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I'm sitting in front of my computer and just made myself of cup of tea ("Brasilianische Limette") and see the water on the top evaporating into the room. But what caught my eye was a small rotating veil. Sometimes the veil tries to move to the center but gets pushed back by a turbulence. Sometimes parts or regions of the veil suddenly disappear.

Why does the veil sometimes disappear instantly? Please explain for someone who has basic physics knowledge ;)

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  • $\begingroup$ At my next cup I will make a foto if it helps. $\endgroup$
    – velop
    Aug 2, 2016 at 9:25
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    $\begingroup$ A photo would definitely make it much easier (make it possible?) to know what you're talking about. $\endgroup$ Aug 2, 2016 at 10:14
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    $\begingroup$ I presume you are talking about a thin veil less than a millimetre above the liquid. In my experience the tea has to be without milk for this to happen and the air needs to be still. My first guess was that water vapour from the tea condenses and forms a cloud layer above the tea, but I get puzzled about why the cloud layer doesn't rise with the rising warm air. A dark liquid is best for this, so it should be strong tea, or even black coffee. (It may be that the milk is only relevant because tea with milk is lighter in colour than tea without). $\endgroup$ Aug 2, 2016 at 10:25
  • $\begingroup$ The full write-up of the study mentioned in the accepted answer can be found here: nature.com/articles/srep08046 $\endgroup$
    – Ulthran
    Jul 13, 2018 at 11:23

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Probably what you saw is the effect explained in this physics arXiv blog : The Mysteries Of White Mist On The Surface of Black Coffee

The effect was first reported by the Japanese physicist Terada in the 1920s.

The arXiv paper, also from a Japanese team, is dated January 2015. They report that the white mist is a cloud of tiny water droplets, about $10~ \mathrm{\mu m}$ in diameter, levitating $10-100~ \mathrm{\mu m}$ above the surface of hot tea or coffee - or indeed any hot-water-based beverage. Since it is a white mist, it is seen more easily against black tea or coffee. I suspect also that after the addition of milk the liquid is not hot enough for the mist to levitate.

The mist is thought to originate from vapour condensing in cooler air above the hot beverage and falling down like rain onto the surface. It floats on rising hot air/vapour from the hot drink and forms a triangular lattice, suggesting that the droplets are charged.

The mist can vanish in about 1/30 of a second, at the same speed as capillary waves - which suggests that it is being swamped by a surface capillary wave. The trigger for such events is one droplet touching the surface, presumably after being hit by a cosmic ray and changing its charge. Other effects include "whiffling" as though wafted by a gentle breeze, a splitting of the lattice by a $1~\mathrm{mm}$ wide rift, and the formation of complex patterns.

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  • $\begingroup$ They don't actually make any presumptions about what causes the droplets to float. There are theories that it is caused by the rising hot air but there are also theories that it is related to their charge or to thermo-capillary convection (aka the Marangoni effect). $\endgroup$
    – Ulthran
    Jul 13, 2018 at 11:25

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