The process of filling a fountain pen, particularly a piston-filler, can be influenced by various factors, including the speed at which you fill the pen. While it's not directly related to reversible processes in thermodynamics, there is a practical reason why filling slowly allows you to draw more ink into the pen.
In a piston-filler fountain pen, the ink reservoir is connected to a piston mechanism that creates a vacuum or negative pressure inside the reservoir. This negative pressure draws ink into the pen when the piston is retracted. When you submerge the pen's nib into an inkwell and slowly retract the piston, the slow and gradual movement allows more time for the ink to flow into the pen through capillary action.
Capillary action is the ability of a liquid to flow against gravity in narrow spaces, such as the feed and nib of a fountain pen. It occurs due to the combination of adhesive forces between the liquid and the solid surfaces (the ink and the pen's feed) and cohesive forces within the liquid (the attraction between ink molecules). When you fill the pen slowly, capillary action has more time to work, allowing the ink to fill the pen more effectively.
On the other hand, if you were to fill the pen quickly, the ink might not have enough time to flow into the pen's feed and reservoir through capillary action. The speed at which you retract the piston creates turbulence and disrupts the smooth flow of ink. As a result, you might not be able to draw in as much ink as you would with a slower, more controlled filling process.
So, while it's not directly related to reversible processes in thermodynamics, the slower filling process allows for better utilization of capillary action, resulting in a more efficient and effective filling of ink into a fountain pen.