Shear vs. Extensional Rheometry

Shear rheology is what people normally think of when describing how viscous a material is. It measures the flow of fluids in sliding motion, a property that can be easily assessed using current analytical tools. Shear rheology properties are crucial for understanding processes such as coating (e.g., paints), spreading (e.g., creams), and pumping, as they quantifies the fluid’s resistance to flow, or how “slippery” it is.

Although the physical properties influenced by extensional viscosity, such as tackiness or stickiness, are familiar to many, extensional rheology itself remains less well-known compared to its shear counterpart. This is surprising, given its relevance in various industrial processes and applications where fluids undergo changes in cross-sectional area, such as fiber spinning, spraying, and food processing.

Extensional rheology plays a crucial role in flows involving nozzles or jets, like injection processes, and in forming droplets (e.g., spraying, ink-jetting) or strings (e.g., dispensing ketchup or soap). However, measuring extensional rheology poses challenges due to the lack of standardized techniques. While specialized instruments are available, interpreting results and understanding their implications can be complex, requiring careful analysis and expertise.

Instruments for Extensional Rheometry

Capillary Breakup Extensional Rheometer (CaBER®)

Cambridge Polymer Group scientists are some of the world’s leading authorities in extensional rheometry, having been involved in the design and construction of two extensional rheometers. We developed a Capillary Breakup Extensional Rheometer (CaBER®), a unique tool for understanding the breakup kinetics of low viscosity fluids when they are stretched. This method provides a rapid and straightforward way to measure the extensional viscosity of polymer fluids, allowing for characterization in scenarios relevant for their usage, such as spraying and coating.

By pulling the fluid into an unstable liquid bridge, the CaBER® monitors the kinetics of the solution as surface tension pinches off the filament while the extensional viscosity resists the forces. This process mirrors droplet formation in spraying or coating flows; therefore this instrument can yield profound insights into fluid behavior. The CaBER® is useful for low viscosity fluids and for probing the conformation of proteins in biofluids.

Although the commercial CaBER® is no longer available, CPG can build custom prototype units and can help in designing an applicable test array using the CaBER®, or other techniques to fully characterize your material’s response to an extensional flow.

Filament Stretching Rheometer (FiSER™)

For more academic level testing, Cambridge Polymer Group has also designed and built the Filament Stretching Rheometer (FiSER™). In contrast to the CaBER®, this device operates on viscous polymer solutions and has found application in topic areas as diverse as entangled polymer solutions to pizza dough. The FiSER™ imposes deformation on the material and directly measures the resulting force, very similar to a conventional shear rheometer but with a focus on extensional flow properties.

Extensional Rheology Testing Services by CPG

Although there is no international standard for testing the extensional properties of fluids, CPG offers comprehensive analytical testing services on these materials and can add a deep understanding of the processes at work to aid in characterization. CPG has performed extensional rheology on a range of materials including:

  • Coating materials
  • Eye-drops
  • Candy and caramel
  • Consumer products
  • Adhesives
  • Inks and paints
  • Biofluids, such as Hyaluronic Acid

Whether through prototype development or advanced testing methods, CPG ensures that clients gain valuable insights into their materials’ behavior. Contact us to speak to one of our experts.