Teapot effect

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.

[1] 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.

[1] Reiner believed the teapot effect could be explained by Bernoulli's principle, which states that an increase in the speed of a fluid is always accompanied by a decrease in its pressure.

With the help of a suitable pot geometry (or a sufficiently high pouring speed) it can be avoided that the liquid reaches the spout and thus triggers the teapot effect.

Since adhesion also plays a role, the material of the spout or the type of liquid (water, alcohol or oil, for example) is also relevant for the occurrence of the teapot effect.

The pressure difference resulting from the Bernoulli equation is then not sufficient to influence the flow to such an extent that the liquid is pushed around the edge of the spout.

Theoretically, this speed could be precisely calculated for a specific can geometry, the current air pressure and the fill level of the can, the spout material, the viscosity of the liquid and the pouring angle.

Since, apart from the fill level, most of the influencing variables cannot be changed (at least not sufficiently precisely in practice), the only way to avoid the teapot effect is usually to choose a suitable geometry for the pot.

The flow can thus resist wetting even when pouring slowly and the liquid does not reach the downwardly inclined part of the spout and the body of the jug.

[6] In part, this only becomes apparent when one considers the normal maximum fill level: the glass carafe on the far right, for example, appears at first glance to be a poor pourer because of its slender neck.

Certain materials are also used there to prevent dripping, for example glass, which can be easily shaped or even ground to create the sharpest possible edges, or Teflon, for example, which reduces the adhesion effect described above.