siphoning

A siphon (from Ancient Greek σίφων (síphōn) 'pipe, tube'; also spelled syphon) is any of a wide variety of devices that involve the flow of liquids through tubes. In a narrower sense, the word refers particularly to a tube in an inverted "U" shape, which causes a liquid to flow upward, above the surface of a reservoir, with no pump, but powered by the fall of the liquid as it flows down the tube under the pull of gravity, then discharging at a level lower than the surface of the reservoir from which it came. That is a gravity siphon, but it is not the only kind.
There are two leading theories about how gravity siphons cause liquid to flow uphill without being pumped and powered only by gravity. The traditional theory for centuries was that gravity pulling the liquid down on the exit side of the siphon resulted in reduced pressure at the top of the siphon. Then, atmospheric pressure was able to push the liquid from the upper reservoir, up into the reduced pressure at the top of the siphon, like in a barometer or drinking straw, and then over.
However, it has been demonstrated that gravity siphons can operate in a vacuum and to heights exceeding the barometric height of the liquid. Consequently, the cohesion tension theory of siphon operation has been advocated, where the liquid is pulled over the siphon in a way similar to the chain fountain.
It need not be one theory or the other that is correct, but rather both theories may be correct in different circumstances of ambient pressure. The atmospheric pressure with gravity theory cannot explain siphons in vacuum, where there is no significant atmospheric pressure. But the cohesion tension with gravity theory cannot explain CO2 gas siphons, siphons working despite bubbles, and the flying droplet siphon, where gases do not exert significant pulling forces, because liquids cannot exert a cohesive tension force if they are not in contact.
All known published theories in modern times recognize Bernoulli's equation as a decent approximation to idealized, friction-free siphon operation. The gravity siphon is explained by the vertical distance between two points of a closed circuit.
The Venturi siphon differs greatly from the gravity siphon, as it does not require a difference in height; it operates due to a fluid traveling at a different speed and pressure. It can be explained by Bernoulli's principle as well. Venturi siphons also differ from gravity siphons in the working fluids: a second fluid is introduced into the streamflow of the first fluid at the throat of the Venturi. For example a liquid can be inducted into a flowing gas, and a gas can be introduced into a liquid. This is the principle behind the carburetor of a gasoline engine, a Venturi siphon that draws gasoline into fast-moving air.

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