Physicists say they have finally solved the 'teapot effect'

The fact that the tea water drips and flows down the pot while pouring turns out to be more complicated than you think.

The fact that tea water clings to the sides of the pot and drips while pouring - known as the teapot effect - is a minor annoyance to regular tea drinkers. But for physicists around the world, it has posed a troubling theoretical problem spanning decades. An Ig Nobel Prize was even awarded to two scientists for figuring out how to create a drip-free teapot spout.

In 2019, it all seemed to come to an end when a team of Dutch physicists came up with a quantitative model to accurately predict the flow rate as tea is poured out of the kettle . But, there is still much work to be done to fill some of the holes in this fascinating theory of physical effects.

Most recently, physicists at the Vienna University of Technology and University College London say they have finally developed a complete theoretical description for the teapot effect. It captures the complex interplay of inertial forces, viscosity and capillary phenomena to redirect fluid flow when certain conditions are met. Gravity, however, has been shown to have less to do with the phenomenon. That means you'll still notice the teapot effect on the Moon, according to the authors, but won't see it if you're pouring tea on the International Space Station.

The researchers presented their theoretical calculations in a paper published in the September issue of the Journal of Fluid Mechanics. And now, they have published the results of experiments they conducted to test the model. theoretical figure. And if you've read this far and you still haven't figured out what it's like to study the story of the tea pouring process, know that the insights gained from these studies can help us stay in control. than the flow of fluids in microfluidic devices, which is particularly important in medical and biological research.

The teapot effect has baffled physicists for decades.

Israeli engineer and scientist Markus Reiner first described the teapot effect in 1956, and pioneered the field of rheology (the study of how liquids flow). Later, engineer and mathematician Joseph B. Keller conducted his own experiments and concluded that the dripping was caused by air pressure and not by surface tension , as many had assumed. determined. He and a Belgian colleague, Jean-Marc Vanden-Broeck, published a report in 1986 - the work that earned them the Ig Nobel Prize in 1999. According to Keller, the pressure of the liquid in the pouring medium is low. than in the surrounding air, and thus push the tea back into the mouth and outside of the spout.

More specifically, at higher flow rates, the liquid layer near the teapot's spout separates so that it flows smoothly and without causing drips. But at lower flow rates, when flow separation occurs, a layer adheres to the surface of the teapot spout, resulting in a trickle flow. The diameter of the spout, the curvature of the spout, and the "wetting ability" (which describes the adhesion of a liquid to the surface of a solid) of the teapot material, are also factors that can affect affects whether or not dripping occurs.

But, they are not the main culprits. In 2010, French physicists demonstrated that the cause was actually a kind of "hydrocapillary effect" that prevents (at slower pouring rates) liquid from separating from the nozzle for a smoother flow. , clean. All other factors play a role in determining how strong the hydrocapillary effect is. Those physicists suggest making the mouth of the faucet thinner and with a sharper tip to reduce water dripping, perhaps even coating the mouth of the faucet with a super-waterproof material.

Next, the 2019 study was conducted by Etienne Jambon-Puillet, then a postdoctoral researcher at the University of Amsterdam. He was fascinated by the way the liquid wrapped around the cylindrical needle in the syringe he had to clean in the lab every day. With a few colleagues, he set up a series of vertical cylinders and shot dyed water jets at them, videotaping how the fluid behaves at different flow rates. They then discovered that the jets always travel in a straight line at high flow rates, and as that speed drops, the water begins to deflect slightly. At an even lower flow rate, the water begins to coil and "cling" to the cylindrical surface before spinning around to form a spiral.

The researchers' new model accurately predicted when the crucial transition from "sticking" to - rather than detaching - a solid surface like a cylinder (or the spout of a teapot) would occur. They conclude that it happens when there is a combination of hydrostatic attraction and wetting, which binds the tea water to the side of the teapot as it flows down.

The flow rate determines the size of those droplets.

And the latest report is consistent with previous studies, which found that a decrease in the critical flow rate would create the teapot effect. According to the authors, water droplets will inevitably form on the sharp edge under the nozzle, ensuring that the area stays wet at all times. The flow rate determines the size of those droplets. At the lowest flow rates, the droplets can be large enough to redirect the entire flow around the rim and the tea flows out along the body of the kettle.

As for the applied forces, inertia helps the fluid to maintain its original direction. Capillary force slows down the flow rate at the warm mouth. Other important factors are the angle of contact between the liquid surface and the wall of the teapot and the water absorption (or wetting) of the teapot material. The smaller the angle - or the more hydrophilic the material - the slower the liquid's water separation.

"Although this is a very common and seemingly simple effect, it is difficult to explain it precisely within the framework of fluid mechanics," said study co-author Bernhard Scheichl from the University of Vienna. . "We have succeeded for the first time in providing a full theoretical explanation of why this droplet forms and why the underside of the rim of the spout is always wet."

Of course, the predictions of a well-functioning theory still have to be tested through experiments. So, Scheichl and colleagues tested their model by pouring water from an inclined teapot, varying the flow rates (high, medium, and very low) and capturing those dynamics with a speed camera. high. And as you can see in the GIF above, you can see exactly how reducing the flow rate below a critical point resulted in wetting the mouth of the kettle and water dripping down the stem, characteristic of the teapot effect.