Extensional rheometry of cellulose ether solutions: flow instability

Abstract

Capillary breakup extensional rheometryof semi-dilute hydroxyethyl cellulose (HEC) solutionswas performed under several step-stretch conditions.The resulting parameters, i.e. terminal steady stateextensional viscosity (gE) and the timescale forviscoelastic stress growth, commonly referred to asthe extensional relaxation time (kE) were found to besensitive to the step-stretch conditions. The kEdecreased with increasing step-strain as opposed tothe gE. Prior to the filament break-up, a ‘bead-onstring’instability was observed close to the mid-plane.It is believed that this instability originated from theaccumulation of viscoelastic stresses near the filamentneck leading to the ‘elastic recoil’ of the extendedpolymer chains. The reasons for this belief arediscussed in detail with the perspective of the pastliterature. Such type of flow instability has beenreported for the first time for a cellulosic system.Various dimensionless numbers were plotted for theHEC solutions and compared with those obtained frompast studies for various biopolymers as well assynthetic polymers