Paper Number
MM2 

Session
Microrheology and Microfluidics

Title
Characterization of consecutive phase transitions in a fibrous colloidal gel using μ²rheology

Presentation Date and Time
October 9, 2017 (Monday) 4:10

Track / Room
Track 6 / Aspen

Authors (Click on name to view author profile)

1. Wehrman, Matthew D. (Lehigh University, Chemical and Biomolecular Engineering)
2. Milstrey, Melissa J. (Lehigh University, Chemical and Biomolecular Engineering)
3. Lindberg, Seth (Procter & Gamble, Process and Engineering Development)
4. Schultz, Kelly M. (Lehigh University, Chemical and Biomolecular Engineering)

Author and Affiliation Lines (in printed abstract book)
Matthew D. Wehrman¹, Melissa J. Milstrey¹, Seth Lindberg², and Kelly M. Schultz^1
1Chemical and Biomolecular Engineering, Lehigh University, Bethlehem, PA 18015; ²Process and Engineering Development, Procter & Gamble, West Chester, OH 45069

Speaker / Presenter 
Schultz, Kelly M.

Text of Abstract
The gelation and degradation of colloidal particles is important in the design of materials used in commercial products, such as consumer and home care products. Of concern is whether the force that induces phase change can overcome processing history, particularly from shear stress. In this work, we characterize a hydrogenated castor oil (HCO) colloidal gel using μ²rheology, multiple particle tracking microrheology (MPT) in a microfluidic device. HCO is a colloidal fiber that gels and degrades in response to osmotic pressure gradients. Using μ²rheology, we determine the scaffold properties and structure during consecutive phase changes on a single sample. The microfluidic device design is a two-level device with two chambers: sample and suction chambers. Channels that deliver solvent from the second level into the sample chamber are spaced 60° around the chamber creating equal pressure around the HCO locking it in place during solvent exchange. MPT is used to characterize HCO during dynamic transitions. MPT measures the Brownian motion of particles embedded in the material and relates this movement to rheological properties using the Generalized Stokes-Einstein relation. μ²rheology measurements of consecutive phase changes of HCO start with a sheared solution of HCO (0.125 wt%) and an unsheared gel (4 wt%). Single samples are measured for 4 – 9 phase transitions. MPT measurements suggest the structure of the HCO is dependent on the starting materials shear history. When gelled, the sheared HCO solution cannot form a tightly associated, entangled network. During degradation, samples that begin as an HCO gel cannot completely degrade entanglements into single colloids. From these results, we conclude that the osmotic gradient is not a strong enough force to break or reform entanglements. Therefore, equilibrium structures depend on the shear history of the starting material, which can have important implications in end use products made with colloidal gel scaffolds.