Paper Number
GG14
Session
Out of Equilibrium Systems: Gels and Glasses
Title
Controlling microstructures in depletions gels: Effects of quenching rate, depth and shear flow history
Presentation Date and Time
October 21, 2019 (Monday) 5:25
Track / Room
Track 7 / Room 306C
Authors
- Colombo, Gabriele (ETH Zurich, Department of Materials)
- Vermant, Jan (ETH Zurich)
Author and Affiliation Lines
Gabriele Colombo and Jan Vermant
Department of Materials, ETH Zurich, Zurich 8049, Switzerland
Speaker / Presenter
Vermant, Jan
Text of Abstract
Depletion flocculated colloidal gels are versatile and industrially relevant systems. Pronounced changes in their mechanical properties often result from subtle, highly localized microstructural rearrangements in a locally heterogeneous, disordered structure, which are difficult to resolve using ensemble averaged techniques as e.g. light or X-ray scattering, but require microscopy techniques. In the present work, we evaluate the microstructural basis of the rheological properties, and in particular of the thixotropy and yielding in gelled colloidal suspensions by directly imaging colloidal gels during transient simple shear deformations and steady state flows. The experimental approach relies on the quantitative study of the gel microstructure using ultra-high-speed, structured illumination confocal microscopy. Model systems of depletion flocculated PMMA-g-PHSA are used, and particular attention is given to controlling residual charges and how to screen those in a controlled manner by diffusion of an organic salt. This way, well controlled initial structures can be produced. Flow experiments are performed using a rheometer with a home-made shear cell for counter-rotation of the lower glass plate, allowing single particles to be imaged for long times at the stagnation plane. Fast axial scanning is implemented using an electric focus tunable lens, circumventing issues of immersion oil squeeze flow typical of mechanical focus repositioning. The unprecedented combination of temporal and 3D spatial resolution of our setup allows the study of intermediate volume fraction gels at high shear rates and with larger than usual fields of view, a critical factor to resolve the large length scales of structural heterogeneity developing in these systems. The results show that the anisotropic nature of a shear flow, combined with the subtle hydrodynamic interactions, first leads to an increased heterogeneity and anisotropy, which will be compared in detail with recent full hydrodynamic simulations.