Paper Number
GG10
Session
Out of Equilibrium Systems: Gels and Glasses
Title
Large-scale structural rearrangement during yielding of heterogeneous colloidal gels revealed by rheo-microscopy
Presentation Date and Time
October 21, 2019 (Monday) 3:45
Track / Room
Track 7 / Room 306C
Authors
- Nguyen, Tuan (UC Santa Barbara)
- Shetty, Abhishek (Anton Paar, Rheology Division)
- Helgeson, Matthew E. (University of California, Santa Barbara, Chemical Engineering)
Author and Affiliation Lines
Tuan Nguyen1, Abhishek Shetty2, and Matthew E. Helgeson1
1UC Santa Barbara, Santa Barbara, CA 93106-5080; 2Rheology Division, Anton Paar, Ashland, VA 14850
Speaker / Presenter
Helgeson, Matthew E.
Text of Abstract
Colloidal gels can show complex behavior during the transition to flow, including rate-dependent yield stresses and strains, thixotropy and shear localization. This behavior is usually hypothesized to arise from structural or dynamical heterogeneity within the gel. However, confirming the role of heterogeneity during yielding remains elusive due to a lack of methods to measure structure and rheology simultaneously. We report novel rheo-microscopy measurements to track the evolution of structure in model heterogeneous colloidal gels during the yielding transition. We employ an established thermoresponsive colloidal system to form pristine gels in situ with controlled length scales of heterogeneity due to arrested phase separation. We probe the evolution of gel structure during yielding in both creep and large amplitude oscillatory shear measurements. The former provide unambiguous measures of changes in gel structure during yielding, whereas the latter allow separation of viscous and elastic contributions to the stress using local measures of the stress trajectory. We find that yielding proceeds through a sequence of processes involving large structural rearrangements at the length scale of heterogeneity. The onset of yielding is marked by a significant overshoot in viscous response coincident with the formation of large fluid voids, suggesting that poroelastic deformation drives the initial transition to flow as well as large-scale concentration fluctuations. These concentration fluctuations are initially isotropic, and become aligned along the vorticity direction at large strains, consistent with what is observed in experiment and simulation on attractive colloidal suspensions. Importantly, we find that the length scale of these flow-induced structures is seeded by the initial length scale of heterogeneity, providing a route to control the rheological signatures of yielding. These results reveal the importance of large-scale structure in understanding the nonlinear rheology of colloidal gels.