Predicting the Stability of Dispersions with a Yield Stress

Introduction

Assessing the long-term stability of a dispersion or emulsion can be both a tedious and time-consuming process; however, it is essential to ensure a product meets quality standards. Formulators often achieve stability through a combination of effects; minimizing interfacial tension, increasing steric or electrostatic repulsion of the dispersed phase and/or by increasing the viscosity of the continuous phase. For dilute dispersions, the combined effect of these factors can often be reflected in the zero shear viscosity, which can give an indication of the rate at which droplets will coalesce and separate, or dispersions will settle. For systems that are more concentrated, formation of a network structure through dispersed phase interaction or particle/droplet jamming may occur. In this case, stability will be largely related to the strength of the network structure, which can be quantified by the yield StressStress is defined as a level of force applied on a sample with a well-defined cross section. (Stress = force/area). Samples having a circular or rectangular cross section can be compressed or stretched. Elastic materials like rubber can be stretched up to 5 to 10 times their original length.stress

For stability, the yield StressStress is defined as a level of force applied on a sample with a well-defined cross section. (Stress = force/area). Samples having a circular or rectangular cross section can be compressed or stretched. Elastic materials like rubber can be stretched up to 5 to 10 times their original length.stress must be greater than the StressStress is defined as a level of force applied on a sample with a well-defined cross section. (Stress = force/area). Samples having a circular or rectangular cross section can be compressed or stretched. Elastic materials like rubber can be stretched up to 5 to 10 times their original length.stress imposed by the dispersed phase under the influence of gravity. This can be estimated from the following equation:

There are a number of experimental tests for determining yield StressStress is defined as a level of force applied on a sample with a well-defined cross section. (Stress = force/area). Samples having a circular or rectangular cross section can be compressed or stretched. Elastic materials like rubber can be stretched up to 5 to 10 times their original length.stress. One of the quickest and easiest methods is to perform a shear StressStress is defined as a level of force applied on a sample with a well-defined cross section. (Stress = force/area). Samples having a circular or rectangular cross section can be compressed or stretched. Elastic materials like rubber can be stretched up to 5 to 10 times their original length.stress sweep and determine the StressStress is defined as a level of force applied on a sample with a well-defined cross section. (Stress = force/area). Samples having a circular or rectangular cross section can be compressed or stretched. Elastic materials like rubber can be stretched up to 5 to 10 times their original length.stress at which a viscosity peak is observed. Prior to this viscosity peak, the material is undergoing elastic deformation. This peak therefore represents the point at which the elastic structure breaks down (yields) and the material begins to flow. 

For a system to be stable, the yield StressStress is defined as a level of force applied on a sample with a well-defined cross section. (Stress = force/area). Samples having a circular or rectangular cross section can be compressed or stretched. Elastic materials like rubber can be stretched up to 5 to 10 times their original length.stress must be sufficient to withstand the stresses imposed by the dispersed particles, but also additional stresses likely to be encountered during product transportation for example. 

This application note shows methodology and data for evaluating the stability of two shower gel (bodywash) products against their capability to suspend bubbles as a product requirement.

Experimental

Results and Discussion

Figure 1 shows the viscosity against StressStress is defined as a level of force applied on a sample with a well-defined cross section. (Stress = force/area). Samples having a circular or rectangular cross section can be compressed or stretched. Elastic materials like rubber can be stretched up to 5 to 10 times their original length.stress curves for the two shower gel samples in the StressStress is defined as a level of force applied on a sample with a well-defined cross section. (Stress = force/area). Samples having a circular or rectangular cross section can be compressed or stretched. Elastic materials like rubber can be stretched up to 5 to 10 times their original length.stress ramp test. Data for Bodywash 2 shows a clear viscosity peak in the StressStress is defined as a level of force applied on a sample with a well-defined cross section. (Stress = force/area). Samples having a circular or rectangular cross section can be compressed or stretched. Elastic materials like rubber can be stretched up to 5 to 10 times their original length.stress ramp test, while data for Bodywash 1 is relatively flat. This implies that Bodywash 2 is exhibiting StrainStrain describes a deformation of a material, which is loaded mechanically by an external force or stress. Rubber compounds show creep properties, if a static load is applied.strain hard-ening associated with a Yield StressYield stress is defined as the stress below which no flow occurs; literally behaves like a weak solid at rest and a liquid when yielded.yield stress while Bodywash 1 is behaving like a liquid with a zero shear viscosity.

