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I have been told that industrial mixing machines (say, for cake batter) switch directions periodically, first stirring in one direction, then the other, because this mixes the material more thoroughly.

I imagine (but don't know for sure) that stirring in only one direction will tend to create helical structures in the mixed material, where each helix is more or less uniform but two helices might be quite different from one another; and that switching directions tends to break up and mingle these helices. Is this at all correct?

Is there a way to quantify the effectiveness of different methods of stirring? If so, how much better is it to stir in alternating directions, and how often should one switch directions?

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Mixing means and requires turbulence.

Single direction stirring can settle into a pretty laminar regime at least some of the time. Abruptly reversing direction would break up that order for a while. SO would abrupt stops and starts or just running the machine in a mode where the motion of the blades has a highly turbulent Reynolds number.

How much reversal do you need? Well, that the crux of it and the devil is in the details. I presume that they settle this question empirically. Engineers can be very simple that way.

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When you say "laminar regime" I imagine concentric cylinders. Is that what you had in mind? If so, why would it help to reverse direction? – MJD May 7 '12 at 13:29
@MarkDominus Exactly. Youtube host some videos of an extreme example: almost completely reversable mixing is a highly viscous material. – dmckee May 7 '12 at 13:32
It occurs to me that the youtube link doesn't actually support my claim. I will edit to be clear that it is turbulence that matters. – dmckee May 7 '12 at 14:04
Well, I think you might also say that the spoon's contact with the liquid and the relative speed between spoon and liquid is what matters. When it's so, you achieve a higher relative speed if you move your spoon and the liquid is moving in the opposite direction at the moment. That's achieved if you alternate the direction. – Luboš Motl May 7 '12 at 14:07

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