Redefining the Rheometer

The way in which materials flow or deform when a force is applied can have a direct impact on product performance. While rheology is often perceived as a complex mathematical subject, on a practical level it is relatively easy to grasp. For example, a yogurt with a high yield stress will sit on a spoon, appearing thick and creamy, while its low yield stress counterpart is thin and watery. A paint may be semisolid in a pot, highly viscous, and difficult to stir, but its behavior transforms during brushing to provide even coverage.

Many similar examples exist in everyday life that demonstrate the link between rheology and performance. Experts often readily identify the meaningful parameters for a given application, but for the less experienced rheologist this can be challenging. A new generation of rheometer recognizes that embedding expert knowledge in the instrument itself is the best way to deliver the most value to the user.

Transforming the user interface

A rheometer provides the means to conduct a range of different tests, all fundamentally based on measuring the response of a sample to applied deformation or force. While instruments are developed and designed to deliver a certain level of performance, the real key is an intuitive, intelligent user interface.

Figure 1 - Kinexus rheometer platform.

Figure 2 - Users are guided at a level appropriate to experience.

Users interact with a rheometer at different levels and in a variety of ways; thus a “one size fits all” approach is limiting. The Kinexus rheometer platform (Malvern Instruments Ltd., Malvern, Worcestershire, U.K.) (Figure 1) guides users at a level adapted to their experience and objectives (Figure 2). For example, someone developing a test, who has little knowledge of rheology, may approach it from the perspective of solving a specific problem. A pharmaceutical suspension is unstable; a coating formulation or process does not give the desired level of finish quality—can rheology explain why?

The intelligent integration of expert applications encourages this approach. An understanding of rheology and its importance in different sectors is distilled into guides that steer users toward the tests to solve their particular problem. This follows through to data presentation, which is tailored to the norms of each industry sector. It becomes much easier for users to identify the correct test methodology, implement procedures, and analyze the data for results that provide real insight and understanding of a specific problem.

A more experienced rheologist knows what to measure, but needs to understand how to get the most from the instrument; hence a different starting point is needed. Here, defined sequences, offering instrument-specific guidance, optimize data quality.

Of course there is much overlap between these complementary approaches—even the same user may select a different starting point according to the application. This ability to invite users in, at a level at which they are comfortable, makes rheology instantly more accessible. Equally important is for users or organizations to be able to customize or design tests, whatever measurement approach they take, to meet their exact requirements.

Data quality

Accessibility is fundamental, but so too is the quality of data produced once a measurement method has been selected or developed. The definition and reproducibility of the entire analytical process are critical.

Use of a standard operating procedure (SOP)-type approach to drive analysis reduces operator-to-operator variability. Procedures are broken down into a series of closely defined steps, many of which are fully automated, the operator being prompted for information or to take an action at the required time. The testing process can be closely prescribed, delivering operational consistency with minimal training, and procedures are easily transferred from instrument to instrument and site to site.

Hardware for the 21st century

The software developments are matched by advances in hardware design, which have a significant impact on the operating environment. With a rotational rheometer, the sample is held in the gap between two measurement geometries, typically two plates, a cone and a plate, or concentric cylinders. The geometries then rotate relative to each other to apply shear to the sample. They are, to some extent, test and material dependent, and switching between geometries is a routine task in many laboratories. Performing this operation using a one-click chuck mechanism is simple, quick, robust, and reliable. Intelligent geometry recognition allows automatic configuration of optimal working setups and, in combination with expert system software, enables guidance as to the most appropriate geometries for a particular sample or application.

Figure 3 - Environmental controllers based around “plug and play” cartridges provide optimized usability combined with automatic system configuration.

Rheological properties are a function of temperature, with some materials being particularly sensitive. These require close control of the test environment, and “plug and play” cartridges ensure easy switching between environmental controllers (Figure 3); all necessary connections are made automatically. Intelligent software recognizes the unit, and its configuration data are automatically uploaded to the rheometer system, simplifying setup and eliminating any risk of using incorrect hardware for a defined test. This combination of integrated SOP-type driven software with innovative hardware has significantly improved sample loading, a critical user interaction that is often the root cause of poor data.

Extending performance

Improvements in core performance make it possible to characterize the rheological properties of a greater variety of materials in more ways than ever before. Rheological investigation of pressure-sensitive adhesives, for example, may require the application of a high degree of torque (rotational force), while weakly structured biological fluids demand more gentle testing. The forces or stress applied to a sample are determined by the control and performance characteristics of an ultralow-inertia, dynamically optimized motor in combination with an ultralow torque bias air bearing. Even very weakly structured fluids can be characterized using small sample volumes—a major advantage for medical and pharmaceutical applications where often only small amounts of high-value material are available. Rotational displacement control is accurate to microradian resolution and better, allowing the application of ultralow deformations (strains) to probe delicate strainsensitive structures.

Performance at higher torque has not been compromised to achieve these gains, and Kinexus spans a range of eight decades of torque. Consequently, a single instrument can measure an extremely wide array of different materials.

In addition to control of rotational force, the normal force acting on a sample can be controlled and measured to high sensitivity. It is important to ensure consistent rheology from the point at which a sample is introduced into the measurement gap in the rheometer, and a highly responsive normal force system also permits optimization of sample loading procedures for different sample types. Materials with rapidly time-evolving structures can be followed accurately due to the high and uninterrupted data rates available over the latest USB2 communications interface.

Improved thermal engineering solutions deliver benefits in ramp rate performance, temperature stability, and minimized thermal gradients through the sample. While these are applicable to all materials and processes, they are particularly advantageous for the extremely temperature dependent. Improvements to the user interface are the single most important development for the industrial user, since they allow full exploitation of rheometer performance.

Conclusion

The performance capabilities of the Kinexus rheometer platform demonstrate how far rheometer technology has developed. While there are marked improvements in core performance, it is the user’s interaction with the system and the accessibility of high-quality data that have been completely transformed. Embedding expert knowledge within the instrument delivers rheological intelligence to even the novice user, making it simpler to access the high-quality data that will add value to a product or process.

Powerful software underpins the most radical changes, providing an applications- led interface, SOP-driven operation, and results presentation tailored to industry norms. The rheometer enables simplified setup and intuitive interpretation of results.

Dr. Carrington is Product Manager Rheology, Malvern Instruments Ltd., Enigma Business Park, Grovewood Rd., Malvern, Worcestershire WR14 1XZ, U.K.; tel.: +44 01684 892456; fax: +44 01684 892789; e-mail: [email protected].

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