A technical note on large normal-stress differences observed in a novel self-assembling functionalized dipeptide surfactant solution

A number of functionalised dipeptides self-assemble in water under specific conditions to give micellar aggregates. The micellar aggregates formed depend on the exact molecular structure and are important to understand as they control the properties both of the micellar phase and also of the gel phase which can be formed from these precursor solutions. Here, we investigate the rheological properties of a functionalised dipeptide which behaves as a surfactant at high pH. This solution has been shown previously to exhibit very “stringy” behaviour, and this has previously been characterised using capillary breakup extensional rheometry (CaBER). In the current technical note, we extend the rheological characterisation of an exemplar precursor solution via small-amplitude oscillatory shear and steady shear. Using a cone-and-plate geometry and a dedicated protocol, we can measure the first normal-stress difference N1 and using a parallel-plate geometry to also measure (N1-N2), subsequently determining the second normal-stress difference N2. In so doing, we confirm that these systems are highly elastic, e.g. for shear rates greater than ~ 30 s−1, corresponding to a Weissenberg number based on the longest relaxation time ~ 330, N1 > 10τ where τ is the shear stress, and also, we find that N2 can be significant, is negative and approximately equal in magnitude to ~ 0.36 ± 0.05 N1. Significant uncertainties associated with the normal-stress difference data led to us using a range of different rheometers (and geometries) and highlight the issues with determining N2 using this two-measurement approach. Despite these uncertainties, the non-negligible value of the second-normal stress difference is demonstrated for these fluids.