Paper Number
IN3 

Session
Flow-induced Instabilities in Non-Newtonian Fluids

Title
Confinement-induced migration of thermoresponsive gel clusters in cylindrical channel flow

Presentation Date and Time
October 11, 2022 (Tuesday) 4:25

Track / Room
Track 5 / Sheraton 2

Authors (Click on name to view author profile)

1. Hsiao, Lilian (North Carolina State University, Chemical and Biomolecular Engineering)
2. Smith, Kristine (North Carolina State University, Chemical and Biomolecular Engineering)

Author and Affiliation Lines (in printed abstract book)
Lilian Hsiao and Kristine Smith
Chemical and Biomolecular Engineering, North Carolina State University, Raleigh, NC 27606

Speaker / Presenter 
Hsiao, Lilian

Keywords
experimental methods; colloids; flow-induced instabilities; gels; microscopy

Text of Abstract
The interplay between flow and attractive interactions in colloidal gels results in complex particle trajectories and velocity profiles that are not entirely predictable from bulk rheological measurements. We use high-speed confocal microscopy to investigate the local velocity of a thermogelling nanoemulsion system as it flows through a cylindrical capillary. The nanoemulsions are comprised of fluorescent poly(dimethyl siloxane) droplets in a continuous phase of sodium dodecyl sulfate, deionized water, and a gelator molecule, poly(ethylene glycol diacrylate). The spatiotemporal evolution of the gel microstructure is obtained by directly visualizing the dispersed phase near the edge of a cylindrical channel. By heating the nanoemulsions to different temperatures above the gel point, attractive interactions between the droplets are increased. We observe the flow of the nanoemulsion gels at a range of shear stresses, from 1.5 – 10 Pa, and find that high shear stresses near the wall result in a fluidized structure, while low shear stresses near the center result in heterogeneous clusters and voids. The large clusters appear to be affected by confined flow and accumulate towards the central axis of the channel, resulting in a lower volume fraction near the channel walls. Cluster size and volume fraction variability in the radial direction are most prominent in the highest temperature experiments where the attractive interactions are the strongest. In high temperature experiments, a distinct transition from sparse, fluidized clusters near the walls to concentrated, large clusters towards the center is observed. These two structural states coincide with a velocity-based transition we previously observed during flow, from higher flow velocity and shear rate near the walls to lower velocity with an increase in variability towards the center of flow.