fluid dynamics In physics, physical chemistry and engineering, fluid dynamics is a subdiscipline of fluid mechanics that describes the flow of fluids – liquids and gases. It has several subdisciplines, including (the study of air and other gases in motion ...

, the tea leaf paradox is a

phenomenon A phenomenon ( phenomena), sometimes spelled phaenomenon, is an observable Event (philosophy), event. The term came into its modern Philosophy, philosophical usage through Immanuel Kant, who contrasted it with the noumenon, which ''cannot'' be ...

where tea leaves in a

cup A cup is an open-top vessel (container) used to hold liquids for drinking, typically with a flattened hemispherical shape, and often with a capacity of about . Cups may be made of pottery (including porcelain), glass, metal, wood, stone, pol ...

tea Tea is an aromatic beverage prepared by pouring hot or boiling water over cured or fresh leaves of '' Camellia sinensis'', an evergreen shrub native to East Asia which probably originated in the borderlands of south-western China and nor ...

migrate to the center and bottom of the cup after being stirred rather than being forced to the edges of the cup, as would be expected in a spiral

centrifuge A centrifuge is a device that uses centrifugal force to subject a specimen to a specified constant force - for example, to separate various components of a fluid. This is achieved by spinning the fluid at high speed within a container, thereby ...

. The correct physical explanation of the paradox was for the first time given by James Thomson in 1857. He correctly connected the appearance of

secondary flow In fluid dynamics, flow can be decomposed into primary flow plus secondary flow, a relatively weaker flow pattern superimposed on the stronger primary flow pattern. The primary flow is often chosen to be an exact solution to simplified or approxim ...

(both

Earth atmosphere The atmosphere of Earth is composed of a layer of gas mixture that surrounds the Earth's planetary surface (both lands and oceans), known collectively as air, with variable quantities of suspended aerosols and particulates (which create weather ...

and tea cup) with "

friction Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. Types of friction include dry, fluid, lubricated, skin, and internal -- an incomplete list. The study of t ...

on the bottom". The formation of secondary flows in an annular channel was theoretically treated by

Joseph Valentin Boussinesq Joseph Valentin Boussinesq (; 13 March 1842 – 19 February 1929) was a French mathematician and physicist who made significant contributions to the theory of hydrodynamics, vibration, light, and heat. Biography From 1872 to 1886, he was appoin ...

as early as in 1868. The migration of near-bottom particles in river-bend flows was experimentally investigated by A. Ya. Milovich in 1913. The solution first came from

Albert Einstein Albert Einstein (14 March 187918 April 1955) was a German-born theoretical physicist who is best known for developing the theory of relativity. Einstein also made important contributions to quantum mechanics. His mass–energy equivalence f ...

in a 1926 paper in which he explained the

erosion Erosion is the action of surface processes (such as Surface runoff, water flow or wind) that removes soil, Rock (geology), rock, or dissolved material from one location on the Earth's crust#Crust, Earth's crust and then sediment transport, tran ...

of river banks and repudiated Baer's law.English translation: The Cause of the Formation of Meanders in the Courses of Rivers and of the So-Called Baer's Law
accessed 2017-12-12.

Explanation

The stirring makes the water spin in the cup, causing a

centrifugal force Centrifugal force is a fictitious force in Newtonian mechanics (also called an "inertial" or "pseudo" force) that appears to act on all objects when viewed in a rotating frame of reference. It appears to be directed radially away from the axi ...

outwards. Near the bottom however, the water is slowed by friction. Thus the centrifugal force is weaker near the bottom than higher up, leading to a secondary circular (helical) flow that goes outwards at the top, down along the outer edge, inwards along the bottom, bringing the leaves to the center, and then up again.

Applications

The phenomenon has been used to develop a new technique to separate

red blood cell Red blood cells (RBCs), referred to as erythrocytes (, with -''cyte'' translated as 'cell' in modern usage) in academia and medical publishing, also known as red cells, erythroid cells, and rarely haematids, are the most common type of blood cel ...

s from

blood plasma Blood plasma is a light Amber (color), amber-colored liquid component of blood in which blood cells are absent, but which contains Blood protein, proteins and other constituents of whole blood in Suspension (chemistry), suspension. It makes up ...

, * to understand atmospheric pressure systems, and in the process of

brewing Brewing is the production of beer by steeping a starch source (commonly cereal grains, the most popular of which is barley) in water and #Fermenting, fermenting the resulting sweet liquid with Yeast#Beer, yeast. It may be done in a brewery ...

beer Beer is an alcoholic beverage produced by the brewing and fermentation of starches from cereal grain—most commonly malted barley, although wheat, maize (corn), rice, and oats are also used. The grain is mashed to convert starch in the ...

to separate out coagulated trub in the whirlpool.

References

External links

* * * See also figure 25 i
figures.pdf
* * Einstein's 1926 article online and analyzed on
BibNum
' (click 'Télécharger' for English) (unsecure link). {{DEFAULTSORT:Tea Leaf Paradox Fluid mechanics Physical paradoxes Albert Einstein Tea Articles containing video clips