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Knowledge of a material's rheological behavior can be of value for research and development, process design, and quality control. This article provides a foundation for understanding viscosity measurement.
Viscosity measurements provide a wealth of information about a material - from how toothpaste squeezes out of a tube to the flow of gasoline in an engine's fuel-injection system. Many companies throughout a range of industries use this material property for quality control management, and almost every liquid and semisolid consumer and industrial product has its viscosity measured.
In the chemicals industry, for example, a viscosity measurement can be used to indicate the endpoint of a polymerization process - as polymer chains get longer, the viscosity of the material in the process vessel increases. In the asphalt industry, the viscosity of bitumen is measured at elevated temperatures, typically around 150°C or higher, to ensure pumpability of the material. In the personal care products industry, the viscosity of haircare products is measured to ensure proper pour behavior (i.e., the product will discharge from its container) and stability (i.e., shelf life) of the product.
In addition to quality control, viscosity measurements can be used in several other ways. They can predict processing performance, such as the startup torque required for a motor that drives a pump to move material from a holding tank, as well as product performance, such as ink transferability in a printing process. Viscosity data are also useful to detect changes in a material that indicate the extent of reaction.
This article discusses the basics of viscosity - how it is measured and how this information can be used. It describes the different types of viscometers, with emphasis on the most commonly used rotational viscometer. The article also explains how to read a flow curve.
Measuring viscosity with rotational viscometers
Before the advent of formal equipment for measuring viscosity, rubbing a material between your fingers had long been accepted practice for evaluating flow behavior. Today, several types of devices are available for measuring viscosity - flow cups (Figure 1, left), capillary tubes (Figure 1, center), and rotational viscometers (Figure 1, right).
Flow cups are simple, easy-to-use instruments based on a straightforward operating model in which the viscosity of a sample is related to the time...