When friction is applied, one plane of fluid moves relative to another plane, and this force is called shear. Viscous materials require greater shear than those less viscous. The definition of viscosity is illustrated in the figure below. If two planes of fluid, separated by dx, are moving in the same direction, at two different velocities, the force required to maintain dv is proportional to the velocity gradient, or:

In which, η is a constant, known as viscosity. The velocity gradient, represented as dv/dx, is the change in the speed of multiple layers of fluid relative to each other, which is defined as shear rate (γ).  The F/A is the force per area, also known as the shear stress (τ). Viscosity can then be calculated as:

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. For instance, the food industry uses viscosity measurements to determine or troubleshoot the appearance, texture or consistency of food products which are factors in a consumer’s sensory experience. 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.

The pharmaceutical industry uses rheology to predict how products will behave during consumer use, such as how a medicine will flow, pour or spread. For example, how well an ointment will cover the skin, or how a cough syrup or other suspension will pour from a bottle.

Other industries also use viscosity measurements. The automotive industry uses measurements for engine lubricants or hydraulic systems. The printing industry tests the properties of semi-liquid inks. The paint and coating industries test coatings and finishes to improve their application properties. The cosmetic industry tests the flowability and spreadability of liquid makeup and creams to enhance coverage performance.  In short, any product or industry which uses liquid in manufacturing or their final product has a use for determining and controlling viscosity.