Why won't a pen work on the International Space Station?

Question

Today, in my physics class, we were discussing the effect of microgravity (specifically on the ISS) on various objects of everyday value. I learnt that a normal pen wouldn't work in space (So they designed a totally new pen ~ Fisher Space Pen).

My Question: Why is it so? As far as I know, a pen works on the principle: due to dynamic friction, the particles of ink or lead stick to the paper and get locked in its irregularities. So how will a micro-g environment affect the frictional force between the pen and paper ?

Thanking you in advance!

Answer 1

Take a ball point pen, and write on a paper such that the pen is upside down (i.e, the paper is above the pen) You'll find that the pen stops working in a short while.

The pen works because gravity creates a pressure gradient which helps force out ink near the nib of the pen. When you hold the pen upside down to write / use in in space, no such pressure gradient is formed and hence the ink has no tendency to be forced/flow out of the nib.

The fisher space pen has a pressure generator (pressurized nitrogen is present in the pen which helps force the ink out, regardless of if the pen is held in any gravitational field)

(Or they could have just used a pencil)