Decoding the Dripping Dilemma: Unraveling the Mystery of the Teapot Effect

  Do you often encounter this annoying phenomenon: no matter how careful you are, the water poured from the teapot always fails to enter the teacup according to the expected trajectory, but flows along the spout, against the wall of the pot, and onto the table? At this time, have you ever complained about the poor quality of the teapot or that you were not careful enough? In fact, this really cannot be blamed on the teapot or the tea pourer. This is a very common physical phenomenon.
  The mysterious “teapot effect”
  In 1965, Israeli physicist Marquez Reina conducted a series of “simple” experiments: pouring tea out of the teapot at different flow rates and observing the subsequent phenomena. Reina poured the tea at increasing speeds, first at a very slow speed and then at the fastest manual speed.
  Based on life experience, we can imagine the amount of tea water that Reina can receive at these different speeds: when the tea water flow rate is very small, almost all the tea water will flow to the bottom of the teapot along the spout and outer wall; as the tea water flow rate increases, the outflowing water will It becomes two parts. One part of the tea begins to get rid of the attraction of the pot wall and flows into the teacup in a parabola shape. The other part still flows along the pot wall to the bottom of the pot. When the flow rate is high enough, the water flow no longer bends towards the pot body. Finally, I was able to get rid of the teapot and pour it into the teacup, but at this time there were still water droplets overflowing from the edge of the spout, and the teacup could not catch all the water.
  Renner calls this phenomenon the “teapot effect,” which refers to the phenomenon where liquid drips down the side of a pot when it’s poured too slowly. The faster the liquid flows, the greater the kinetic energy it has, and the better it can maintain the direction of movement when the water comes out. But why is the liquid “attracted” to the spout when the liquid flows slowly? At that time, Reina and his colleagues believed that this was the result of the “tug of war” between surface tension and air pressure. At a lower flow rate, the surface tension of the liquid was greater, and it would adhere to the spout and wall when flowing out of the spout. As the flow rate increases, the surface tension of the liquid is smaller than the surrounding air pressure, which pushes the tea out of the spout, so the water flows smoothly and there are fewer attached water droplets.
  However, with the deepening of research, scientists have discovered that using surface tension to explain the “teapot effect” is not comprehensive. Many researchers have identified more factors related to the “teapot effect” besides surface tension, including the flow rate of the liquid, the radius of curvature of the edge of the teapot spout, the material of the teapot, etc. By adjusting these factors, the “teapot effect” can be avoided. These factors mean that the “teapot effect” is not only related to the surface tension of the liquid, but also has more secrets hidden behind it. How to solve the mystery?
  ”Visualization” of the “teapot effect”
  Physicists have been studying the principles behind this phenomenon for many years. It was only recently that a joint scientific research team from the University of Amsterdam in the Netherlands and the Technical University of Vienna in Austria succeeded in fully understanding it theoretically. They believe that the reason for this effect is inertial force and capillary force (capillary effect refers to the phenomenon that fluid can flow inside a capillary tube without the help of external force or even overcoming gravity. Capillary force is the relationship between the surface tension of liquid and the molecules of liquid and solid. The complex interplay of the adhesion forces between them leads to the “teapot effect”.
  Although the “teapot effect” is a very common and seemingly simple phenomenon, it is very difficult to explain it quantitatively. Therefore, during the experiment, the researchers also used high-speed cameras to film the process of pouring tea at different speeds, so that They can confirm the process of “from quantitative change to qualitative change”. The researchers erected a series of vertical glass cylinders 3 millimeters in diameter and sprayed them with dyed water, videotaping how the liquid behaved at different flow rates.
  In the experiment, the researchers first sprayed a capillary water flow with a diameter of 0.5 mm from one side of the cylinder at an inclination angle of 30° to the other side of the cylinder. Consistent with daily life, as the initial flow rate of water flows changes, the paths they attach to the cylinder surface and fall are different: at high flow rates exceeding 1 ml/s, the cylinder has little effect on the straight trajectory of the capillary flow; As the flow rate is reduced, the water flow begins to slowly tend to deflect around the cylinder; when the flow rate is reduced to about 0.5 ml/s, the water flow changes from simple deflection to coiling, almost completely attached to the cylinder. .
  The researchers then repeated the experiment with glass tubes of different diameters, as well as with cylinders made of Teflon, a stable, low-viscosity material that coats non-stick pans. They found that the same behavior was observed in either case: once the jet completely adhered to the solid, a liquid spiral formed, the exact shape of which depended on the jet’s initial velocity and geometric angle.
  When water is poured from a teapot, the droplets eventually collect on the sharp edge under the spout. The speed of the water determines the size of these droplets. At the lowest flow rate, the droplets may be large enough to pull the entire stream over the edge, and the tea will flow down the wall of the pot. The essence is that the capillary force defeats the inertial force. The inertial force ensures that the flowing liquid tends to maintain its original direction. The capillary force slows down the flow rate of the liquid at the spout and forms larger water droplets to resist the inertial force.
  The researchers also considered the extent to which gravity plays a role in the “teapot effect,” but concluded that it was not as decisive compared to the other forces involved. They note that gravity does determine the direction of the fluid jet, but its strength is not important for the development of the effect. This means there will still be a “teapot effect” on the moon, but it won’t spill if you pour it on the ISS.
  Of course, the results of this experiment also explain how other previously discovered factors contribute to the “teapot effect.” Among them, the most critical thing is the wettability of the spout material, which is the ability of the liquid to wet the solid surface. The reason why materials such as glass and ceramics are easily adhered to by water is because there are many small pores on them that produce capillary action, pulling the liquid and increasing the capillary force of the liquid, causing the liquid to flow down the spout. Therefore, the “teapot effect” can be eliminated by coating the surface of the teapot with a superhydrophobic material with a lotus leaf structure (these materials have been applied to clothing with waterproof requirements such as raincoats, diving suits, and jackets) to reduce its wettability. . In addition, the smaller the contact angle between the liquid surface and the wall of the teapot, the slower the liquid will detach, that is, the more water will hang on the wall. Designing the edge of the spout to be thin and sharp will help reduce the “teapot effect”. For example, metal teapots are less likely to leak.
  In addition to daily pouring, some industrial processes such as pouring, printing and extrusion will also encounter this “teapot effect”. Once the flow rate is too slow, the liquid will “stick” to the edge of the container as mentioned above, which is a waste of raw materials. It can also damage the instrument. The model developed by the researchers successfully predicts the threshold flow conditions for water coiling, and may become an important tool in helping teapot manufacturers and printer manufacturers solve this annoying “teapot effect.”

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