(607c) Extensional Flow Affecting Shear Rheology: Experimental Evidence and Comparison to Models

Real world flows are kinematically mixed, yet despite this, we consider rheology under pure shear or extensional conditions by default. In this work, we show that for an example shear thinning system, shear viscosity is not only a function of shear rate, but both shear and extension rate simultaneously, with their relative straining directions being important. As summarised in figure 1, this is achieved through a novel two-phase flow experiment that measures shear stresses not at the wall, where extension rates are zero⁽¹⁾, but in-situ and at defined locations within an extensional flow field. Data is then compared against transient shear control experiments. More recent results are also presented (time permitting) comparing this data against the Carreau-Yasuda and FENE models, highlighting their limited ability to capture this dependence. This work draws a parallel between our knowledge that liquid crystals display anisotropic viscosities when oriented⁽²⁾ to polymer melts and solutions which may be oriented under extensional flow. This dependence directly affects velocity fields in extensional flow situations (such as observed by Trebbin, et al.⁽³⁾) and has relevance in situations from polymer extrusion through to the design of extensional rheology experiments that contain mixed flows.

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