In general, viscosity is the quantification of the internal friction of a fluid. The analysis area of a spindle makes complete contact with a sample and stress (or shear) is applied either by rotation or oscillation of the spindle. The viscometer measures the drag, or resistance, placed on the spindle and produces results reported in units of centipoise (cP) or milliPascal seconds (mPa-s). The size and shape of the spindle, rotational speed, sample container, and temperature of the fluid (25°C by default) all greatly affect a viscosity measurement, so it is important to have consistent analysis conditions across analyses and instruments. This testing plays a crucial role in the research, development, and process control of liquid and semi-liquid products.

Viscoelastic materials pose a bit more of a challenge when trying to gain a full picture of material behavior. Classic viscosity measurements may give information about the viscous fluid properties, but won’t be able to give information about the elastic properties of such a material. In this case, oscillatory tests are preferred. An oscillatory test will partially rotate back and forth in a sinusoidal manner. The sample will take time to respond to the rotation with some amount of lag—a phase shift. The angle of the phase shift can determine at what shear rates viscous properties dominate the sample’s behavior, and what shear rates the elastic properties dominate sample behavior. This is useful for materials that go through periods of low strain, then periods of high strain—think of lotion sitting in a bottle, then being pumped through a nozzle for application.

Many industries use rheology testing as a quick and reliable way to analyze important factors that can affect the performance, quality, and/or efficiency of a product, both during manufacturing and for quality control testing of final consumer goods. The food, cosmetic, and pharmaceutical industries use viscosity measurements to determine or troubleshoot the appearance, texture or consistency of products, and determine how product will flow, pour or spread. Rheology is also used to determine how a material will move through production, such as moving viscous liquid through pipes or other equipment during the manufacturing process. In short, any product or industry which uses liquid in manufacturing or their final product has a use for determining and controlling viscosity.