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Author Topic: Why do tea leaves rotate in the middle of the cup when stirred?  (Read 6558 times)

Offline Tommo

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I noticed when stirring a cup of tea that the teabag had burst and allowed some of the tea leaves to exit the bag.  When I looked in the cup I saw that all the tea leaves were whizzing around in the center of the cup in a column. 

I would have thought that centrifugal forces would have thrown the tea leaves out to the edges of the cup.

My colleagues and I discussed the probable reasons for this and we thought that one possible cause was that the tea leaves were lighter than the surrounding water and therefore were pushed into the middle.  We discounted this theory as there were leaves on the bottom of the cup clearly they were heavier that the water and the above explanation did not apply.

Can anyone explain why the tea leaves migrate to the center and not to the perimeter

MOD EDIT: Hi Tommo, and welcome! Great question BTW.
« Last Edit: 12/01/2010 20:12:32 by Tommo »


 

Offline yor_on

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Brownian motion?
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Or and the tea leaves surfaces binds together with the help of the water molecules/steam??

===

Btw: Does it work the same in all kind of teacups, no matter their shape?
« Last Edit: 10/01/2010 03:31:58 by yor_on »
 

Offline RD

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Possibly lower pressure in the centre because of the curvature of the surface caused by the rotation,
i.e. the height of liquid above the centre of the cup is lower than at the perimeter,
 (here the curvature is exaggerated compared with a typical stirred cuppa)...

http://books.google.co.uk/books
« Last Edit: 10/01/2010 05:29:20 by RD »
 

Offline Geezer

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Nice DD, but, according to the the experimental results, the tealeaves are arranged in a column. This would suggest that they have a range of densities. If they were all more dense than the water, why would they not all be at the bottom of the column?

The effect seems to affect tealeaves with a range of saturations equally.
 

Offline Tommo

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I have found that the technique of using a rotating body of liquid is used in water filtration.  If a body of water containing suspended particles is required to be filtered is pumped into the top of a circular vessel in such a way as to create a vortex within the vessel.  It can be seen that all the suspended particles collect in a rotating column in the center of the vessel.  As more and more particles enter the vessel the suspended particles slowly drop to the bottom where they can be removed by opening an exit port located below the rotating column.  Clean water can be removed through an opening near the bottom of the vessel.

This technique is used to filter suspended particles from fish ponds.
 

Offline Tommo

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Dear DiscoverDave,

What I have seen in the past works like this:

Water is pumped from the fish pond up into a plastic barrel (size dependent on your pond).  The barrel resides above the surface of the pond and preferably to one side.  The water is pumped from the pond and injected into the barrel just below the top so that it forms a vortex.  Water is extracted at the bottom of the barrel where it exits the barrel at an angle which preserves the vortex.  The exit pipe is thus placed so that it runs up the outside of the barrel where air is allowed to enter the pipe (this stops the barrel emptying due to syphoning action).  Once the unit is running the barrel is always kept full of water and when looked upon, the water will be swirling around inside.  Once the unit is up and running, suspended particles migrate to the center and fall to the bottom over a period of time into a collection chamber (this may be in the shape of an upside down funnel) where, from time to time, a valve is opened to allow the collected particles to be flushed away (say onto the garden), the valve is then closed.  The frequency of opening the valve may be frequent at first (say once per day), reducing this as required as the pond is cleared.  Normally, the output from the particle filter is then fed into a biological filter (such as a UV filter and bacterial filter) before returning back to the pond. 

These units are available commercially but for some reason are very expensive to buy. 

Hope that this was of use to you.
 

Offline yor_on

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RD do you mean that gravity should have an effect or?
Ah, okay the rotation..

Awh sh* I missed that one. Look? When you stir something you make a rotating moment right, a little like frame dragging, and the center of that spiraling motion will always be the center, won't it.

But shouldn't that 'force' work outwards, pushing things to the perimeter, like a tumbler?
 

Offline Geezer

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How about this?

Initially, the tealeaves are not saturated. When you put tealeaves in water, they float for a while until they saturate and sink to the bottom of your cup/teapot.

So, while the tealeaves are still boyant, they are simply floating in the water. When you create a vortex, the leaves are still floating at the boundary between the air and the water within the vortex. Hence the observed column.

If this is true, the tealeaves will drop out of the vortex when they are completely saturated and they lose their buoyancy. (Which seems to be confirmed by the observations.)

We know it's extremely difficult for a floating object to escape from a vortex (think whirlpool) because there is a continuous flow of water into the top of the vortex. The water level at the top of a whirlpool is always lower than the surrounding water. To escape, the floating object has to "swim uphill".

I will attempt to pinch some of Mrs G's Earl Gray and conduct an experiment.
 

