The teapot effect, also known as dribbling, is a fluid dynamics phenomenon that occurs when a liquid being poured from a container runs down the spout or the body of the vessel instead of flowing out in an arc.
Markus Reiner coined the term "teapot effect" in 1956 to describe the tendency of liquid to dribble down the side of a vessel while pouring. Reiner received his PhD at TU Wien in 1913 and made significant contributions to the development of the study of flow behavior known as rheology. Reiner believed the teapot effect could be explained by Bernoullis principle, which states that an increase in the speed of a fluid is always accompanied by a decrease in its pressure. When tea is poured from a teapot, the liquids speed increases as it flows through the narrowing spout. This decrease in pressure was what Reiner thought to cause the liquid to dribble down the side of the pot.
However, a 2021 study found the primary cause of the phenomenon to be an interaction of inertia and capillary forces. The study found that the smaller the angle between the container wall and the liquid surface, the more the teapot effect is slowed down.