Characterization of extensional rheological filament stretching with a dual-mode Giesekus model

A new, simple, formulation that describes capillary thinning as predicted by a two-mode Giesekus model is derived, and its application in analysing data from extensional rheometry (capillary thinning) experiments is discussed. An algorithm is presented that can be used to fit the expressions obtained from the Giesekus model to extensional rheometry data. Examples of data fitting are given for an idealised data set, for measurements obtained for aqueous solutions of 6 wt% 900,000 molecular weight polyethylene oxide, and for biological fluids obtained from pitchers of Nepenthes Rafflesiana. Good fits to the data were obtained, with coefficients of determination in excess of 0.98. For each data set, it was possible to calculate values of extensional viscosity and relaxation time for each of the two modes, allowing quantitative comparison of different fluids or of the same fluid as it ages.The authors are grateful to Dr Ulrike Bauer, University of Bristol, for access to experimental data on pitcher plant fluids obtained in the field in Brunei Darussalam between August and September 2014. The authors wish to acknowledge Edward Bosson and Tom Hoier for their data analysis of pitcher plant fluids that identified the initial region of filament thinning, colloquially termed the ‘hockey stick’.This is the author accepted manuscript. The final version is available from Wiley via http://dx.doi.org/10.1002/aic.1518