I’m talking about cheap ballpoints where the ink is contained in a translucent plastic tube. There does not appear to be any sort of plug on the end opposite the point. However, the ink level clearly moves toward the tip as the pen is used (over time). I have some fancier ballpoints where the ink reservoir is a larger tube and the ink appears to be more fluid. It is actually wet on the page. However, again, the ink does not flow out the open end of the pen.
So, why doesn’t the ink pour out? What mysterious force is holding it in and pressing it against the ball in the tip?
Same reason as why it doesn’t pour our around the ball – it’s a fairly viscous ink, and held together by surface tension. Capillary action prevents it from flowing out the open tube, or around the ball at the other end until the ball revolves to break the tension and spread it where the pen moves on the paper.
How do you know this? The viscosity of the ink can hardly be the reason - viscosity creates forces proportional to flow rate, and so can only slow the movement. The ink is able to run out rapidly enough for writing, and yet left in a drawer for years will not drain out. The surface tension could be pretty hard to make work right; if the surface tension of the ink is below the critical surface tension of the metal it would be able to spread spontaneously. The critical surface tension of metals is usually over 50 and maybe a couple hundred newton meters, isn’t it? It would be hard to give the ink more than 20 or maybe 30, especially if it is a complex mixture.
Of course, there must be some reason, but I am curious if it can be what you say.
Perhaps surface tension is important, but it is really a competition between the two ends of the tube, as the top end of the plastic tubing is open also, and generally the body of the pen has a vent hole. Maybe the capillary effect of the large tube prevents the ink from leaving, except that the paper (which has even finer capillarity) is able to wick it out. Thus the mechanism might be more like the mechanism by which a chamois will dry a cloth.
Another thing that lets you write with a ballpoint pen but without it leaking is that the ball acts as a sort of plug. When you press on the ball, as when writing, it widens the gap between the ball and casing a bit.
I feel I must add, in the interest of completeness, that my childhood research showed that if you remove the tube from the pen casing and suck on the open end hard enough, the ink will eventually exit through that open end into your mouth.
“In the case of ballpoint pen ink, the ink is very thick and quick-drying. It is thick so that it doesn’t spill out of the reservoir, but thin enough that it responds to gravity. That is why a normal ballpoint pen cannot write upside-down – it needs gravity to pull the ink onto the ball.”
But if it’s thin enough to respond to gravity why doesn’t it eventually run out of the open end when left upside down?
I think we’re getting closer here. I believe there’s a plug at the upper end of the ink-containing tube, made of a waxy substance. It follows the ink down as the level decreases. In every ball-point I’ve seen (or at least those with transparent ink tubes), there’s a bit of a different-colored substance at the end.
My personal research went a little bit further. You can also suck on the small “vent” hole in a standard transparent Bic Biro. The ink will flow out of the tube, into the barrel of your pen, and out the vent hole… It does taste nasty, but I must have destroyed dozens of pens that way, and used a small forest of paper towels cleaning up the mess.
I have emailed Bic and have left voice messages to the corporate offices, I haven’t heard back from either. I even called the “press” division, with no response. If I ever do get a response I’ll post it.
I still remember when the first ballpoint pens came out, sometime before 1950 (since it preceded my going to HS that year). They did leak, often horribly. They cost $15 (think $150 today) and it was not legal to sign checks with them. Obviously today the ink and the ball are engineered to avoid that with the consequence that they will sometimes stop up and there is nothing to do but throw them away.
Amusingly, in school, we were not permitted to use fountain pens and had to use steel pens with an inkwell. After ballpoint pens became common (and better) we were allowed fountain pens, but of course, not ballpoint pens. As though there had to be something forbidden. Now fountain are sufficiently rare as to seem almost as an affectation.