Extensional rheometry of cellulose ether solutions: flow instability

Capillary breakup extensional rheometry of semi-dilute hydroxyethyl cellulose (HEC) solutions was performed under several step-stretch conditions. The resulting parameters, i.e. terminal steady state extensional viscosity (eta (E) ) and the timescale for viscoelastic stress growth, commonly referred to as the extensional relaxation time (lambda (E) ) were found to be sensitive to the step-stretch conditions. The lambda (E) decreased with increasing step-strain as opposed to the eta (E) . Prior to the filament break-up, a 'bead-on-string' instability was observed close to the mid-plane. It is believed that this instability originated from the accumulation of viscoelastic stresses near the filament neck leading to the 'elastic recoil' of the extended polymer chains. The reasons for this belief are discussed in detail with the perspective of the past literature. Such type of flow instability has been reported for the first time for a cellulosic system. Various dimensionless numbers were plotted for the HEC solutions and compared with those obtained from past studies for various biopolymers as well as synthetic polymers.