Viscosity

Viscosity plays a central role in the processing and handling of liquids, pastes and viscous materials and is regularly measured and controlled in various industries.

Viscosity is usually measured with special instruments known as viscometers or rheometers. There are two main types of viscosity:

1. Dynamic viscosity (η)

It describes the internal friction of a fluid as it flows. Dynamic viscosity is stated in pascal-seconds (Pa·s). Typical measuring methods include:

• Rotational viscometer: measures the resistance of a liquid to a rotating movement.

• Falling ball viscometer: determines the time a ball takes to sink through a liquid.

2. Kinematic viscosity (ν)

It is the ratio of the dynamic viscosity to the density of the fluid and is measured in square millimetres per second (mm²/s). A typical measuring instrument is the capillary viscometer.

The viscosity of a material is influenced by various factors:

• Temperature: as temperature rises, the viscosity of liquids typically decreases, whereas that of gases increases.

• Pressure: higher pressure can increase the viscosity of liquids.

• Material composition: additives or fillers can change viscosity significantly.

Controlling and adjusting viscosity is of decisive importance in many industries:

Plastics and rubber industry

• Extrusion: materials with optimum viscosity ensure even shaping.

• Compression moulding: the flow properties determine how the mould fills and the quality of the end product.

Automotive industry

• Lubricants: the viscosity of oils affects lubricity and engine efficiency.

• Paints and coatings: consistent viscosity ensures even application.

Food and pharmaceutical industry

• Processing: viscosity determines the flowability of doughs, creams and liquids.

• Quality control: products such as syrups or ointments must have a constant viscosity.

Viscosity differs according to the type of material:

Material	Typical viscosity	Examples
Liquids	Low to medium (e.g. 0.001 to 1 Pa·s)	Water, oil, paints
Pasty materials	Medium to high (e.g. 10 to 1,000 Pa·s)	Adhesives, rubber compounds
Solids with flow properties	Very high (e.g. >1,000 Pa·s)	Bitumen, polymer melts

Precise control of viscosity brings numerous advantages:

• Process optimisation: efficient production and consistent quality.

• Cost savings: less material loss and lower energy consumption.

• Product safety: ensuring the functionality and durability of end products.

Despite its importance, viscosity control often involves challenges:

• Temperature dependence: fluctuating temperatures can strongly affect viscosity.

• Homogeneity: inconsistent material mixes lead to inconsistent viscosity.

• Measuring accuracy: high-precision instruments are required to obtain reliable data.

• How is the viscosity of a material controlled? Through temperature control, adding additives or adjusting the material composition.

• Which unit is used for viscosity? Dynamic viscosity is measured in pascal-seconds (Pa·s) and kinematic viscosity in mm²/s.

• Why is viscosity important? It affects the flowability, processability and final quality of a material.

• Can the viscosity of solids be measured? Yes, for materials such as polymer melts or bitumen this is done with special instruments such as rheometers.

Viscosity is a central parameter in materials science and manufacturing technology. It has a significant influence on the processing, quality and function of materials and end products. Precise measurement and control of viscosity allow production processes to be optimised and product properties improved, making it an indispensable part of modern industrial applications.