Capillary action | Day 201

I’ve been having problems with my Parker fountain pen. First, I bought official black ink cartridges which turned out to cause problems with the writing flow – a known fact about Quink amongst fountain pen aficionados. Then I bought arguably better ink cartridges, only to find that they don’t fit the proprietary Parker cartridge size. I don’t think I’ve spent enough time on fountain pen forums or I would have known better. Sigh.

Anyway, having spent a good hour online researching pens and inks and so on, I started wondering about the science behind how fountain pens work.

It’s likely that in biology class at school you once learned that plant roots suck water out of the soil through what’s known as capillary action. It’s what causes liquids to move along the surface of a solid material, because the molecules of the liquid are attracted to the molecules of the solid. And one by one, as the water molecules keep together, they slowly creep along the surface they are sticking to.

But it’s not just plants that use this property to their advantage. Capillary action also happens when you wipe something up with a paper towel. In fact, a paper napkin and a glass of coloured liquid is often used to demonstrate this process. I found it extremely difficult to find a video of this that I liked, but here’s one aimed at kids that actually does a pretty good job. “Cool water experiment” indeed.

See how the water just creeps along the paper? This is exactly why fountain pens have that tiny slit on the nib. Like this one.

fountain pen macro photo
By Leo Reynolds CC-BY-NC

As fountain pen authority Richard Binder explains in his glossary:

[Capillary action is the] drawing of a liquid into a narrow space; occurs when the adhesive force between the liquid and the surface of the solid forming the space exceeds the cohesive force between the molecules of the liquid itself. Capillary action draws ink from a pen’s reservoir into and through the feed, and thence along the nib’s slit to the tip of the nib, from which it can draw the ink onto the surface of the paper when the pen is used to write.

Now you know. And you also know to be careful before you buy fountain pen cartridges online. Best to check with The Fountain Pen Network first.

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