Introduction
Many complex fluids, such as network forming polymers, surfactant mesophases and concentrated emulsions do not flow until the applied 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 exceeds a certain critical value, known as 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. Materials exhibiting this behavior are said to be exhibiting yield flow behavior. 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 is therefore defined as 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 that must be applied to the sample before it starts to flow. Below 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 the sample will deform elastically (like stretching a spring), above 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 the sample will flow like a liquid.
Most fluids with 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 can be considered as a structural skeleton that extends over the entire volume of the system. The strength of the skeleton is governed by the structure of the dispersed phase and its interactions. Normally, the continuous phase is low in viscosity, however high volume fractions of a dispersed phase can increase the viscosity by a thousand times and induce solid-like behavior at rest. When a complex fluid that exhibits yield behavior is sheared at low shear rates, in the range between 0.01 -0.1 s-1 and below its critical 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, the system is subjected to work hardening. This is characteristic of solidlike behavior and results from elastic elements being stretched in the shear field. When such elastic elements approach their critical 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, the structure begins to break down causing Shear ThinningThe most common type of non-Newtonian behavior is shear thinning or pseudoplastic flow, where the fluid viscosity decreases with increasing shear.shear thinning (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 softening) and consequent flow. 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 this catastrophic breakdown of the structural skeleton occurs is 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.
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. 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 ramp is often employed as it is an easy and quick means of determining 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, however, a more accurate method is to perform series of CreepCreep describes a time and temperature dependent plastic deformation under a constant force. When a constant force is applied to a rubber compound, the initial deformation obtained due to the application of the force is not fixed. The deformation will increase with time.creep tests and look for changes in the gradient of the compliance versus time curve [1].
Depending upon the nature of the material being tested, the CreepCreep describes a time and temperature dependent plastic deformation under a constant force. When a constant force is applied to a rubber compound, the initial deformation obtained due to the application of the force is not fixed. The deformation will increase with time.creep response can be quite different as is illustrated in Figure 1.
Since the actual change of 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 will be dependent upon the applied 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, it is usual to talk about the compliance rather then the 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. The CreepCreep describes a time and temperature dependent plastic deformation under a constant force. When a constant force is applied to a rubber compound, the initial deformation obtained due to the application of the force is not fixed. The deformation will increase with time.creep shear compliance (J) can be determined from the preset 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 (σ) and the resulting deformation (γ) through:
Using this notion, CreepCreep describes a time and temperature dependent plastic deformation under a constant force. When a constant force is applied to a rubber compound, the initial deformation obtained due to the application of the force is not fixed. The deformation will increase with time.creep curves generated using different stresses can be directly compared. All J(t) curves overlap with each other independent of the applied 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 as long as 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 is within the Linear Viscoelastic Region (LVER)In the LVER, applied stresses are insufficient to cause structural breakdown (yielding) of the structure and hence important micro-structural properties are being measured.linear viscoelastic region. When this criterion is no longer met, the material is considered to have yielded. This is illustrated in Figure 2 from which it can be deduced for the sample under test that the 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 is between 3 and 4 Pa since at 4 Pa, the curve no longer follows the same profile. This Application Note shows methology and data from multiple CreepCreep describes a time and temperature dependent plastic deformation under a constant force. When a constant force is applied to a rubber compound, the initial deformation obtained due to the application of the force is not fixed. The deformation will increase with time.creep testing for a moisturizing lotion.
Experimental
- A commercial moisturizing lotion was used as the sample under evaluation.
- Rotational rheometer measurements were made using a Kinexus rheometer with a Peltier plate cartridge and a cone and plate measuring system2, and utilizing standard pre-configured sequences in rSpace software.
- A standard loading sequence was used to ensure that the sample was subject to a consistent and controllable loading protocol.
- A series of CreepCreep describes a time and temperature dependent plastic deformation under a constant force. When a constant force is applied to a rubber compound, the initial deformation obtained due to the application of the force is not fixed. The deformation will increase with time.creep tests were run at seven different applied stresses between 30 Pa and 66 Pa.
- Each CreepCreep describes a time and temperature dependent plastic deformation under a constant force. When a constant force is applied to a rubber compound, the initial deformation obtained due to the application of the force is not fixed. The deformation will increase with time.creep test was stopped after a set time (120 s), and a recovery test of equal time was subsequently performed between the CreepCreep describes a time and temperature dependent plastic deformation under a constant force. When a constant force is applied to a rubber compound, the initial deformation obtained due to the application of the force is not fixed. The deformation will increase with time.creep tests.
- All rheological measurements were performed at 25°C unless specifically stated.
Results and Discussion
Figure 3 compares Creep Compliance (J)Creep Compliance (J) is the shear strain divided by shear stress as a measure of instantaneous continuous flexural deformation of a material under static stress loading over time, reported in Pa-1: J(t)=γ(t)∕σcreep compliance (J) with time at all seven stresses. Below 42 Pa, the compliance curves are superimposed and there does not appear to be an increase in compliance with time, suggesting that no flow is occurring below this 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 i.e. the material is behaving as a viscoelastic solid.
At 48 Pa, there is a noticeable change in gradient indicating time dependent behaviour and hence viscous flow. This is perhaps more clearly demonstrated in Figure 4, which shows the final compliance at each 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 following the 120 second CreepCreep describes a time and temperature dependent plastic deformation under a constant force. When a constant force is applied to a rubber compound, the initial deformation obtained due to the application of the force is not fixed. The deformation will increase with time.creep test. It can be inferred from the latter chart that the emulsion product has 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 between 42 and 48 Pa.
To achieve a more precise estimate of the 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, it would be necessary to repeat the test with small incremental increases in 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 between these two values and evaluate in a similar manner.
Conclusion
For the moisturizing lotion tested, the maximum stress where the compliance is within the Linear Viscoelastic Region (LVER)In the LVER, applied stresses are insufficient to cause structural breakdown (yielding) of the structure and hence important micro-structural properties are being measured.linear viscoelastic region is 42 Pa, while at 48 Pa the 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 is exceeded. The 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 therefore has a value between 42 Pa and 48 Pa. For a more precise value of 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 this material, further test iterations within this narrow stress band are required. Multiple CreepCreep describes a time and temperature dependent plastic deformation under a constant force. When a constant force is applied to a rubber compound, the initial deformation obtained due to the application of the force is not fixed. The deformation will increase with time.creep testing to derive 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 is an accurate method, but can require multiple iterations and correct user interpretation.
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 artifacts relating to slippage at the geometry surface.