On Fri, 24 Oct 2003 18:01:13 -0700, Robert Walsh <[EMAIL PROTECTED]> wrote:
>> Here's an interesting link to follow. It seems Pedro Duque brought a >> regular ballpoint pen with him to ISS and it works just fine in >> microgravity! >> >> http://radio.weblogs.com/0105910/2003/10/24.html >> >> I wonder what other "truths" are mere fears? > >I was wondering about this. Here's the thing: my regular ball-point pen >doesn't work too well when it's used upside-down here on Earth (I >haven't yet managed to check it in orbit.) After a short while, it >starts to run dry and I need to hold it upright and shake it a few times >to get it started again. So those magic pens have a use down here at >least. There's nothing magical about pens and microgravity. Ball point pens work, or fail to, because of the relationship between diffusion, viscosity, and gravity. The inks are also chosen based on how shear-thinning they are. You write with the pen. The ball rolls, and applies a shear force to the ink next to it in the cartridge. The ink thins and adheres to the ball. By the time the ink-coated ball touches the paper, the ink has returned to its normal viscosity, and adheres more strongly to the paper than the ball. As the ink near the ball is taken up, more ink diffuses down from the cartridge. Ink is fairly viscous, and diffusion of such large molecules is pretty weak (there are less of them), so gravity is enough to overcome diffusion and keep the ink away from the ball. You can't write for long upside down. But in the absence of gravity, it's just as if you were holding the pen exactly horizontal in a gravity field, and it will work fine. You can verify this by holding the pen upside down at various angles from the horizontal, and seeing at what angle you eventually dry out the ball. (BTW, internal pressure is not the only means to make ball point pens write upside down. Cartridges filled with porous material to aid diffusion also work well. The porous material needs to be in near contact with the ball, of course.) -R -- Tip Of The Day: Never Pet A Burning Dog. --Lance M. Bryant, Capt USMC _______________________________________________ ERPS-list mailing list [EMAIL PROTECTED] http://lists.erps.org/mailman/listinfo/erps-list
