In quality control the structure of products plays a vital role: In the personal care industry, the application behavior and long-term stability of creams is an important parameter. The same applies to lotions for pharmaceutical applications, for instance. In the food industry, the flow behavior of sauces, like ketchup, is also closely monitored. This article discusses the determination of these parameters in different industries using rheology.
Rheology in use
Rheology in the personal care industry
The rheological properties of creams and lotions are largely influenced by the ingredients and by the manufacturing process. Whether the consumer perceives the product as being pleasant must first be investigated in sensory tests, directly on the skin [1, 4]. As soon as the ideal consistency of a cream has been found, this consistency needs to be determined quantitatively and documented in order to use this as a reference for later products. This requires parameters such as structure or spreadability, which can be measured in rotational and oscillatory tests with rheometers.
Evaluating the structure with rheology
The structure is an important parameter for the evaluation of the skin cream’s behavior when it is extracted (e.g. pressed out of a tube) and for the skin feel . The structure is also significant for processing and when planning the production plant. The structural strength of a sample can be determined with an oscillation measurement in the form of an amplitude sweep. Whereas the parameter G′ (storage modulus) describes the elastic behavior of a sample, G″ (loss modulus) represents the viscous portion. If G′ is above G″ in the linear viscoelastic range* (LVE range, see fig. 1), the sample shows gel-like behavior at the preset frequency. This means that the sample will only start to flow when influenced by additional external forces.
Figure 1 shows two measurements on a lotion and a skin cream. The measurement reflects what can be felt immediately: The cream has a higher structural strength. The measurement shows that the values for storage and loss modulus of the lotion are clearly lower than the values for the cream (see table 1). It is surprising that even lotions – which seem quite fluid – show gel-like behavior. This is an intended property as the product should be easily applicable on the skin and not drip off.
*The linear viscoelastic range describes the part of the measuring curve in an oscillatory measurement in which the structure of the sample is not destroyed. This means that the sample does not change during the measurement and G′ and G″ remain constant.
The yield point τf is an appropriate parameter to evaluate the structural strength. The structural strength correlates with the force that is required to be applied on the sample to make it flow. The evaluation is done via the cross-over point of G′ andG″ versus shear stress τ.
The results show that the samples can be clearly differentiated and in this way their applicative behavior can be predicted. In practice it has proved useful to have good and known samples as reference measurements.
Evaluating the spreadability
The flow behavior of creams can be evaluated using viscosity measurements. Thereby it is assumed that the sample can be spread easier at a low viscosity value [1, 2].
Cosmetic creams and lotions generally show shear-thinning behavior. This means that the viscosity is not a constant value, but depends on the shear intensity. This correlation is described using the shear rate. How strong the shear is during spreading depends on the sample and application. A sun cream, for example, is applied more quickly and with more force than an ointment used on wounds and in this way the sun cream is exposed to considerably higher shear rates.
Figure 2 shows the viscosity curve of the above-mentioned samples. Both samples were measured at 37 °C to simulate their behavior on skin. The viscosity values of the skin cream are higher during the whole measuring process. At the beginning, at lower shear rates, the difference is quite clear but it decreases with increasing shear rates (see table 1). It can therefore be expected that the difference in spreadability of the cream and lotion decreases with increasing shear stress.
Rheology in the pharmaceutical industry
Some widely used rheological tests in the pharmaceutical industry are:
- Flow and viscosity curves
- Flow or yield point determination
- Measurement of the structural regeneration, also called “thixotropy“ testing
- Determination of the temperature-dependent behavior
- Temperature swing test
These measuring methods are useful for optimizing the manufacturing process (homogenizing, pumping, filling etc.), the long-term stability of the dispersion, and also the subsequent end-use of all kinds of gels, creams, and lotions.A viscosity curve for sunscreen, for instance, means that the viscosity is determined under different shear conditions with one test. One condition would be at rest (at low shear rates for simulating the behavior in the tube); another would be at high shear rates (for simulating the behavior when shaking the tube or during stirring or pumping in the production process).
The flow or yield point determination is important for finding out how much force is needed to squeeze the sunscreen out of its tube. This can also be tested by the rheometer quickly, reliably, and easily under different conditions. Sunscreen should be easy to squeeze out of its tube, regardless of the temperature.
Rheology in the food industry
Flow behavior plays a vital role in the food industry. Classic examples are sauces. Typically ketchup mainly consists of tomatoes, vinegar, salt, and a number of different herbs. During the manufacturing process the ketchup should be easy to mix and easy to fill into bottles. Consumers expect ketchup to flow easily out of the bottle but to not run off the food or the plate once out of the bottle.
Also here it is the yield point which is responsible for the behavior of ketchup when we squeeze the bottle and ketchup flows out. It is assumed that the ketchup is in a solid state until the yield point is reached and then it begins to flow. As soon as the pressure in the bottle drops, the ketchup returns to its previous solid state and remains in the bottle.
To simulate the flow behavior and regeneration behavior after squeezing out the ketchup a rheological test called a step test is used. This test has three phases. The first test phase describes the behavior at rest. The second phase shows the structural decomposition and the third phase describes the structural regeneration.
Depending on requirements the step test can be carried out with a ball-bearing rheometer as a rotational test with set shear rate or with a more sensitive air-bearing rheometer carried out as an oscillatory test with set deformation and angular frequency.
In rotational tests a constant and very low shear load is set in the first test interval (behavior at rest) and third interval (regeneration). In the second interval (shear load) a constant high shear load is applied in order to simulate the shearing process which occurs when the ketchup is squeezed out of the bottle or filled into the bottle.
After shear load, Ketchup 1 clearly shows a complete structural regeneration quicker than Ketchup 2, despite Ketchup 1 having a lower viscosity in the second phase. Ketchup 2 shows a considerably slower structural regeneration, which is not yet complete even after the end of the measurement duration of 600 s.
In oscillatory tests the first and third phases have a constant set angular frequency and a consistent deformation within the linear viscoelastic range. The second interval is carried out with rotation at a constant shear rate in order to simulate the shearing process.
Despite its high viscosity at rest, Ketchup 1 has a lower viscosity under shearing (pouring out of the bottle) than Ketchup 2. The structural regeneration of Ketchup 1 takes longer than the time required by Ketchup 2. This means that Ketchup 1 has a longer time to flow which results in a layer thickness which is thinner than that of Ketchup 2.
The step test in rotation or oscillation used on a quickly adjusting MCR rheometer with a highly dynamic measuring drive provides the capability to determine the time-dependent structural regeneration of ketchup. Anton Paar’s rheometers MCR 72 and MCR 92 provide a high accuracy combined with simple operation.
Rheology enables evaluations of the behavior of products throughout the industries. The best choice for these investigations is a rheometer which provides both rotational and oscillatory measurements. With these kinds of measuring devices, samples for both quality control and research can be tested and evaluated within a short time.
 Yao, M.L. and J.C. Patel, 2001, Rheological characterization of body lotions, Appl. Rheol. 11, 83-88.
 Min-Sun Kwak, Hye-Jin Ahn and Ki-Won Song, Rheological investigation of body cream and body lotion in actual application conditions, Korea-Australia Rheology Journal, 27(3), 241-251 (August 2015)
 Brummer, R. and S. Godersky, 1999, Rheological studies to objectify sensations occurring when cosmetic emulsions are applied to the skin, Colloids Surf. A 152, 89-94.
 Moravkova, T. and P. Stern, 2011, Rheological and textural properties of cosmetic emulsions, Appl. Rheol. 21, 35200