Abstract
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 (η E ) and the timescale for viscoelastic stress growth, commonly referred to as the extensional relaxation time (λ E ) were found to be sensitive to the step-stretch conditions. The λ E decreased with increasing step-strain as opposed to the η 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.
Highlights
Extensional rheometry of polymeric, nanoparticulate and associating fluids has been a subject of interest for many researchers for last three decades following the realization that the deformation regimes in many industrial and biological processes have a significant extensional component (Duxenneuner et al 2008; Patruyo et al 2002)
The rate constants associated with the exponential and linear capillary thinning allow the determination of an extensional relaxation time kE and quasi-steady state terminal extensional viscosity gE respectively
This study aims to investigate the capillary thinning of semi-dilute hydroxyethyl cellulose (HEC) solutions with high-speed imaging
Summary
Extensional rheometry of polymeric, nanoparticulate and associating fluids has been a subject of interest for many researchers for last three decades following the realization that the deformation regimes in many industrial and biological processes have a significant extensional component (e.g. coating, atomization, flow through porous media and sensory perception in the case of food products) (Duxenneuner et al 2008; Patruyo et al 2002). Extensional deformation is encountered whenever a fluid undergoes a change in its cross-sectional area of flow. This includes but is not limited to flow through converging channels, capillary entrance flows and free surface capillary thinning (Gonzalez et al 2005). In CaBER, a fluid filament is subjected to step-strain and the mid-plane diameter of the filament undergoing capillary driven thinning is recorded as a function of time (Fig. 1). From the perspective of processability, breaking up of an initially stable fluid filament is a process which has commercial applications such as spraying and atomization of pesticides, applying paints and adhesives, coating and food processing operations e.g. container filling (McKinley 2005b)
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