There are many different applications that require the knowledge of extensional viscosity of a specific material, typically an elastomer or a polymer. This is sometimes also called the elongational viscosity and it is a measurement of how resistant a particular material is to stretching.
Think of this as the amount of stretch or pull that can be applied to any type of rubber or elastic material. The material is only stretched or pulled along the length in a horizontal line without any type of twisting or forces applied to the length through the vertical. This would then include shear force, not just the extensional force required to measure this type of viscosity.
Since most materials are much more resistant to elongational force than shear force, this is often a critical element in choosing one material over another. Internally, during the stretching processing, long molecules actually realign to create the measured level of extensional viscosity. The more the molecules are intertwined and linked with each other, the more resistant they are to being deformed (stretched).
Differences to Consider
There are many different factors that can influence extensional viscosity. This is particularly important with the non-Newtonian fluids or those that act differently under stress and force.
Newtonian fluids, such as water, act in a predictable and consistent way based on the temperature and the type of pressure. With rubbers, polymers and other types of “semi-solid” types of liquids, the elongation changes over time as well as with force, so there is no option to develop a coefficient for viscosity for these materials.
With the use of rheometers and other testing devices, the specifics properties of materials can be measured throughout time and with different levels of force. This information can then be used to assess the effectiveness of a particular polymer or rubber mixture in a particular application, ensuring safety and durability under the required operating conditions.