Viscosity vs. rheology: why it is important to formulators

Rheology is the more complex study of the flow of liquids, gels, creams or ointments that exhibit some level of flow. While simple Newtonian fluids can be characterised by a single value of viscosity, most creams we deal with daily change their viscosity with changing shear – they are described as Thixotropic. The issue is every spindle and speed combination produces a different amount of shear to the cream and as a result you are on different parts of a thixotropic curve (see Fig 1). Thus when you use a viscometer you cannot compare formulations at different spindle/speeds or even the same formulation at a different spindles/speeds during a stability trial as you are applying different amounts of shear. Only true Newtonian solutions are independent of the amount of shear applied. 

The difference between a viscometer and rheometer principally is that a rheometer can perform a much more comprehensive range of tests such as yield stress measurements to describe how a formula behaves at rest through to a continuous full viscosity flow curve with thixotropy calculations and even temperature profiling (how the viscosity property changes with temperature). 

A rotational viscometer is a simple low cost instrument that rotates a spindle in a single direction making it suitable for simple process and is highly suitable for quality control where the result is a simple pass/fail result, but a cone and plate rheometer allows far greater characterisation of flow and deformation behaviour for the formulator. Both instruments are complimentary and it is not uncommon to see viscometers and rheometers in the same lab.