Definition
Rotary rheometer is a sophisticated analytical instrument used to measure the rheological properties of materials, that is, to study the flow and deformation behavior of substances under the action of external forces. It detects the viscosity, elasticity, and viscoelastic response of materials by applying a controlled rotational motion, making it suitable for testing a wide range of material morphologies, from low-viscosity fluids to high-viscosity soft solids.
Principle
The core working principle of a rotary rheometer is based on applying a defined shear deformation or shear stress to a sample and accurately measuring its response. Instruments typically consist of a drive system, a measurement geometry system such as a parallel plate, cone plate, or coaxial cylinder, and a temperature control system. The drive motor applies rotational torque to create angular displacement or angular velocity for the upper measuring part, while the lower measuring part or sample cup is usually stationary. By measuring the generated torque and angular displacement, combined with the dimensional parameters of the geometric system, the shear stress and shear rate can be calculated, and then the rheological parameters such as viscosity of the sample can be obtained. For dynamic oscillation test, the instrument applies a sinusoidal change strain or stress, and by analyzing the phase difference and amplitude ratio of stress and strain, the dynamic viscoelastic parameters such as energy storage modulus and loss modulus of the material can be obtained.
Measurement method
The measurement modes of rotary rheometers are mainly divided into two categories: steady-state measurement and dynamic oscillation measurement. Steady-state measurements include controlling shear rate mode and controlling shear stress mode. In controlled shear rate mode, the instrument sets and maintains a constant rotational speed to measure the stress required to maintain the deformation. In the controlled shear stress mode, a constant torque is applied to measure the resulting rotational speed. These two modes are commonly used to draw flow curves, i.e., curves of shear stress versus shear rate.
Dynamic oscillation measurement is the application of a small amplitude of oscillatory shear without damaging the sample structure to study the linear viscoelastic zone properties of the material. Frequency scanning can understand the time dependence of the material, strain or stress scanning can determine its linear viscoelastic zone, and temperature scanning can study the thermal behavior of the material, such as phase transition and curing process.
Influencing factors
The accuracy and repeatability of measurement results are affected by a variety of factors. Temperature is the primary factor, and a precise temperature control system is required to ensure uniform and stable sample temperature. The selection and installation of measurement geometry systems is critical to selecting the appropriate plate spacing, cone angle, or cylinder clearance based on sample characteristics and ensuring that they are clean, parallel, and well-aligned. Samples should be prepared and loaded in a manner that avoids the introduction of bubbles or a history of pre-shear. The calibration status of the instrument itself, inertia effects of the shaft system, and edge effects can also interfere with measurements, especially when testing at low viscosity or high frequency. The setting of test parameters, such as shear rate range, oscillation strain amplitude and equilibrium time, needs to be reasonably selected according to the specific sample and research purpose.
Applications
Rotary rheometers have a wide range of applications. In the food industry, it is used to analyze the flow characteristics and texture of sauces, chocolates, doughs, etc. In the petrochemical field, it is used to characterize the viscosity and viscoelastic behavior of lubricating oils, asphalt and polymer melts. In the coatings and inks industry, it is used to study its workability, leveling and settlement resistance. In the field of daily chemicals, it is used to evaluate the stability and usability of products such as shampoos and lotions. In the field of polymers and composites, it is a key tool for studying polymer processing properties, curing kinetics and interfacial behavior of composites. In addition, rotary rheometers play an important role in many industries such as building materials, electronic paste, and ceramic processing.
Instrument selection
When selecting a rotary rheometer, a systematic evaluation is required. First, you need to define your test needs, including the viscosity and modulus range of the sample to be tested, the desired shear rate or frequency range, and whether you want to focus on steady-state flow or dynamic viscoelastic testing. The torque and normal force measurement range and resolution of the instrument should meet the requirements. The capabilities of the temperature control system, including temperature range, temperature control accuracy, and temperature rise and fall rate, need to be matched to the experimental protocol. Whether the instrument has the necessary environmental control options, such as solvent traps or humidity control, also needs to be considered. Measure whether the types and materials of the geometry system are complete to suit different samples. The operating software of the instrument should be comprehensive and easy to use, and the data processing capabilities should meet the requirements. In addition, the long-term stability of the instrument, ease of maintenance, and technical support capabilities are also important considerations.