We all have poured liquid from a bottle. Wine, whiskey, cognac, milk or maybe even water. When liquid runs out and air can flow back in, all is fine. When we tilt the bottle more the flow-rate increases; still all is well. Above a certain amount, the flow begins to fluctuate, and bubbles occur, rising from the opening of the bottle, up to the (alas, ever-expanding) air volume above the liquid. The underlying principle: liquid can only come out when air can get in.
Most of us will have accepted this as a fact of life and even enjoyed the sound of glugging. Since you are here and read this, I assume, you are inquisitive enough to want to know why.
I drew a stylised bottle (a hermetically sealed container, diagram 1) with its opening sitting on a liquid surface. Some of you may have performed this experiment when you fill a bottle through holding it under water and pull it up with the opening remaining under the water surface. You are amazed that the water doesn’t run out. It’s otherwise known as the birdbath. If you have not, try it, but I suggest you use water, why waste the good stuff.
Let’s observe this a bit closer. After you filled the bottle with water and you turn it upside down, so the opening is closed by the water, a small amount of water escapes and then, the flow of water stops.
Gravity and the volume of water cause its weight and the force Fwater, which causes the water to flow out of the bottle. Simultaneously the pressure Pvacuum of the volume of air above the water reduces below that of the surrounding air Pair. This pressure difference results in the force Fvacuum, and when it’s equal to the force Fwater, the water stops running out.
This is a stable situation, which only changes when circumstances change.
Let’s do this. When lifting the bottle upwards for a small amount, the air-pressure difference sucks in one or several bubbles of air into the bottle. Therefore, the vacuum lessens and so does its resulting force Fvacuum. Subsequently, a certain amount of water runs out and the vacuum increases until Fvacuum is again strong enough to counteract the force Fwater created by the column of water.
The above-mentioned change of circumstances in regards to a birdbath can be induced when we ask a chicken to drink some water, or when the writer begins to write and use up the ink stored in the feed.
Please note: With every amount of water flown out, the weight of water lessens, therefore the vacuum required to hold it is less. When we repeat this, the amount of water running out per bubble in reduces.
I recommend you read this repeatedly until you surely understand it. It is one of the essentials of the function of a feed and demonstrates the process and variation of parameters the feed has to compensate.
And please note further: The same principle of physics apply to any other liquid such as the wine mentioned above, whiskey, and cognac. If you want to prove this point, use a drinking glass as the collecting receptacle and try not to spill too much.
And for chicken and other feathered creatures, please note: when you drink water from the little tray and its niveau drops, the same principle applies and the water in the tray gets replenished.
And a final note for fountain pen users: The same applies to ink.
Fountain Pens and Bottles
Now, I will translate the above into fountain pen physics, with one exception. Since we have not handled capillaries and surface tension, I will use for ink transport a thin pipe, rather than a capillary. I will talk about the difference in the next chapter.
Diagram 2 is very similar to diagram 1 – the bird bath. The lower water surface is replaced by a piece of paper, saturated enough to withstand the outside air-pressure thus preventing air to enter through the bottleneck. The equilibrium of forces can be achieved and sustained. The flow of ink stops.
(Technically speaking: If the piece of paper is very absorbent like blotting paper and if it is large enough and you wait long enough, the container will drain, eventually.)
As you move the bottleneck along the paper to a dry area, the pressure difference can push a bubble inside, and a small amount of ink can escape from the bottle (the same birdbath process as in diagram 1). The hygroscopic characteristic of the paper and its capillary action contributes to it, but about this, later.
Don’t get too excited. What I described is only a thinking model. But mind you, it may well have been the function of early fountain pens. The resulting line of writing would have been rather blotchy. However, having said this, if we can make these burps of ink small enough and more frequent then the flow will be more continuous, and we have achieved one of the functions of the feed.
Since we are curious, let’s leave behind what we know and explore characteristics of liquids and solids, which help the feed to fulfil its function, namely: surface tension and capillaries, in as far as they concern ink and the surface of the feed.