Is your viscometer nearing the end of its life? Do you feel that your trial and error 'rheology-light approach to formulation is becoming dated? Are there longstanding product performance problems that you’re failing to gain traction with, where you believe rheology may hold the answer?
If the answer was yes to any of these questions, then the user may need to evaluate the capabilities of a modern rheometer against a low-cost viscometer.
The design and functionalities of a rheometer have advanced significantly over the last decade. Modern instruments come with a wide range of test capabilities, wrapped in software that enables them to be accessed by novice users. Such systems promote the industrial adoption of rheology and provide more cost benefits than that can be achieved with viscometers, from research and development, through formulation and into production. This article discusses the top five measurements performed by rheometers that make them superior to viscometers.
A Broader Measurement Range – Extend Your Viscosity Flow Curve
A broader measurement range enables relevant data to be obtained by exposing the sample to conditions that are similar to the conditions applied during product manufacture or use. For many industrial products, viscosity is a key performance-defining parameter.
The viscosity of a Newtonian fluid is independent of the applied shear rate, for example, water. Conversely, non-Newtonian materials exhibit lower viscosity at higher shear rates (shear thinning), or less commonly exhibit viscosity increase with applied shear rate (shear thickening). Knowing the properties of a material under the conditions that are routinely applied helps to understand its behavior when used or processed. Obtaining this information is a challenging task for non-Newtonian materials compared to Newtonian materials.
Generating a flow curve, which is a graph of viscosity as a function of shear rate or shear stress, is one of the simplest methods to analyze viscosity. A sample may be subjected to different shear stresses, and the resulting shear rates measured at each applied stress or the sample can be subjected to a controlled shear rate and the resultant shear stress is measured. A far more comprehensive flow curve can be generated using a rotational rheometer due to its ability to cover a wide range of shear rates and stresses compared to a rotational viscometer. Specifically, it is possible to generate data at the very low shear rates relative to storage and movement under gravity (Figure 1).
Figure 1. By spanning a much wider range of shear rates than a viscometer, a rotational rheometer is able to provide data of direct relevance to more processes.
Opthalmic viscosurgical devices (OVDs) are gels or viscoelastic solutions applied during eye surgery. They are generally aqueous polymeric solutions consisting of one or more of the following constituents: chondroitin sulfate, hyaluronic acid, and/or methyl cellulose. The ISO 15798:2013 standard that covers OVDs recommends rheological measurement due to the effect of product rheology on in-use performance. The behavior of a fluid within the anterior chamber and when administered through a cannula into the eye can be determined by performing steady state testing at shear rates in the range of 0.001-100 s-1. A typical rotational viscometer cannot access the lower end of this range.
The flow curve data for three OVD formulations, each with a varying hyaluronic acid concentration (15 mg/ml, 18 mg/ml and 25 mg/ml) is shown in Figure 2. Malvern Panalytical’ Kinexus rotational rheometer was used to measure the steady state viscosity at the different shear rates using cone-plate measurement geometry at 25 °C. The shear-thinning behavior exhibited by the samples is similar, but with more concentrated solutions exhibiting higher viscosity. Interestingly, all of the samples exhibit Newtonian behavior at very low shear rates. This characteristic cannot be detected with a viscometer. It reveals that the OVDs do not have a gel-like structure when at rest in the eye, but remain fluid like.
Figure 2. Equilibrium flow curve data shows the shear-thinning behavior of 25 mg/ml (#), 18 mg/ml (+) and 15 mg/ml () HA solutions, and their tendency towards Newtonian behavior at the very low shear rates typical of at rest conditions within the eye.
Relevant Yield Stress Measurement – Generate Accurate Data for Every Sample Type
The consumer appeal of many products is defined by their yield stress, which is the input stress required to break down any solid-like (network) structure in the product and make it flow. Accurate and relevant measurement of yield stress with an appropriate technique is helpful to achieve faster and more effective formulation. After viscosity, yield stress is probably the most routinely measured rheological property, because many consumer products gain value from having one.
Many cosmetics and a number of foodstuffs, for example, yoghurt, mayonnaise, tomato ketchup, are rich and thick in a pot or on a plate (at rest). However, cosmetics can be easily applied over skin or foodstuffs can be easily dispensed due to their more liquid-like behavior upon application of shear. Yield stress and yield strain (strain at which yielding occurs) can also be used as a measure of strength and brittleness, respectively.
Yield stress varies as a function of the temperature and timescale during which the stress or deformation is applied. Various techniques are available to measure the yield stress. Rotational rheometers can provide far more relevant yield stress data than a rotational viscometer by enabling application of the broad range of these methods. Applying a stress ramp is one of the simplest methods with a stress-controlled rotational rheometer. This approach includes subjecting the sample to a gradually increasing stress, measuring the resultant strain, and calculating a peak viscosity (Figure 3). The impact of timescale on measured values can be assessed by carefully controlling the stress ramp rate or by making measurements at different ramp rates.
Figure 3. Applying a stress ramp to a sample with a yield stress detects the point at which viscosity passes through a maximum, structure in the sample starts to break down, and the material begins to flow as a liquid. The stress at which this occurs is the yield stress.
Oscillatory testing may also be used to generate yield stress data by means of an amplitude sweep. In an amplitude sweep, the sample is subjected to a sinusoidal strain or stress profile of steadily increasing amplitude. When the material microstructure is intact the elastic modulus, G’, remains constant with strain or stress, but above the yield stress a rapid drop in the elastic modulus is observed when the structure breaks down.
The region where G’ does not change is called the Linear Viscoelastic Region (LVER). The edge point of LVER can be can be considered as defining the yield point, although some define it as stress or strain at which G’’ exceeds G’ cross, although this is less preferable. Typically, the true yield point happens somewhere between these two transition points and this can be correlated with a peak in the elastic stress (stress component of G’) when plotted against strain.
Figure 4. An amplitude sweep identifies the LVER of a sample, the region in which it exhibits solid-like behavior; a longer LVER is indicative a greater structure in the sample.
Stress ramp test data for three tomato ketchup samples (value, supermarket own brand, and branded) is shown in Figure 5. Malvern Panalytical’ Kinexus rotational rheometer was used to perform the tests with a 40 mm serrated parallel plate geometry. The results reveal that each sample exhibited a yield stress, with the branded product showing most structure with the highest yield stress of 22 Pa. This implies that the branded product will require more force to dispense but will be satisfyingly non-drip during use.
Figure 5. The high yield stress of the branded tomato ketchup (red), relative to the supermarket (blue) and value (green) alternatives is indicative of greater structure and suggests that it will be less prone to spreading on a plate or dripping during use.
This information has been sourced, reviewed and adapted from materials provided by Malvern Panalytical.
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