Cone-partitioned plate device measures normal forces with better temperature range performance

Determining the first (N1) and second (N2) normal stress difference is a crucial part of understanding a polymer’s response. Measuring them, however, remains a challenge in experimental rheology, where responses are not in equilibrium and stress and strain relationships are nonlinear. The cone-partitioned plate (CPP) fixture provides a method to extract both types of normal stresses, but is cumbersome, and using a newer CPP geometry based on two partitions, called CPP3, has been too complicated and lacks adequate temperature control.

Researchers have developed a new fixture based on the CPP3 concept to measure normal forces with improved temperature control over a wider range. Named CPP3+, the device by Athanasiou et al. is incorporated into the oven of a commercial rheometer, unlike more cumbersome CPP3 fixtures, uses the rheometer’s tranducer and a piezoelectric sensor to measures normal stresses quickly with high-quality transient signals.

“With this and other CPP-based developments by different groups including ours, we believe that the community has reliable tools available to attempt measuring N1 and N2 in different soft materials but also detect instabilities through signal fluctuations,” said author Dimitris Vlassopoulos. “This is important because there are few data available on N1, and especially N2.”

In such systems, instabilities at the sample’s outer boundary affect measurements as shear rates increase. The CPP fixture delays them, enabling measurements over a range of shear rates.

They confirmed feasibility with samples of polystyrene and poly(n-butryl acrylate), and found the fixture cannot be used above 80 degrees Celsius, and that N2 data is sensitive to outer boundary instabilities.

The group plans to boost the CPP3+ fixture signal stability by combining it with confining outer partitions, and further analyzing the complex CPP geometry.

Source: “A double partition cone-plate piezorheometric tool to measure normal stress differences,” by Thanasis Athanasiou, Katerina Peponaki, and Dimitris Vlassopoulos, Journal of Rheology (2025). The article can be accessed at https://doi.org/10.1063/8.0001001 .