Paper Number
ET1 

Session
Advanced Experimental Techniques/Methods in Rheology

Title
Using rheo-microscopy to directly correlate structural and mechanical aging in a thermoreversible colloidal gel

Presentation Date and Time
October 15, 2018 (Monday) 9:50

Track / Room
Track 5 / San Felipe Room

Authors (Click on name to view author profile)

1. Nguyen, Tuan (University of California, Santa Barbara)
2. Helgeson, Matthew E. (University of California, Santa Barbara)

Author and Affiliation Lines (in printed abstract book)
Tuan Nguyen and Matthew E. Helgeson
University of California, Santa Barbara, Santa Barbara, CA

Speaker / Presenter 
Nguyen, Tuan

Text of Abstract
Colloidal gels at moderate volume fraction coarsen and age through a complex coupling of kinetic processes including aggregation, phase separation and glass-like arrest, significantly influencing their final structure and mechanical properties. Understanding this coupling is necessary for rheological design, yet requires advanced methods for simultaneous structural and mechanical characterization. Here, we utilize rheo-microscopy on a well-characterized colloidal system with thermoreversible attractions to understand how thermal quenching influences the development of colloidal gel structure and rheology through the coupling of gelation and arrested phase separation. Synchronized linear viscoelastic measurements and bright-field optical microscopy, along with custom image analysis, resolve morphological and structural features of phase separation at length scales much larger than individual colloids. The results provide a unique window into the relationship between rheological aging and coarsening of large-scale structure. For example, we find that varying the quench rate and depth provides control over the relative kinetic time scales for gelation and arrest. Time-resolved texture analysis microscopy reveals a linear growth rate in the dominant length scale of phase separation that is consistent with late-stage spinodal decomposition. Differential dynamic microscopy analysis reveals mechanisms by which the coarsening occurs, namely the intermittent storage and release of elastic stresses driven by the advection of phase separation. Finally, directly comparing the structural and rheological data provides new insight into the features of gel rheology that are dominated by large-scale structure as opposed to more local measures. Overall, these observations improve our understanding of thermally processed colloidal gels and demonstrate the utility of rheo-microscopy for characterizing kinetic processes in arrested fluids.