Fascinating Demonstration.

Fascinating demonstration.

This video demonstrates one of my favorite effects: the reversibility of laminar flow. Intuition tells us that un-mixing two fluids is impossible, and, under most circumstances, that is true. But for very low Reynolds numbers, viscosity dominates the flow, and fluid particles will move due to only two effects: molecular diffusion and momentum diffusion. Molecular diffusion is an entirely random process, but it is also very slow. Momentum diffusion is the motion caused by the spinning inner cylinder dragging fluid with it. That motion, unlike most fluid motion, is exactly reversible, meaning that spinning the cylinder in reverse returns the dye to its original location (plus or minus the fuzziness caused by molecular diffusion).

Aside from being a neat demo, this illustrates one of the challenges faced by microscopic swimmers. In order to move through a viscous fluid, they must swim asymmetrically because exactly reversing their stroke will only move the fluid around them back to is original position. (Video credit: Univ. of New Mexico Physic and Astronomy)

More Posts from Coolbrzblu and Others

coolbrzblu

5 years ago

Want to play this so much..

Performance capture in Death Stranding

Record-Breaking Galaxy Cluster Confirms Dark Matter Universe

“But this new cluster is just 2.6 billion years old, and seems to be undergoing the very transition where a collection of galaxies falls into a bound structure for the first time, from a protocluster to a true galaxy cluster. This marks the first time astronomers have ever detected such an event: of the exact moment that a protocluster transitions to a true cluster. The fact that so many total galaxies (seventeen!) were discovered localized together at the same redshift (z=2.506) was a big hint, but the final piece of evidence came from the X-rays, where the diffuse emission engulfing the entire set of objects shows, beyond a shadow of a doubt, that this really is a galaxy cluster!”

There was once a time early on in the Universe where there were no stars, no galaxies and no clusters of galaxies at all. While stars and galaxies form very early on, after only tens or hundreds of millions of years, it takes billions of years for the first clusters to form. Yet even if we were to look back into the Universe’s past up to ten billion years, the clusters we see are already well-evolved and quiet. We had never seen a set of galaxies fall in and actively form a cluster before. We’d never seen the protocluster/cluster transition before. And we’d never found one from when the Universe was between two and three billion years old: when our dark matter theory predicts the first great clusters ought to form. Until, that is, now.

Come see how the Chandra X-ray observatory just found a record-breaking cluster that confirms our greatest picture of the Universe’s history!

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coolbrzblu

5 years ago

Good memories.