“Gel ink is viscous, but it’s not thick enough to prevent it from flowing. This means it can evaporate over time”
These are three different, unrelated phenomena. In a Newtonian fluid, no matter how large the viscosity, the fluid will flow when a shear stress is applied (think honey).
A gel is instead typically a non-Newtonian fluid, i.e. a certain threshold stress has to be reached before motion sets in (think toothpaste).
I don’t really see why the author then relates evaporation to viscosity. Shielding the fluid of interest from mixing with the surroundings will prevent evaporation (close the bottle of water and it will remain full, leave it open and then it will evaporate eventually).
> Shielding the fluid of interest from mixing with the surroundings will prevent evaporation (close the bottle of water and it will remain full, leave it open and then it will evaporate eventually).
If you close the other end, pressure will prevent the ink from flowing out the tip.
Thinner inks spread out in a thinner layer - I can stick a little ball of gel ink on a picture and even make a little texture in it, but this doesn't happen as well with thinner ink. The thinner layers have more surface area, making evaporation faster.
You can see the effect with paint, too: Spray paint vs normal wall paint, for example. Also, when dehydrating fruits and vegetables, thin slices dry out faster (I know this isn't viscosity per se, but still with the surface area and thickness).
I'm going to guess that there are other things at play here too (how water gets trapped, etc), but I really don't know the science behind it.
But not fountain pen cartridges (the “international format” seems not to be an ISO standard). However there are ISO standards for the inks themselves, which can be the contents of these cartridges.
Unsurprisingly there are also national ink standards. For example certain contracts like marriage licenses in Germany are only legal if signed with one of two DIN standard inks that are certified to last at least a century.
> These thickeners can range from particulate silica, alumina, titanium dioxide, or even powdered clay.
If it were any of those, I imagine the stopper would be hazy, unless the refractive index of the matrix were very close to the aggregate. Curiosity abounds, the mind boggles.
Might be a silicone (polydimethylsiloxane) based grease, with various silicone oxides and/or polymers as thickeners. Now I wanna take one apart.
Pentel's Energel refills have two followers: first a transparent then a solid white one. Pilot's G1/G2 refills' stoppers are grease yellow / translucent orange.
Similarly Faber Castell's Super True Gel refills are stopped by a composite follower. A transparent one, then a white one.
Parker's gel refills' follower sometimes leaks and it is messy.
When I was a boy my parents gifted me a Cap-O-Matic space pen. It's a fairly generic looking pen with a small space shuttle logo in the middle of it but uses their patented pressurized ink cartridges. Of course like any boy I took it to school to show it off and use it until someone tried to steal it. It was super precious to me at the time so I stored it away in my precious items tin lunch box under my bed. It stat there for a couple of decades, forgotten through time.
Like any well prepared person I keep a pen in the glove box of my car. The problem is they last only a couple of years before they leak or dry out which means they never work when I actually need them. One day I'm back home with my parents and I find my old tin and the Fisher space pen in it. It still worked perfectly and I figured if it's suppose to handle the rigors of space then a car glove box shouldn't be a problem. It wasn't.
After about a decade in the glovebox of my old car it found its way to the glove box of my new car (which I guess is kinda old when compared to a car from today). It still works perfectly.
You (and others) might be pleased to know that you can get the space pen as a Parker refill that fits many different pen bodies. You can also get the very popular uni jet stream ink in the same refill type. I suggest a Kaweco sport as the pen body of choice
They didn't really indicate why the end is sealed with a fluid as opposed to something physical. I suspect it's because flowing ink with a solid stopper would create a vacuum and quickly cease to function. A stopper is fluid and so can flow along with the ink. The space between the stopper and the shell is probably vented.
Yes, it's such an obvious question that's not addressed in the article. I was thinking about why they don't pinch the plastic tube closed at the point where the ink stops being filled. The following statement in one of the patents[1] seems to confirm your explanation:
The aims of the arts are to make the ink follower follow the ink smoothly.
Not vented so much as just not air tight-- no reason to make it air tight, and the inside of the pen is by no means sealed, as you can tell if you've ever taken one apart (or had it explode inside your bag). The parts just don't fit that well together. Which I guess is a vent in a sense, although that's probably not the main design consideration. The (obvious) exception being the space pens, which do have some pressure chambers. They solve the vacuum issue by being pressurized, which has the nice benefit of letting them write upside down.