Offline Tommo

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To understand the problem I would suggest that a tea bag is sacrificed and a small amount of tea leaves be placed in a glass of cold water.  Stir the water (don't go too fast as to cause air to be sucked into the vortex) and it can be seen that despite water entering or leaving the vessel the leaves will congregate in the middle as a column (depending on their saturation), if left to soak and the experiment repeated, the leaves will attain a distributed position in the column depending on there mass and absorption.  Repeat the actions above only this time use hotter water.
« Last Edit: 10/01/2010 22:48:52 by Tommo »
 

Offline yor_on

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Okay, think i got the 'force' of it.

When you stir your teacup you create a 'vortex' a spiraling force sucking in all matter to the center of it.

". A mass of fluid, especially of a liquid, having a whirling or circular motion tending to form a cavity or vacuum in the center of the circle, and to draw in towards the center bodies subject to its action; the form assumed by a fluid in such motion; a whirlpool; an eddy. "

As there is no hole in the bottom of the teacup the only air you get in the middle will be from the rotating movement of your spoon. I was thinking wrongly when comparing it to a tumbler. A tumbler resembles you spinning around holding a string with some heavy object at the end. In that case the force is directed outwards.

A vortex is more like a diminishing spiral where the force is directed inwards. Does this make sense?
 

Offline Tommo

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Why do tea leaves rotate in the middle of the cup when stirred?
« Reply #10 on: 15/01/2010 14:01:01 »
yor_on.  I see what you mean about the tumbler action.  Any objects will be thrown outwards and compressed against the edge, this is what is expected from the tea cup but in actual fact the opposite occurs hence the confusion. 

I think we can look at the 2 components of the vortex independently, horizontal and vertical.

Horizontal.  I think the horizontal components of the tea leafs lay in the fact that the the center of the vortex travels slower than the outer edges.  If we can imagine the tea leave is suspended in the water, as the tea starts to rotate in the cup, the side of the tea leaf that faces the outside of the cup will experience a faster flow of tea than the side facing the inside of the cup.  This will have the property of causing the tea leaf to rotate.  A low pressure zone will be set up on the inside of the tea leaf and as stuff flows from high pressure to low pressure the tea leaf will have a tendency to be pulled towards the center (similar to the actions of a football curving when kicked from a spot kick.  Here the ball will curve in the direction opposite to the side that was struck.

Vertical.  The vertical components of the tea leafs depend upon their abortion of liquid.  The more liquid they absorb the denser they become and the lower they will settle.

Settling zone.  Where the tea leafs settle depends on both the vertical and horizontal components.  When the tea leafs become saturated, i.e. they cannot absorb any more liquid they sink to the bottom of the cup.  The horizontal components will 'push' them into the center forming an inverted cone.  The tea leafs will be 'stacked-up' in the center of the vortex depending on buoyancy of each tea leaf.   

An interesting demonstration you can perform yourself.  Using a cereal bowl full with water, sprinkle tea leafs on top and let them soak up the water.  Stir the water and let everything settle in the bottom.  Take a tea spoon and drag it through the pile of tea leafs.  It will be noticed that 2 vortexes (one either side of the spoon) will form, 1 in either half of the bowl, and will each vortex take on all the properties of the single vortex i.e. there will be 2 piles of tea leafs containing halt each of the total amount of leafs.  The point behind this demonstration is to show that the site where the tea leafs pile up is independent of vessel shape but will come to rest in the center of the vortex(es)
 

Offline yor_on

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Why do tea leaves rotate in the middle of the cup when stirred?
« Reply #11 on: 16/01/2010 10:46:12 »
Beautifully put, that last one Tommo. It's a direct proof of the concept of a vortex and how it's 'force' is directed inwards, towards its center. If it wasn't so then you would not get two 'pillars' from one simple movement with your spoon. I'm sure one could make that one into a really intriguing riddle too :)

'How can you by one moment prove/ create... ' or similar ::))
 

Offline Tommo

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Why do tea leaves rotate in the middle of the cup when stirred?
« Reply #12 on: 17/01/2010 11:32:11 »
I seem to have got to the bottom (no pun intended) of the tea leaf phenomenom.  It is all to do with a property called 'Secondary Flow'.  Albert Einstein notice this phenomenon and became a topic of discussion for him and it basically goes like this:

If a body of liquid / air is in rotation then if we draw lines between points of equal pressure we end up with rings within rings.  Each ring is at one pressure.  Flow is normally in the direction of the ring but if it is a column (Primary Flow), the flow at the bottom is subject to friction on the ground.  A second flow can be found from one ring to another (higher pressure to lower pressure) caused by the difference in pressure between rings (Secondary Flow).

So as the tea leaf saturates it falls to the bottom of the vessel.  Pressure caused by Secondary Flow makes all the tea leafs migrate to the center of the vortex.  The same phenomenon can be found if watching a small dust devil in Autumn where the dry fallen leaves rotate at the corner of buildings.

For further reading and more detail, go to newbielink:http://en.wikipedia.org/wiki/Secondary_flow [nonactive]
 where more in depth knowledge is located.
« Last Edit: 17/01/2010 13:32:58 by Tommo »
 

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Why do tea leaves rotate in the middle of the cup when stirred?
« Reply #12 on: 17/01/2010 11:32:11 »

 

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