We all have poured liquid from a bottle. Wine, whiskey, cognac, milk or maybe even water. As liquid runs out and air can flow back in, all is fine. When we increase the flow, at a certain amount the flow-rate begins to fluctuate and bubbles occur, rising from the opening of the bottle, up to the (ever increasing) air volume above the liquid.
Most of us will have accepted this as a fact of life and even enjoyed the sound. Since you are here and reading this, I assume, you are inquisitive enough wanting to know why.
I drew a stylised bottle (a hermetically sealed container) with its opening sitting on a liquid surface. Some of you may have performed this experiment, otherwise known as the birdbath. After you filled the bottle with water and you turn it upside down so the opening is closed by the water-surface, a small amount of water escapes and then, the flow of water stops.
Gravity and the volume cause the weight of water and the force Fwater, which causes the water to flow out of the bottle and the pressure Pvacuum of the volume of air above the water reduces to a level lower than that of the surrounding air Pair. This pressure difference results in the force Fvacuum. When this force is 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. We lift the bottle upwards for a small amount. This allows the higher air-pressure to push one or several bubbles of air inside the bottle. Therefore, the vacuum lessens and so does its resulting force Fvacuum. Subsequently a certain amount of water runs out until the vacuum increases until Fvacuum is again strong enough to hold the column of water.
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 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 above mentioned wine, whiskey, and cognac. If you want to prove this point, use a glass as the collecting receptacle.
And as a final note: The same applies to ink.
Fountain pens and small 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 small 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 into the bottleneck. The equilibrium of forces can be achieved and sustained. The flow of ink stops.
As you move the bottle along the paper, the opening of the pipe will move to a section of dry paper. Now, the outside air-pressure can push a bubble inside and a small amount of ink can escape from the bottle. (The hygroscopic characteristic and of the paper and its capillary action contributes to it, but about this, later.)
Don’t get too excited. This is only a thinking model. This may have well been the function of earlier feeds. 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 make the feed function, namely: surface tension and capillaries, in as far as it concerns ink and the surface of the feed.