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Dr. Rut Besseling School of Physics & Astronomy (Room 2607) James Clerk Maxwell Building Campus map University of Edinburgh Mayfield Road Edinburgh EH9 3JZ, UK rbesseli.at.ph.ed.ac.uk tel: +44 131 650 5234 / 5216 (office / lab) fax: +44 131 650 5902 |
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Methods: Primarily confocal microscopy and rheology. We have coupled a fast confocal scanner with a rheometer, allowing to measure rheological response (e.g. flow curves, moduli, creep) and simultaneously image the evolution of the micro-structure (in 3D) during these tests. We analyze the images in IDL to obtain both global dynamics and local particle dynamics under flow. In collaboration with Eric Weeks (Emory, Atlanta), we have developed a new method, Correlated Image Tracking, to follow particles in fast flow fields (see the tutorial or the paper). For a general description of particle tracking methods, click here.
| Flow of Emulsions: Emulsions and foams are fruitfull model systems to study 'jamming' of soft frictionless spheres and to test various predictions for the behavior around the so-called J-Point. Using fast confocal rheo-imaging we study the global flow properties, shear induced diffusion and the dynamics of force networks in various compressed emulsions. The movie on the right shows a projection of 3D confocal images of an oil in water/glycerol emulsion, sheared at rate of 0.2 s-1. See also Gijs Katgert's webpage. |  |
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Colloidal glasses and dense liquids under shear: Dense suspensions of (hard-sphere) colloids are a model system to study the dynamical 'slowing down' and disordered arrest typical for glass forming systems. Deformation, and in particular flow of these soft glasses is poorly understood on the microscopic level. In imaging-experiments under slow, glassy flow (Pe ≤ 1 ) of dense colloids, we have observed their 3D microscopic relaxation dynamics (left figure and paper), novel slip-behavior (paper)) as well as shearlocalization, depending on the wall roughness. Click here for more details. |
| Microfluidic flow of pastes: The flow of dense colloids (pastes) in 'non-rheometric' geometries such as microfluidic channels is largely unexpored. Experiments by Lucio Isa (now in ETH, Zurich), using narrow channels (width ~20-50x the particle diameter) and relatively fast flow (Pe > 1 ), have revealed a range of surprsing phenomena. One prime result is a scaling of velocity profiles in the channel with the flow rate. This differs markedly from typical yield-stress fluid behavior. Instead, the flow is very similar to granular pipe flow, showing the importance of 'contact-dynamics' under these conditions (see the paper). For the smallest channels and smooth walls, the flow also exhibits remarkably periodic fluctuations beyond a threshold speed, related to 'jamming' in self-organized constrictions (see the paper). |  |
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Rheology of colloid-polymer mixtures: The nature of yielding and flow-induced breakup of aggregates in physical gels is of strong practical and fundamental interest. In a number of pilot experiments on a colloid-polymer depletion gel, we have used fast rheo-imaging to establish the relation between microstructure during flow and bulk rheology of the system (left figure). Near the yield stress, the system shows partial breakup of clusters as well as slip (regardless of boundary roughness), while at large flow rate we observe a complete network breakup. These and similar experiments are being continued in collaboration with G. Petekidis and P. Ballesta, Crete. |
| Shear thickening: At sufficiently large stress or flow rate, dense colloids show a dramatic increase in viscosity, know as 'thickening' (right figure). There are competing theories for the cause of this phenomenon, either based on strong hydrodynamic forces and clustering, or a theory based on stress induced jamming. We try to disentangle the microscopic mechanism for thickening by simultaneous cone-plate rheology and fast confocal confocal (3D) imaging. In experiments started by Lucio Isa, we have observed strong confinement effects and fluctuations at the onset of thickening, and managed to track the 3D microstructure during thickening. |  |
Most recent publications/preprints:
- Quantitative imaging of concentrated suspensions under flow
L. Isa, R. Besseling, A. B. Schofield, and W. C. K. Poon, Review, to appear in 'High Solid Dispersions' ed. M. Cloitre, Adv. in Polymer Science (Springer). pdf - Velocity oscillations in microfluidic flows of concentrated colloidal suspensions
L. Isa, R. Besseling, A. N. Morozov, and W. C. K. Poon, Phys. Rev. Lett. 102, 058302 (2009). pdf - Quantitative imaging of colloidal flows
R. Besseling, L. Isa, E. R. Weeks, and W. C. K. Poon, Adv. Coll. Int. Sci. 146, 1 (2009). pdf - Slip and flow of hard-sphere colloidal glasses
P. Ballesta, R. Besseling, L. Isa, G. Petekidis, and W. C. K. Poon, Phys. Rev. Lett. 101, 258301 (2008). pdf