Sticky walls slow mixing – from Physics World

Anyone who has mixed paint, cake batter or any other viscous fluid
knows that the sides of the container must be scraped occasionally
because unmixed fluid tends to stick there. Although the effect of
sticky walls on mixing is difficult to quantify, physicists in France
and the UK have now done experiments and calculations that reveal how
mixing is inhibited by the walls of a vessel. The results could lead to
a better understanding of mixing in a wide range of disciplines from
geophysics to microfluidics (Phys. Rev. Lett. 99 114501).

Read more in Physics World or PRL

I like most the following part:

By studying time-sequences of the images and via numerical simulation,
the team concluded that “laminar chaotic mixing” was occurring in the
vessel. This is a well-known process by which fluids are mixed together
gently without any turbulence.The team was able to describe this power-law mixing in a theoretical
model based on the “baker’s map” — the process by which a baker kneads
dough by first stretching it and then folding it together. If this
process is repeated, it leads to chaos. The effect of the sticky walls
was modelled in terms of a periodic injection of strips, representing
unmixed material, into the folds of the baker’s map.While Thiffeault admits that engineers already have a good practical
understanding of mixing, he says that industry could benefit from
having a deeper knowledge of the physical processes involved. One field
that could benefit from a better understanding of the effects of
surfaces on chaotic mixing is microfluidics – which is concerned with
the transport and mixing of fluids along very thin channels. The
surface to volume ratio of microfluidics vessels is very large and
therefore mixing is a great challenge to those designing such devices.

Mixing bowl – Physics World – physicsworld.com

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