ElI5: Why a pen sometimes refuses to write on a specific part of the paper but works perfectly fine everywhere else?
In: Other
At least two reasons I can think of
For a ball pen: The ball may be dried out. After rolling it a bit, it may work again normally.
Fat (or other dirt) on the paper may be the issue
If it’s a ballpoint pen perhaps there isn’t enough ‘tooth’ or grip on those sections of the paper to get the ball rolling.
I think it is usually fat. The paper was touched by (your?) fingers (they are always a bit fat; that is where fingerprints come from). The fat is slippery and the ball won’t roll.
A standard ball point pen requires friction. Initially there is only ink on the part of the ball that is inside the pen. Friction between the paper and the ball causes the ball to roll which brings the inked part of the ball to the paper (and inks more of the ball). When your pen isn’t working it means the ball isn’t rolling or the pen is out of ink.
Ball point pens work on paper because the surface of the paper is rough enough (at the microscopic level) to cause friction with the ball. Unfortunately, when a pen fails to write, your attempt to write presses down on the paper and often smooths the surface of the paper. This is why it often works better to try writing/scribbling on another section of paper (where it is still rough)
A pen might not initially write because the ink has dried in the “joint”. The friction between the ball and paper might not be enough to break the friction of resistance. Scribbling to get the pen working again works because you are applying a larger force to break up the dried ink.
The way a ball point pen works is that the tip has a ball in it (hence the name), and as the ball rolls across the page it picks up ink on the top of the ball then deposits it down on the page as it rolls over. If this ball happens to get stuck, it will slide across the page and not leave any ink, so like any normal person you find a scrap piece of paper and scribble on it to get the ball rolling again. However when you try to write over the area where the pen stopped working in the first place, you’ve already crushed the paper & made it smooth, and that smoothness means that there’s little to no friction to keep the ball rolling, making it difficult to write there.
Skin oils, more often than not. Your hands/fingers/arm have pressed into the paper on that spot, making it difficult for the ink to bond to the paper. You’ll get this a lot using Post-Its if you pull the sheet off first, stick it to the table/desk top, and then start writing. You’ll notice where you ran your finger across the top of the Post-It to help it adhere to the table/desk top is the space where you have a difficult time writing.
Did you moisturize your hands then rest part of your hand on the paper when writing and now can’t write on that spot?
Ballpoint pens operate with a freely-spinning ball at the tip (thus the name). As you drag the tip of the pen across a surface, the friction on the ball causes it to spin, “dipping” it into the inside of the pen (where there is ink) and then transferring it to the paper. Imagine an ink-coated tire leaving marks wherever it rolls.
Microscopically, paper’s surface is rough or kind of “furry” – the plant fibers that make up the paper all stick up and make it a textured surface. This creates the friction that the ballpoint pen needs to roll. If a section of the paper is “flattened,” then it doesn’t create enough friction for the ballpoint pen to roll, and the ball stays stuck without any ink. The tip of the pen still gets dragged over the paper, but it never deposits any ink.
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