💥 How does cornstarch manage to flow through a pipe?

2024-07-16 06:00:08

Transported in pipes, rheothickening fluids, such as cornstarch in water or certain concretes, should in principle solidify when subjected to high pressures. However, this is not what is observed in practice.

Researchers from the Institut universitaire des systèmes thermiques industriels and Chryso France have discovered that the stresses are actually focused on a short section of the pipe, allowing other parts of the flow to remain fluid. Published in the journal PNASthis work concerns the numerous industrial uses of rheothickening fluids.

Composed of particles of about ten microns in suspension, rheothickening fluids flow like conventional viscous liquids under low stresses, but solidify as soon as they are vigorously sheared. This behavior comes from the repulsion between the particles, which slide under low stress and rub under high stress. This is why we sink into a pool of water mixed with cornstarch if we enter it gently, but not if we run fast enough. These materials, such as high-performance concrete before setting, chocolate paste or even certain magmas, nevertheless manage to circulate in pipes despite the pressure that should solidify them. Of course, the Speed stops increasing proportionally to the stress when the latter exceeds the level which should in principle solidify the Allbut the flow persists, without fluctuations in flow rate.

Researchers from the University Institute of Industrial Thermal Systems (JUSTCNRS/Aix-Marseille University) and the company Chryso France revealed and explained the mechanism of this very particular flow: the constraints are in fact concentrated on a short section of pipe, allowing the rest of the flow to remain fluid. This jamming zone, as short as the diameter from the pipe, leave passer the flow while going against the current.

Because shear-thickening fluids are dense suspensions, they are not transparent. The researchers were only able to observe the flow over the hundred microns closest to the pipe wall. They added fluorescent tracers that gave them the velocity gradients of the flow, along the wall and toward the center of the pipe.

They thus identified the wedging zone, called a frictional soliton because the grains rub against it and it rises up the flow like an isolated wave. The soliton extends continuously upstream while disintegrating downstream. There is only one soliton at a time in the pipe and it only appears when the pressure brings the stresses above the threshold of solidification. Once the soliton reaches the top of the pipe, it disappears, only to reform immediately at the bottom and resume its ascent.

The soliton forms at the end of the pipe as soon as the valve is opened. It moves upstream, spreading as quickly upstream as it disintegrates downstream, without ever blocking the flow.
© A. Bougouin

Pressure measurements along the pipe confirm that the pressure loss is concentrated in the section where the soliton is located, allowing the other sections to remain fluid. This behavior has been verified for several suspensions, consisting of grains of various composition, shape and size.

This work could help establish flow laws for many industrial applications, such as in construction or for the food industry. To do this, it remains to understand what happens when the flow also passes through bends and reservoirs.

References:
A frictional soliton controls the resistance law of shear-thickening suspensions in pipes.
Alexis Bougouin, Bloen Metzger, Yoël Forterre, Pascal Boustingorry & Henri Lhuissier.
PNAS2024.

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#cornstarch #manage #flow #pipe