A comparison of three different methods for measuring both normal stress differences of viscoelastic liquids in torsional rheometers

• Published in: Rheologica acta Vol. 48; no. 2; pp. 191 - 200
• Main Authors: Alcoutlabi, Mataz, Baek, S. G., Magda, J. J., Shi, Xiangfu, Hutcheson, S. A., McKenna, G. B.
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer-Verlag 01-03-2009; Springer; Springer Nature B.V
• Subjects: Characterization and Evaluation of Materials; Chemistry and Materials Science; Complex Fluids and Microfluidics; Cross-disciplinary physics: materials science; rheology; Exact sciences and technology; Fluid dynamics; Food Science; Fundamental areas of phenomenology (including applications); Materials Science; Measurement methods; Mechanical Engineering; Methods; Non-newtonian fluid flows; Normal stress; Original Contribution; Physics; Polymer Sciences; Pressure distribution; Pressure sensors; Rheological measurements; Rheology; Rheometers; Shear flow; Shear rate; Shearing; Soft and Granular Matter; Stress concentration; Techniques and apparatus; Tensors; Torsion; Viscoelastic fluids; Viscoelastic liquids; Viscoelasticity; Viscoelasticity; Normal stresses; Rheology; Pressure sensors; Viscoelastic fluid; Liquids; Shear flow; Measuring methods; Experimental study; Normal stress; Cone plate viscosimeter; Rheometer; Parallel plate; Rheological properties
• ISSN: 0035-4511; 1435-1528
• DOI: 10.1007/s00397-008-0330-z

A novel pressure sensor plate (normal stress sensor (NSS) from RheoSense, Inc.) was adapted to an Advanced Rheometrics Expansion System rheometer in order to measure the radial pressure profile for a standard viscoelastic fluid, a poly(isobutylene) solution, during cone–plate and parallel-plate shearing flows at room temperature. We observed in our previous experimental work that use of the NSS in cone-and-plate shearing flow is suitable for determining the first and second normal stress differences
N
1
and
N
2
of various complex fluids. This is true, in part, because the uniformity of the shear rate at small cone angles ensures the existence of a simple linear relationship between the pressure [i.e., the vertical diagonal component of the total stress tensor (Π
22
)] and the logarithm of the radial position
r
(Christiansen and coworkers, Magda et al.). However, both normal stress differences can also be calculated from the radial pressure distribution measured in parallel-plate torsional flows. This approach has rarely been attempted, perhaps because of the additional complication that the shear rate value increases linearly with radial position. In this work, three different methods are used to investigate
N
1
and
N
2
as a function of shear rate in steady shear flow. These methods are: (1) pressure distribution cone–plate (PDCP) method, (2) pressure distribution parallel-plate (PDPP) method, and (3) total force cone–plate parallel-plate (TFCPPP) method. Good agreement was obtained between
N
1
and
N
2
values obtained from the PDCP and PDPP methods. However, the measured
N
1
values were 10–15% below the certified values for the standard poly(isobutylene) solution at higher shear rates. The TFCPPP method yielded
N
1
values that were in better agreement with the certified values but gave positive
N
2
values at most shear rates, in striking disagreement with published results for the standard poly(isobutylene) solution.