Teapot effect
Under teapot effect refers to the phenomenon that the fluid , especially if the pot is still very busy and you will pour very carefully to avoid splashes, on the spout or spout download and the pot body, rather than running out in an arc.
Regardless of its name, it occurs on all types of jugs , cans , some glasses and even on horizontal well pipes .
research
Around 1950 researchers from the Technion Institute in Haifa ( Israel ) and from New York University tried to explain this effect scientifically. In fact, there are two phenomena that contribute to this effect: on the one hand, the Bernoulli equation is used to explain it, on the other hand, the adhesion between the liquid and the nozzle material is also important.
According to Bernoulli's explanation, when pouring out the liquid is pressed against the inner edge of the spout, because the pressure conditions at the end, the edge, change significantly; the surrounding air pressure pushes the liquid towards the nozzle. With the help of a suitable jug geometry (or a sufficiently high pouring speed) it can be avoided that the liquid reaches the spout and thus triggers the teapot effect. Laws of hydrodynamics ( fluid mechanics ) describe this situation, the relevant ones are explained in the following sections.
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.
In this context, the Coandă effect is sometimes mentioned, which, however, is rarely cited in the scientific literature and is therefore not precisely defined. Often several different phenomena seem to be mixed up in this.
Continuity equation
In hydrodynamics, the behavior of flowing liquids is illustrated by flow lines . They run in the same direction as the flow itself. If the flowing liquid hits an edge, the flow is compressed to a smaller cross-section. It only does not break if the flow rate of liquid particles remains constant, regardless of where an imaginary cross-section (perpendicular to the flow) is. So just as much mass has to flow in through one cross-sectional area as flows out of another. One can now deduce from this, but also observe in reality that the flow becomes faster at narrow points and the streamlines bundle. This fact describes the " continuity equation for non-turbulent flows".
Bernoulli's equation
But what happens to the pressure conditions in the flow if you change the flow velocity? The natural scientist Daniel Bernoulli already dealt with this question at the beginning of the 18th century . Based on the continuity considerations mentioned above, he linked the two quantities pressure and speed . The key message of the Bernoulli equation is that the pressure in a liquid drops where the speed increases (and vice versa): flow according to Bernoulli and Venturi .
impact
The pressure in the flow is thus reduced at the edge of the can nozzle. However, since the air pressure on the outside of the flow is the same everywhere, a pressure difference arises that pushes the liquid to the edge. Now, depending on the materials used, the outside of the nozzle is wetted during the flow process . At this point, additional interfacial forces occur : the liquid runs as a narrow trickle along the spout and jug until it separates from the bottom.
The undesirable teapot effect only occurs when pouring slowly and carefully. When pouring quickly, the liquid flows out of the spout in an arch without dripping, so it is given a relatively high speed with which the liquid moves away from the edge (see outflow speed according to Torricelli ). The pressure difference resulting from Bernoulli's equation is then insufficient to influence the flow to such an extent that the liquid is pressed around the edge of the nozzle.
Since the flow conditions can be described mathematically, a critical outflow velocity is also defined. If it falls below it when pouring, the liquid flows down the jug; she drips. This speed could theoretically be calculated precisely for a certain can geometry, the current air pressure and the filling level of the can, the spout material, the viscosity of the liquid and the pouring angle. Since, apart from the level, most of the influencing variables cannot be changed (at least not sufficiently precise in practice), the only way out to avoid the teapot effect is usually to choose a suitable geometry for the jug.
Another phenomenon is the reduction of the air pressure between the nozzle and the liquid jet due to the entrainment of gas molecules (one-sided water jet pump effect), so that the air pressure on the opposite side would push the liquid jet to the nozzle side. However, under the conditions that usually prevail when pouring tea, this effect will hardly appear.
consequence
Regardless of its fashionable appearance, a good jug should have a spout with a tear-off edge (i.e. not have a rounded edge ) to make it more difficult to walk around the edge. And - even more important - after the edge, the spout should first lead upwards (regardless of the position in which the jug is held). This would force the liquid to flow upwards when it was poured around the edge of the spout, but this is prevented by gravity . The flow can thus oppose the wetting even when pouring out slowly and the liquid does not reach the downward sloping part of the spout and the jug body.
The illustration on the left shows three vessels with poor pouring behavior. Even in a horizontal position, i.e. H. when standing on the table, the lower edges of the spouts do not point upwards. To the right of this are four vessels with good flow behavior, which results from well-shaped tips. Here the liquid rises at less than 45 ° at the lower edge of the spout. In some cases, this only becomes apparent when you take the normal maximum filling level into account: the glass carafe on the far right, for example, appears at first glance to be a poor pourer because of its slim neck. However, since such vessels are generally filled at most up to the edge of the round piston part, an advantageous ascent on the neck is then obtained when pouring horizontally. This is also important for the thermos jug (to the left of the carafe) in contrast to the sheet metal jug to the left: apart from the slightly more favorable angle of the spout even when the carafe is standing, the significantly lower maximum fill level (if you want to close the lid) requires a significantly higher one Tilt angle and thus an even steeper upward angle for the liquid when pouring. In the case of the lower two jugs on the right, the high position of the spout (above the maximum filling level) means that the vessel has to be tilted quite far before pouring, so that the spout then also upwards directly after the edge (against gravity) shows.
To avoid the teapot effect, the jug can be filled less, so that a larger tilt angle is necessary at the beginning. However, the effect or the ideal fill level again depends on the can geometry.
The teapot effect does not occur with bottles, because the slim bottle neck always points upwards when pouring; the current would therefore have to “flow uphill” a long way. Bottle-like containers are therefore often used for liquid chemicals in the laboratory. Certain materials are also used there to prevent dripping, for example glass, which is easy to shape or even grind to create the sharpest possible edges, or for example Teflon , which reduces the adhesive effect described above.
literature
- H. Dittmar-Ilgen: How the cork crumbs get to the wine glass . Hirzel-Verlag, ISBN 3-7776-1440-8 , p. 21: Always trouble with dripping jugs.
- F. Mugele: What to do if the teapot drips? Physik Journal 9, 2010, p. 18.
- C. Duez, C. Ybert, C. Clanet and L. Bocquet: Wetting controls separation of inertial flows from solid surfaces. Phys. Rev. Lett. 104, 084503 (2010)