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Multiphase and Particle-laden Flows

Carbon nanotube manufacture

Carbon nanotube synthesis reactor close-up

Carbon nanotube synthesis reactor swirl

 

 

 

These images were taken in a carbon nanotube (CNT) synthesis reactor, whereby gaseous precursors react to form catalyst particles and eventually carbon nanotubes.

 Credit: Adam Boies

 

 

 

Top: As the carbon nanotubes continue to react, they agglomerate to form an aerogel which can be pulled from the reactor for industrial production of CNT sheet. 

The web formation is the aerogel as it forms in the reactor. It is composed of CNTs 10 nm in diameter that agglomerate to form structures the eye can see >1 μm.

Click on the image to show a close-up of the central region.

Bottom: The "swirl" is a deposition on the reactor wall during the process that gives us some indication of the flow dynamics within the reactor. The reactor diameter is 5 cm.

 


 

High-density colloids

 Force chains in shear-thickening fluid

Credit: Romain Mari

Mixtures of solid particles immersed in a fluid (called suspensions) often display a complex flow behaviour, especially at high solids loading. In particular one finds non-Newtonian fluids whose viscosity depends strongly on the flow rate. One striking example is the shear thickening phenomenon, where the viscosity increases dramatically in a narrow range of flow rate. This can famously be observed in mixtures of cornstarch powder dispersed in water: a kitchen experiment clearly shows the mixture turning from liquid-like to pasty flow or even solid fracture when stirred with increasing force. Shear thickening marks a transition from a situation where the grains do not touch, and can slide past each other smoothly thanks to a lubricating fluid film between them, to a situation where the lubrication films break and the grains make frictional contacts, impeding their relative motion.

We are developing coarse-grained mathematical models for dense suspension flows, informed by extensive numerical simulations at the particle scale. In simulations, the emphasis is on how qualitative changes in the microscopic interactions between solid particles influence the macroscopic mechanical behaviour. The picture shows a simulated shear-thickening suspension subject to a shear as indicated by the arrows, highlighting in yellow the contact forces created between the blue grains. The width of each yellow segment is proportional to the force carried. These forces organize in chains, in a manner reminiscent to the contact forces in a sand pile, except that these force chains bear a very clear signature of the shear direction, with a majority of them lying along a 45 degree angle with the vertical direction. These force chains are responsible for the increase of the viscosity of the system.