Investigating the Phenomenon of Milk and Coffee Stirring

[Aquafire]

Milk Please..!

Part of my early morning ritual involves making myself a nice cup of coffee.

I pour my aromatic brew into the cup and then add a little sugar.

In the process of getting the sugar to dissolve, I give the contents a clockwise stir, turning the teaspoon, (not too vigorously, not to slowly) but just enough to cause a little centrifugal dip in coffee as it turns. (alternately you can say the centrifugal force pushes the coffee slight up the sides of the cup, with the centre of the cup being the lowest point)...I am happy with either...

Anyway, as this is happening, I take the milk carton, open it, and begin to pour milk in a thin stream directly over the top of the centre of the cup. This I do at a height of about 8 to 10 inches above.

Then a delightful and to my mind exciting thing happens.

In the process of pouring the milk into the centre of the cup, it seems to collect not only enough force, so that I can virtually see to the bottom of the cup; but even more wonderfully, a gap of about 3 to 5 millimeters opens up between the descending milk stream and the rotating coffee into which the milk is being poured.

In due course, it seems to me, that the wall of rotating coffee and milk, actually picks up a bit more rotational speed while the milk pushes down, out and up against the sides.

Eventually after about half a second or so, the air gap, dividing the stream of milk from the rotating coffee collapses inwards...thus obliterating that wonderful vortexed wall space.

What I'd like to know, is what is causing the gap between the descending milk stream and the rotating wall of mixing coffee and milk?

If you don't know what I am describing, go out and give it a shot...

For me, its one of those everyday mysteries...for which I do not have a full explanation...

So...please, if you can shed some light on this phenomena, then I am all ears...

Cheers

Aqua

 

[Valerie Park]

The gap between the descending milk stream and the rotating wall of coffee and milk is caused by the Bernoulli Effect. This effect occurs when a fluid (in this case, milk) is moving through a pipe or tube at different speeds. The faster moving fluid has lower pressure than the slower moving fluid, which creates a gap between the two streams. Additionally, the centrifugal force of the rotating wall of coffee and milk also helps to create the gap.

 

[charng]

I find this observation of milk and coffee stirring to be quite fascinating. It seems that the addition of milk to the rotating coffee creates a gap between the two liquids, which eventually collapses. This phenomenon could be due to a number of factors, including the difference in density and viscosity between the milk and coffee, as well as the speed and direction of the stirring motion.

One possible explanation is that the milk, being less dense than the coffee, creates a layer on top of the coffee that is pushed away by the centrifugal force of the stirring motion. This creates a gap between the two liquids, which eventually collapses due to the difference in viscosity and the forces acting on the liquids.

Another factor to consider is the height at which the milk is poured into the cup. By pouring it from a height of 8 to 10 inches, you are creating more force and energy in the milk stream, which could contribute to the creation of the gap between the milk and coffee.

Additionally, the shape and size of the cup could also play a role in this phenomenon. The shape of the cup and the direction of the stirring motion may create a specific vortex pattern, which could affect the behavior of the milk and coffee.

In conclusion, while there may not be a definitive answer to this everyday mystery, it is clear that there are multiple factors at play in the phenomenon of milk and coffee stirring. Further experimentation and observation could help to better understand the dynamics of this process and provide a more complete explanation. Thank you for sharing your observation and sparking curiosity in the scientific community.