2D and 3D Rheology

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Imaging and Rheology of Bulk Dense Suspensions

Understanding the deformation and flow, or rheology, of dense suspensions poses deep fundamental challenges and has wide-spread consequences on many industrial applications. In particular, non-linear phenomena such as shear thickening or thinning, non-uniform shear profiles (shear banding), wall slip and yielding are all of crucial importance to understand how dense particulate flow. Moreover, when the flow geometry has dimensions comparable to the size of the particles, confinement effects also become relevant.

Shear Thickening of Dense Granular Pastes

In this study we have established a direct link between the occurrence of the continuous and discontinuous shear-thickening transition in the dense granular suspensions and the friction between the flowing grains. In particular, we have demonstrated that shear thickening occurs when the liquid film providing hydrodynamic lubrication between grains breaks down and the grains enter into direct contact. Depending on the system's solid fraction and boundary lubrication coefficient, flow in the boundary lubricated regime can or cannot be sustained, giving rise to continuous or discontinuous shear-thickening flows, respectively. Moreover, we have found that the critical shear rate and volume fraction for discontinuous thickening can be tuned by tuning the boundary lubrication friction coefficient by adsorbing polymers on the particle surface. Polymers that give lower boundary friction allow for denser system to flow without discontinuous thickening.

a) Sediments of suspensions of quartz particles coated by different polymers. The polymers giving the lowest friction give the densest sediment. b) Flow curves of quartz suspensions as a function of volume fraction showing the transition from continuous to discontinuous shear thickening. c) Flow curves of quartz suspensions coated by the different polymers. Lowering the friction delays the onset of shear thickening. Figure from [29].
(a) - Sediments of suspensions of quartz particles coated by different polymers. The polymers giving the lowest friction give the densest sediment. (b) - Flow curves of quartz suspensions as a function of volume fraction showing the transition from continuous to discontinuous shear thickening. (c) - Flow curves of quartz suspensions coated by the different polymers. Lowering the friction delays the onset of shear thickening.

Imaging and Flow of Dense Suspensions

We have studied a variety of such non-linear phenomena by using confocal microscopy and particle tracking both in conventional model rheological geometries (e.g. cone-plate) as well as in microchannels. Direct observation of the suspension microstructure and dynamics at the single-particle level combined with controlled stress measurements enables unprecedented insight in understanding the physics of dense flows. The experiments required the development of both new instrumentation and image analysis software.

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a - 3D velocity profile of a dense colloidal suspension flowing in a square microchannel.
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b - Velocity oscillations in the flow of dense colloidal suspensions under confinement.

Colloidal monolayers at liquid-liquid interfaces under shear

The macroscopic response of colloidal systems to shear and deformation is directly coupled to their microscopic structural evolution. We investigate the steady shear response of dense binary monolayers of charged colloids spread at a water/oil interface. Colloids trapped at a liquid-liquid interface are an ideal model system to study the response of 2D crystals and glasses to external perturbations. In addition to 2D confinement, the presence of a liquid-liquid interface triggers the formation of crystalline regions with large lattice constants. A micro-fabricated magnetic disk is placed at the interface and rotated/oscillated in a controlled fashion by external magnetic fields.
Upon rotation of the microdisk, we observe two distinct regimes of motion. At small shear rates, the monolayers flows via discreet hopping of the particles, i.e., the shear induced motion is defect-mediated. At larger applied rates, the particles flow continuously and form alternating layers of small and big particles. Hopping Regions reveal a higher resistance to flow compared to the Flowing Regions, where spatial organization into layers reduces dissipation.
Upon oscillatory shear flows, the mechanical properties of the 2D-colloidal crystal strongly depend on the amplitude and frequency of the applied perturbation: large amplitudes and small frequencies lead to irreversible plastic rearrangements that alter the monolayer crystalline microstructure.

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Picture 1 - (a) Trajectories of particles in the Hopping (black) and in the Flowing (white) Regimes. (b) Long-exposure image of particle layers forming under steady shear. (c) Flow curves calculated for monolayers at different densities. Arrows denote the position of the FR-HR transition.
 
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23.07.2017
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