Extensional flow affecting shear rheology: Experimental evidence and comparison to models

Abstract

Real world flows are kinematically mixed, yet despite this, we consider rheology under pure shear or extensionalconditions by default. In this work, we show that for an example shear thinning system, shear viscosity is not only afunction of shear rate, but both shear and extension rate simultaneously, with their relative straining directions beingimportant. As summarised in figure 1, this is achieved through a novel two-phase flow experiment that measures shearstresses not at the wall, where extension rates are zero , but in-situ and at defined locations within an extensionalflow 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 tocapture this dependence. This work draws a parallel between our knowledge that liquid crystals display anisotropicviscosities when oriented to polymer melts and solutions which may be oriented under extensional flow. Thisdependence directly affects velocity fields in extensional flow situations (such as observed by Trebbin, et al. ) andhas relevance in situations from polymer extrusion through to the design of extensional rheology experiments thatcontain mixed flows.