Rheometry For Viscoelastic Properties

Rheometry is used to determine the viscoelastic properties of materials, that is the qualitative and quantitative relationships between stresses and strains and their derivatives. Viscoelasticity is the property of a substance exhibiting both elastic and viscous behavior. Viscoelastic materials behave like both solids and liquids when undergoing deformation. When stress is applied, it causes temporary deformation if quickly removed and permanent deformation if the stress is maintained.

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How rheometry works

The viscoelastic properties of a material are measured by a rheometer (Figure 1). A rheometer is a device used to measure how a liquid, suspension, or slurry flows in response to applied forces. In one type of method, a test specimen is placed in a die cavity, closed, and maintained at an elevated temperature. The die cavity is constructed by two dies, one of which is oscillated through a rotary amplitude. This produces a sinusoidal torsional strain in the test specimen, resulting in a sinusoidal torque, which measures the viscoelastic quality of the test specimen.

Figure 1: A rotational rheometer (Source: Wikipedia, CC BY-SA 4.0)

These viscoelastic measurements are made based on (1) a frequency sweep, in which the frequency is programmed to change in steps under constant strain amplitude and temperature conditions, (2) a strain sweep, in which the strain amplitude is programmed to change, or (3) a temperature sweep, in which the temperature is programmed to either increase or decrease. Rheological properties are measured for each set of frequency, strain, and temperature conditions.

Strengths of the rheometry technique

  1. Sample preparation is easy.
  2. Can test both Newtonian & non-Newtonian fluids.
  3. Allows high shear rates without flow instabilities
  4. Prevents evaporation and contamination of samples

Limitations of the rheometry technique

  1. The tests are carried out till the specimen’s failure. 
  2. High energy requirement. 
  3. Requires expertise to carry out. 
  4. Limited maximum applied strain. 

Uses of the rheometry technique:

Characterization of viscoelastic property data is useful for quality control, process control, and manufacturing. The study of flow properties of liquids is important for many industrially important substances, such as cement, paint, and chocolate, which have complex flow characteristics. Rheometry has applications in materials science, engineering, geophysics, physiology, human biology, and pharmaceutics. 

Viscoelastic property data is useful in the preparation of pharmaceutical products such as ointments, creams, pastes. Viscoelastic properties are used as a criterion to maintain the quality of the product. 

Sample requirements:

Small sample size requirement (26µL – 100µL). In one type of test, the test specimen should be circular, with a diameter smaller than the test chamber of the instrument to be used. 

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