In some instances, viscoelastic liquids may show a slight peak in viscosity even though they do not possess a true Yield StressYield stress is defined as the stress below which no flow occurs; literally behaves like a weak solid at rest and a liquid when yielded.yield stress. In this case, user discretion may be required or alternatively confirmation using an alternative test such as a CreepCreep은 일정한 힘 아래에서 시간과 온도에 따라 달라지는 소성 변형을 나타냅니다. 고무 화합물에 일정한 힘이 가해지면 가해진 힘에 인해 발생하는 초기 변형이 고정되지 않습니다. 변형은 시간이 지남에 따라 증가합니다creep test or shear rates test to confirm the presence of a zero shear viscosity2.

The measured Yield StressYield stress is defined as the stress below which no flow occurs; literally behaves like a weak solid at rest and a liquid when yielded.yield stress for Bodywash 2 was 4 Pa.

Using Equation 1, we can predict that the StressStress is defined as a level of force applied on a sample with a well-defined cross section. (Stress = force/area). Samples having a circular or rectangular cross section can be compressed or stretched. Elastic materials like rubber can be stretched up to 5 to 10 times their original length.stress imposed by a 100 μm diameter air bubble would be approxi-mately 0.65 Pa, hence a yield StressStress is defined as a level of force applied on a sample with a well-defined cross section. (Stress = force/area). Samples having a circular or rectangular cross section can be compressed or stretched. Elastic materials like rubber can be stretched up to 5 to 10 times their original length.stress of 4 Pa should be suf-ficient to suspend the bubble phase, although additional stresses encountered during transport and potential reduction in network strength due to increased temper-ature also need consideration.

Since Bodywash 1 does not have a yield StressStress is defined as a level of force applied on a sample with a well-defined cross section. (Stress = force/area). Samples having a circular or rectangular cross section can be compressed or stretched. Elastic materials like rubber can be stretched up to 5 to 10 times their original length.stress, an accu-rate value of zero shear viscosity is needed to evaluate stability, such as analysis from a CreepCreep은 일정한 힘 아래에서 시간과 온도에 따라 달라지는 소성 변형을 나타냅니다. 고무 화합물에 일정한 힘이 가해지면 가해진 힘에 인해 발생하는 초기 변형이 고정되지 않습니다. 변형은 시간이 지남에 따라 증가합니다creep test. Data from this test showed the zero shear viscosity to be 8 Pas and from this figure, a bubble rise rate of approx. 6 cm a day was predicted for a 100 μm bubble. This is clearly unac-ceptable to maintain long-term stability of the dispersed system, and the incorporation of a yield StressStress is defined as a level of force applied on a sample with a well-defined cross section. (Stress = force/area). Samples having a circular or rectangular cross section can be compressed or stretched. Elastic materials like rubber can be stretched up to 5 to 10 times their original length.stress would be necessary to give the required long term stability and shelf-life for a bubble-suspending product.

1) Viscosity against shear StressStress is defined as a level of force applied on a sample with a well-defined cross section. (Stress = force/area). Samples having a circular or rectangular cross section can be compressed or stretched. Elastic materials like rubber can be stretched up to 5 to 10 times their original length.stress curves from a StressStress is defined as a level of force applied on a sample with a well-defined cross section. (Stress = force/area). Samples having a circular or rectangular cross section can be compressed or stretched. Elastic materials like rubber can be stretched up to 5 to 10 times their original length.stress ramp test for shower gels with (Bodywash 2) and without (Bodywash 1) associative thickener – the viscosity peak shown by Bodywash 2 data is indicative of a Yield StressYield stress is defined as the stress below which no flow occurs; literally behaves like a weak solid at rest and a liquid when yielded.yield stress of 4 Pa

Conclusion

Two shower gel products were compared using a Yield StressYield stress is defined as the stress below which no flow occurs; literally behaves like a weak solid at rest and a liquid when yielded.yield stress ramp test. Bodywash 2, which contains an associative thickener, was shown to have a Yield StressYield stress is defined as the stress below which no flow occurs; literally behaves like a weak solid at rest and a liquid when yielded.yield stress capable of suspending gas bubbles. Bodywash 1, which contains no additional thickener, had a zero shear viscosity that was insufficient to promote long-term stability. The test therefore offers a fast and convenient way of predicting suspension stability for a given particle size and DensityThe mass density is defined as the ratio between mass and volume. density.

Please note...

that a parallel plate geometry can also be used – with this geometry being preferred for dispersions and emulsions with large particle sizes. Such material types may also require the use of serrated or roughened geometries to avoid artefacts relating to slippage at the geometry surface.