Paper Number
CS49 

Session
Colloids & Suspensions

Title
High speed confocal microscopy of sheared colloidal gels

Presentation Date and Time
20 April 2018 (Friday) 17:10

Track / Room
Track 1 / Sirene

Authors (Click on name to view author profile)

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

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

Speaker / Presenter 
Colombo, Gabriele

Text of Abstract
Colloidal gels represent an interesting family of soft materials. Despite the wide range of possible technological applications, the microstructural details underlying their typical solid-to-liquid transitions upon shear remain poorly understood, therefore posing challenges for industrial formulations. This stems from their hierarchical, strongly heterogeneous structure, lying at the roots of their bulk mechanical properties. The rheology of colloidal gels is very sensitive to the applied flow history and typically shows a complex behavior including a yield stress and thixotropy. Dramatic changes in mechanical properties may well result from subtle, highly localized microstructural changes, which are difficult to resolve using scattering experiments. In this work, we intend to 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. Such anisotropic structural features are observed here in sheared model, depletion colloidal gels, as evidenced by the Fourier transform power spectra and by real space analysis.