Measurement of second normal stress coefficient

One of the rewards of writing a retrospective review such as this is the opportunity to marvel at the careful, subtle and elegant work which has been done in some sense as a follow-on to one s own work. An example of this is the remarkable work by Magda et al. [32]. Using small flush-mounted pressure transducers to measure the normal stress profile in the radial direction, combined with measurements of total normal thrust, they were able to obtain excellent meaurements of N, which they compared to the predictions of the Doi theory as extended to multidomain fluids [61-63]. 

The behavior of vs. shear rate for isotropic solutions of PBLG + m-cresol was negative and monotonically increasing in magnitude for all shear rates. This was the first measurement of for solutions of rod-like polymers. The ratio of —NJN was constant at 0.29, which 

An interesting experimental observation was noted by Magda et al, [32] regarding the effect of smoothness of the rheometer plates. For a solution of flexible polymers, two sets of plates gave identical linear normal stress profiles, as required to extract from a combination of point normal force measurements and total thrust measurements. However for the lyotropic solutions, only the smoother plate yielded a linear profile. They concluded that although it is possible to impose 

In addition to the obvious visual similarity, in fact they describe six points of detailed agreement (which will be easier to digest if the reader refers to the original figures), reproduced here verbatim  

The plots of Nj and versus shear rate intersect twice, both times at negative values of the stress at the second intersection the stress is more negative than at the first and at both intersections the stress becomes more negative as the concentration increases. 

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