Non-Drip
There are essentially two things that need to be considered when designin a teapot that is supposed to be leaking and dripping proof.
- The architecture of the pot can cause overflow or leakage.
- The shape of the spout can cause liquid to run down the under side of the
spout causing the teapot to drip when pouring tea.
Avoiding Leakage and Overflow
In order to model whether the dimensions of my teapot would cause it to leak from underneath the lid when pouring, I drew some sketches with differing spout heights and used a set square to model water levels in the teapot. Given that my teapot is rather small and has a big opening at the top, the spout's end should not be much higher than the top of the body, as the pot would then have to be tilted very steeply to pour thus causing it to leak from the top. Hence I decided to go for a spout that ends at the same height as the top of the pot.
Avoiding a Dripping Spout
Many teapots suffer from a dripping spout, i.e. when pouring tea, the brew flows around the lip of the spout and runs down the underside of the spout.
In his 1957 paper "The Teapot Effect" Joseph B. Keller explained the reason for this to be as follows.
"When the liquid flows around the lip its velocity is greatest at the lip. By Bernoulli's principle the pressure is then lowest there. Consequently the surrounding air or other fluid pushes the liquid against the lip by virtue of its atmospheric pressure. This enables the flow to turn the corner. Once the flow has turned the corner it continues along the under side of the spout rather than falling because the surrounding air supports it. This upside-down flow, although unstable, will travel quite far down the spout before its instability results in detachment, if the liquid layer is thin. When detachment occurs the jet will fall backwards since its velocity is directed away from the lip."
In an interview, he outlines two simple remedies to this problem.
I drew the sketch below to illustrate this effect.
In his 1957 paper "The Teapot Effect" Joseph B. Keller explained the reason for this to be as follows.
"When the liquid flows around the lip its velocity is greatest at the lip. By Bernoulli's principle the pressure is then lowest there. Consequently the surrounding air or other fluid pushes the liquid against the lip by virtue of its atmospheric pressure. This enables the flow to turn the corner. Once the flow has turned the corner it continues along the under side of the spout rather than falling because the surrounding air supports it. This upside-down flow, although unstable, will travel quite far down the spout before its instability results in detachment, if the liquid layer is thin. When detachment occurs the jet will fall backwards since its velocity is directed away from the lip."
In an interview, he outlines two simple remedies to this problem.
- If the teapot spout points up and then straight down at the pouring end, then
the tea will flow back into the pot when the pot is turned upright again
and a drip would be almost impossible. - If the lower lip of the spout is sharp, as is the case with metallic teapots, then the trouble is ameliorated. The sharp edge would help prevent the tea from turning the corner. It’s still possible to drip but less likely if the tea is coming with any force (Brown 2013).
I drew the sketch below to illustrate this effect.
Furthermore, in order to test the shapes and lip forms of different spouts I created the following model of a vessel with six spouts. Two spouts point down, two spouts point straight, and two spouts point upwards. For each spout shape, there is one version with rounded edges and one version with sharp edges.
A 3D print of this model was used to test the spouts.
None of the spouts seriously suffered from the teapot effect. However, I observed that when pouring through the forward and upward pointing rounded spouts, the liquid does sometimes turn around the corner of the lip. However, the liquid then does not flow down the underside of the spout, as the texture of the 3D printed model is probably too rough to allow it to do so. Furthermore, the downward pointing rounded, and all three sharp-edged spouts achieved very satisfying results. I concluded that sharp edges will suffice to avoid the teapot effect. For aesthetic reasons, i stuck with an upward pointing spout for the prototype.