July 10, 2014

Rheometry is more than just viscosity


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Rheometers are instruments that impose a highly controlled deformation to a fluid while measuring the force required to maintain that deformation (or vice versa). Viscosity, a parameter that indicates a fluid's resistance to flow, is normally the main property that people think of when using a rheometer. However, rheometers can provide a great more information than simply viscosity.

Rotational shear rheometers confine a fluid between a top geometry, either a flat plate or a cone, and a fixed flat platen.  The instrument then either rotates the geometry at a series of specified velocities (shear rates), providing shear viscosity as a function of shear rate, or oscillates the top geometry at a series of specified rotational frequencies, providing the elastic and viscous modulus as a function of frequency.  The latter test is particularly useful for probing the viscoelastic properties of materials as a function of deformation rate, such as relaxation times, moduli, dynamic viscosity, and normal force, or tracking time-related phenomena, such as gelation and curing times.

Extensional rheometers act more like load frames, pulling a fluid in a tensile deformation while measuring force and cross-sectional area. These instruments report the extensional viscosity as a function of strain and strain rate, which can vary by orders of magnitude for non-Newtonian fluids and polymer melts depending on the molecular weight, solution concentration, temperature, and strain rate.

This extensional viscosity can be markedly higher than the shear viscosity for the same fluid and is therefore important for filling and pumping of these complex fluids.  Often, the relaxation time of the material is also determined, which can dictate if the material will behave more like a solid or a liquid in response to the deformation rate.

These properties can be used to determine optimal process conditions, such as extrusion rates, fiber spinning rates, and mixing behavior. Additionally, these properties influence the consumer perception of products that are eaten, smoothed on, or otherwise applied in a tactile fashion.

A priori characterization of these products by rheometry can screen out products that have viscoelastic behaviors that are known to have poor responses in consumer test panels.

Three case studies were presented at a rheology meeting by CPG scientists that explore how rheometry can be used to assess consumer perception of materials.
Link to presentation