Paper Number
IN26 

Session
Flow-induced Instabilities in Non-Newtonian Fluids

Title
Shear-banding in multicomponent solutions with nonionic surfactants near the cloud point

Presentation Date and Time
October 13, 2022 (Thursday) 9:25

Track / Room
Track 5 / Sheraton 2

Authors (Click on name to view author profile)

1. Alexander, Nathan P. (University of Delaware, Chemical & Biomolecular Engineering)
2. Wagner, Norman J. (University of Delaware, Chemical and Biomolecular Engineering)

Author and Affiliation Lines (in printed abstract book)
Nathan P. Alexander and Norman J. Wagner
Chemical & Biomolecular Engineering, University of Delaware, Newark, DE 19716

Speaker / Presenter 
Alexander, Nathan P.

Keywords
colloids; flow-induced instabilities; polymer solutions; spectroscopy; surfactants

Text of Abstract
At temperatures approaching the cloud point, mixtures of nonionic tri-block copolymer in brine become unstable beyond a critical shear rate, resulting in the separation of the mixture into distinct regions (bands) stacked in the velocity gradient direction. Here, we examine gradient shear-banding and shear-induced alignment in semi-dilute, aqueous wormlike micellar solutions comprised of 10mM Synperonic P84 and 2M sodium chloride at temperatures in the vicinity of the cloud point. Two-dimensional small angle light scattering patterns were acquired under shear using a 50mm quartz parallel disk and Peltier plate capable of temperature control within 0.1C. Scattering patterns were recorded simultaneously with steady shear data for shear rates spanning 0.01-1000s^-1 at temperatures equal to T=39C or 41C. For both temperatures, the scattering patterns were isotropic in the absence of shear, but became increasingly anisotropic with increasing shear rate, with high intensity regions located perpendicular to the flow direction, indicating enhanced alignment of wormlike micelles in the direction of flow. Steady shear rheological data acquired via parallel disk, cone-and-plane, and concentric cylinder geometries indicate Newtonian behavior at T=25C and 30C, while broad stress plateaus, suggesting shear-banding, were observed at higher temperatures. In addition, time-dependent turbidity data was acquired via scans over the entire sample vial. The results indicate that clouding in P84/NaCl/water mixtures occurs uniformly in space and slowly over long time scales on the order of several days when the temperature is quickly raised from T=39C to 40C, which is several degrees below the reported cloud point T=43C. However, when the temperature was raised from T=39C to 41C, maximum turbidity is achieved on a shorter time scale on the order of several hours. Furthermore, strong temperature hysteresis was observed in both cases.