2 would be expected to display the greatest stability and Lotion 1 the least.
Combining the yield stress and zero-shear viscosity data we can identify a desirable combination of structural strength and low-shear flow behaviour that can be used as a benchmark for future formulations.
If you would like to learn how these and other methods can help you with your product formulation needs please contact us.