Characteristic mechanical impedance of rheometers with axial symmetry. A theoretical and numerical analysis
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Mechanical impedance
Coaxial
A fully automated, robotically operated rheometer was developed. The full functionality, modularity and accuracy of the rotational rheometer are available, which means the modern principles of high‐throughput screening are brought to full function on the rheometer. The basic rheometer setup remains as modular as before including the ability to run all test modes the rheometer offers with the difference that the high‐throughput rheometer now performs all measuring steps automatically. In addition, the standard and proven environmental chambers of the rheometer are available. The rheometer itself runs by the standard rheometer software and the measurement data and analysis results can be transferred to a monitoring database. The sample loading and the cleaning of the geometries is assisted by a sample preparation unit and a cleaning station, respectively. The sample throughput is further maximized by the use of multiple geometries allowing the simultaneous rheological measurement by the rheometer and the cleaning of the geometries at the cleaning station by the robot. The High‐Throughput Rheometer (HTR) and its special adaptation to different applications like dispersions and polymer melts are described.
Rheometry
Laboratory automation
Sample (material)
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Cheese viscoelasticity is commonly measured using steady uniaxial compression, steady uniaxial extension, shear and compressive creep, and stress relaxation in shear and compression. Viscoelastic properties for many cheeses have also been studied using small amplitude oscillatory shear (SAOS). However, there is little on the measurement of nonlinear viscoelastic properties. The large deformation test usually conducted on cheese to study nonlinear viscoelasticity is uniaxial compression, but this test hardly departs from linear behavior. In this work, the nonlinear viscoelasticity of four cheese varieties was studied using large amplitude oscillatory shear (LAOS). A sliding plate rheometer incorporating a shear stress transducer was used. The data were evaluated using spectral analysis, and results are presented mainly in the form of shear stress versus shear rate loops.
Stress relaxation
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The strain and stress distributions in the sample contained in two new rheometers, the Orthogonal Rheometer and the Balance Rheometer, are discussed. In these rheometers we can measure the complex modulus as the constant force acting on the circular plate (the Orthogonal Rheometer) or as the constant torque acting on the rotational driving shaft (the Balance Rheometer). Although our discussion is restricted to linear viscoelastic materials, any special constitutive equation does not need to be used. The driving torque for the steady rotational motion of the apparatus is also calculated.
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Abstract A rotational benchtop Rheometer with vane spindles can be used to measure the static yield stress behavior of materials. By running at different rotational speeds, the Rheometer data can be equated with the viscoelastic information determined by an oscillating rheometer. The rotational Rheometer offers a less expensive method suitable for Quality Control needs.
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Fluid flow patterns in traditional rotational rheometers are generally well known and rheological parameters such as viscosity can be easily calculated from experimental data of single phase fluids and analytical solutions of the patterns. However, when the fluid is a suspension, where some of the particles are as large as 2 mm in diameter, these rheometers need to be modified. The distance between the shearing planes needs to be increased, which necessitates additional physical confinement of the fluid. This causes the flow pattern to be not analytically soluble leading to an inability to correctly compute the viscosity. This paper presents a modified parallel plate rheometer, and proposes means of calibration using standard oils and numerical simulation of the flow. A lattice Boltzmann method was used to simulate the flow in the modified rheometer, thus using an accurate numerical solution in place of the intractable analytical solution. The simulations reproduced experimental results by taking into account the actual rheometer geometry. The numerical simulations showed that small changes in the rheometer design can have a significant impact on how the rheological data should be extracted from the experimental results.
Shearing (physics)
Lattice Boltzmann methods
Suspension
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Nanochemistry
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A new rheometer measuring viscoelasticity of liquids at high frequencies with a surface loading method was constructed and applied to a waterborne polymer. An impedance head that could simultaneously detect force and acceleration was employed in this rheometer. A small glass plate was attached to a vibrator through the impedance head. The mechanical impedance of liquids was evaluated from difference between the transfer functions for the two cases, one with the plate being immersed in the sample and the other with the plate in the air. Then, the storage and loss moduli were calculated from the mechanical impedance as functions of frequency in the range of 102 to 104 rad s-1. As a calibration, the moduli data for a solution of polystyrene in diethylphthalate obtained with this rheometer were compared with those with a conventional rheometer. Agreement of the two sets of data indicated a good performance of the new rheometer. Viscoelasticity of aqueous solution of associating polymer was determined at frequencies 103-104 rad s-1 with this rheometer and at 10-1-103 rad s-1 with a conventional rheometer. This associating polymer solution was found to possess single relaxation viscoelasticity.
Mechanical impedance
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In evaluating the stiffness of skin surface, internal structures such as bone and muscle often affect the measurements. In the present paper, acoustic random vibration is used to estimated the viscoelasticity of a silicone-gel model. This viscoelasticity, which includes two different stiffness strata, is first estimated using a mechanical impedance spectrum, which describes the relation between the depth and viscoelasticity of internal objects. This method is applied to the depth of a silicone-gel tumor model measured by ultrasound imaging and the viscoelasticity of internal gel can be accurately estimated.
Mechanical impedance
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