Rheological studies of the shear deformation and flow properties of complex fluids are often performed in a torsional flow device. However, during strong flows of highly viscoelastic materials, such as polymer melts, a phenomenon known as edge fracture instability often destabilizes the interface between the fluid and the air, leading to unreliable measurements of these materials’ flow properties.

With the aim of improving the accuracy of these measurements, Hemingway and Fielding developed a mathematical expression for the onset of edge fracture, and explained the physical mechanism by which edge fracture instability arises. This new theoretical understanding could help rheologists predict when edge fracture will occur during experiments.

The authors also presented an edge fracture mitigation strategy based on their work. They suggest immersing a flow cell in a bathing fluid that meets specific criteria to help alleviate edge fracture instability.

Knowing when edge fracture will occur, and potentially being able to mitigate it, could help experimental rheologists achieve and measure stronger flows than before, and in turn allow theorists to develop better constitutive models for these highly viscoelastic materials.

Additionally, this new expression for the onset of edge fracture includes the shear-rate derivative of the second normal stress difference, a quality that is typically challenging to measure. Because the second normal stress difference is contained in the authors’ expression, it may provide a new method for rheologists to measure this quality in a material.

Source: “Edge fracture instability in sheared complex fluids: Onset criterion and possible mitigation strategy,” by Ewan J. Hemingway and Suzanne M. Fielding, Journal of Rheology (2019). The article can be accessed at https://doi.org/10.1122/1.5095717.