Coffee Grains Mysteriously Moving: A Puzzling Sight

[TuviaDaCat]

once whlie stirring on the top of cup filled with black cofee i saw something really cool, yet disturbing...

the grains floating on the upper surface, in the middle if the plain were randomly moving, and at a certain distance from the cup edge it rapidly move toward the edge and sticked to it.
then i remembered that once in a museom i saw tiny polystyrene in the room's atmosphere, and it acted exacly the same as the cofee grains.

 

[tommyers]

I once read a book titled 'choas' by James Gleick and as the name suggests it decribes the theory of choas.

In the beginning chapters of that book it spoke about the work of Lorenz, and his work on predicting weather systems, in particular convection currents. Lorenz described the sometimes choatic motion (swirling) of the convection current in a cup of hot coffee!

If the coffee is just warm it will cool without hydrodynamic motion. However if the coffee is hot then hot coffee from the bottom of the cup will rise to the cooler top surface and cool and fall back to the bottom of the cup whilst more hot coffee rises to the top etc... this hydrodynamic process creates complex convection currents and he stated that they were noticable if one added a little cream to the coffee, I guess you saw the same effect using your coffee granuals.

The same phenomenon can occur in rooms that contain the same convection currents i.e from the baking sun or radiators. This may also explain your polystyrene observations.

Regards

Tom

 

[TuviaDaCat]

well, it can't be connected to thermodynamics, i suspected someone would suggest that, which is pretty much legitimate lane to explore, which is the reson i mentioned the polystyrene case. the room was at about 20 degrees, and the water were simply put in a metal container. so i doubt is about temperature differences.
i think there was some very short explanation on the phenomenon, it think it had something to do with the surface curvature, and einstein was mentioned also...
i guess thay if Albert's name was mentioned it must be something big and scary...