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I've had a rather interesting fascination with fountain pens. The mechanism is a thing of beauty, but my attention was recently caught by a rather insignificant phenomenon which I've used a lot in my experience with them. That of soaking leftover ink. All the bright folks here who've written with one must know that a fountain pen soaks ink at times(I do this with the leftover ink in the pot cap to dry it up), and this happens till a certain amount has entered the nib(it varies, of course with the tank airspace and possibly other parameters)

I've looked up how a fountain pen works, and to the best of my knowledge, I couldn't find a clear mechanism which shows why this happens. enter image description here

I have a vague idea of how it could work:

The four chambers in the jib/nib are built for ink($I_1, I_2 \& I_3$)/air($A_1$) exchange. $A_1$ replenishes the vacuum that would have otherwise formed while the tank got devoid of ink while writing(here $A_1$ does not fill up with ink). Now when I'm soaking some up through $I_1, I_2 \& I_3$, there is no place for air to enter $A_1$ and somehow ink goes up through $A_1$ too alongside $I_1, I_2 \& I_3$(or it opposes the flow). Now, this is where it's all fuzzy for me, assuming what I've put forward is logically true. What phenomenon could cause ink to be pushed through $A_1$?

Is it possibly a minute pressure difference between the tank and where the end of the nib(aka the capillary) is placed, that causes this unprompted flow?

Some clearing up will be appreciated. Thank you:)

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  • $\begingroup$ To clarify, you are asking how the ink gets into a dry fountain pen? $\endgroup$ – QuIcKmAtHs Nov 15 '18 at 13:01
  • $\begingroup$ You could say that, though I've seen this happen with partially or completely(with some air bubbles obviously) filled tank and a wet nib. $\endgroup$ – Abhinav Nov 15 '18 at 16:39
  • $\begingroup$ Im sorry, but im still confused. what exactly is your query? $\endgroup$ – QuIcKmAtHs Nov 16 '18 at 11:50
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The working of fountain pen can be described simply as a controlled leak. This, unlike ball pen or gel pens, uses both gravity and capillary action to draw the ink in the required amount. There are a good number of differences that makes a fountain pen unique.

enter image description here

In this fountain pen the ink is controlled not to leak without a ball. The nib tip is a different heavy metal (commonly used is iridium) which is electrically welded to the nib. In older days, even alloys of gold were used. The main properties needed for the tip material, are resistance to corrosion, chemical attack and abrasion.

If you notice the nib carefully, you can see a slit from the nib tip up to about half the length, terminated by "breather" hole - this hole can be of fancy shapes. The slit is placed exactly above the similar groove on the feed. The breather hole is quite a misnomer. It doesn't play the function of breathing air. Instead it acts as a "stress-relief", to prevent the nib from cracking up via the slit.

Now when you drag the nib tip on the paper, the ink flows down from the reservoir due to gravity up to nib. The fins of the feed, acts as a temporary reservoir and controls the flow of ink via the tip. Now while the ink comes down, the air flows up (to fill in the space left by ink in top), through the slit and the channel made in the feed. All this happens simultaneously.

From the feed, both capillary action and gravity plays role in pulling the ink out through the nib tip while dragging on paper. Both adhesive forces - between ink molecules and the slit, as well as the cohesive forces - between different molecules of ink, takes part actively in making the flow a smooth one.

This pen cannot soak ink inside. If ink goes in then the air inside should come out. But air cannot come down because its density is very less compared to that of ink.

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