Paper Number
SC12 

Session
Suspensions, Colloids and Granular Systems

Title
High speed confocal imaging of sheared colloidal gels

Presentation Date and Time
October 9, 2017 (Monday) 4:35

Track / Room
Track 3 / Crystal C

Authors (Click on name to view author profile)

1. Colombo, Gabriele (ETH Zürich, Department of Materials)
2. Vermant, Jan (ETH Zurich)

Author and Affiliation Lines (in printed abstract book)
Gabriele Colombo and Jan Vermant
Department of Materials, ETH Zürich, Zürich, Zürich 8093, Switzerland

Speaker / Presenter 
Colombo, Gabriele

Text of Abstract
Colloidal gels are an important class of soft materials with a wide range of technological applications, exploiting the combination of solid-like rest structure and liquefaction under mechanical stress of these thixotropic materials. Even though such complex flow behavior is widespread in a number of consumer products and industrially relevant systems, the changes in microstructure underlying thixotropy remain poorly understood. Recent scaling arguments propose a dependence of the mechanical properties of gels under flow on subpopulations of rigid, isostatic clusters of closely packed particles. Such a scaling does not rely on fractal geometry or glassy dynamics, which only take into account ensemble averaged descriptors and are therefore insensitive to highly localized events, which may determine the rheological response. In this work, we intend to test these ideas of cluster rigidity and evaluate the microstructural basis of thixotropy, varying the packing behavior in model colloidal gels by changing the particle aspect ratio slightly. The experimental approach relies on the quantitative study of the gel microstructure using high-speed confocal microscopy. Microscopic studies under flow are performed using a stress controlled rheometer with a home-made shear cell for counter-rotation of the lower plate, allowing single particles to be located and tracked for long times at the stagnation plane. The stress is directly measured, so that the link between microscopic observations and nonlinear rheology can be established. We also intend to clarify the role of microstructural anisotropy under flow, resulting in the butterfly scattering patterns observed for colloidal gels. Recent simulation approaches also show the emergence of large structural elements in the vorticity direction of flow, whose relevance for the rheological properties of gels was recently shown by two-dimensional oscillatory measurements on a model thixotropic gel.