I imagine they've made it viscous/non-Newtonian/whatever (not sure on the technical fluid mechanics term) enough such that gravity pushes/pulls it down towards the tip but prevents it from flowing the other way.
Edit 1 (just before getting in the shower): Removed references to air pressure, as I don't think that makes a difference.
Edit 2 (post shower): Air pressure might a difference after all, it depends on where the seal is. If there's an air tight seal on the tip end (which there might be, if that's how it's kept from leaking) then air pressure might add to the "only goes one direction" effect.
[+] [-] bernulli|4 years ago|reply
These are three different, unrelated phenomena. In a Newtonian fluid, no matter how large the viscosity, the fluid will flow when a shear stress is applied (think honey).
A gel is instead typically a non-Newtonian fluid, i.e. a certain threshold stress has to be reached before motion sets in (think toothpaste).
I don’t really see why the author then relates evaporation to viscosity. Shielding the fluid of interest from mixing with the surroundings will prevent evaporation (close the bottle of water and it will remain full, leave it open and then it will evaporate eventually).
[+] [-] aqme28|4 years ago|reply
If you close the other end, pressure will prevent the ink from flowing out the tip.
[+] [-] andreareina|4 years ago|reply
or even pitch: https://en.wikipedia.org/wiki/Pitch_drop_experiment
[+] [-] Broken_Hippo|4 years ago|reply
You can see the effect with paint, too: Spray paint vs normal wall paint, for example. Also, when dehydrating fruits and vegetables, thin slices dry out faster (I know this isn't viscosity per se, but still with the surface area and thickness).
I'm going to guess that there are other things at play here too (how water gets trapped, etc), but I really don't know the science behind it.
[+] [-] throw_away|4 years ago|reply
[+] [-] gumby|4 years ago|reply
Unsurprisingly there are also national ink standards. For example certain contracts like marriage licenses in Germany are only legal if signed with one of two DIN standard inks that are certified to last at least a century.
[+] [-] DrAwdeOccarim|4 years ago|reply
[+] [-] sneak|4 years ago|reply
Highly patented!
[+] [-] kortex|4 years ago|reply
> These thickeners can range from particulate silica, alumina, titanium dioxide, or even powdered clay.
If it were any of those, I imagine the stopper would be hazy, unless the refractive index of the matrix were very close to the aggregate. Curiosity abounds, the mind boggles.
Might be a silicone (polydimethylsiloxane) based grease, with various silicone oxides and/or polymers as thickeners. Now I wanna take one apart.
[+] [-] bayindirh|4 years ago|reply
Similarly Faber Castell's Super True Gel refills are stopped by a composite follower. A transparent one, then a white one.
Parker's gel refills' follower sometimes leaks and it is messy.
[+] [-] EricE|4 years ago|reply
[+] [-] GloriousKoji|4 years ago|reply
When I was a boy my parents gifted me a Cap-O-Matic space pen. It's a fairly generic looking pen with a small space shuttle logo in the middle of it but uses their patented pressurized ink cartridges. Of course like any boy I took it to school to show it off and use it until someone tried to steal it. It was super precious to me at the time so I stored it away in my precious items tin lunch box under my bed. It stat there for a couple of decades, forgotten through time.
Like any well prepared person I keep a pen in the glove box of my car. The problem is they last only a couple of years before they leak or dry out which means they never work when I actually need them. One day I'm back home with my parents and I find my old tin and the Fisher space pen in it. It still worked perfectly and I figured if it's suppose to handle the rigors of space then a car glove box shouldn't be a problem. It wasn't.
After about a decade in the glovebox of my old car it found its way to the glove box of my new car (which I guess is kinda old when compared to a car from today). It still works perfectly.
[+] [-] powersurge360|4 years ago|reply
[+] [-] tryonenow|4 years ago|reply
[+] [-] computator|4 years ago|reply
The aims of the arts are to make the ink follower follow the ink smoothly.
[1] https://patents.google.com/patent/EP1002663A1/en
[+] [-] ExtraE|4 years ago|reply
I imagine they've made it viscous/non-Newtonian/whatever (not sure on the technical fluid mechanics term) enough such that gravity pushes/pulls it down towards the tip but prevents it from flowing the other way.
Edit 1 (just before getting in the shower): Removed references to air pressure, as I don't think that makes a difference.
Edit 2 (post shower): Air pressure might a difference after all, it depends on where the seal is. If there's an air tight seal on the tip end (which there might be, if that's how it's kept from leaking) then air pressure might add to the "only goes one direction" effect.