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What Is the Transparent Material at the End of a Gel Pen Refill? (unsharpen.com)
144 points by thomas 8 days ago | hide | past | favorite | 46 comments

“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.

Maybe, if it’s not about a bottle of water where we don’t want anything to flow out, one could use some sort of Transparent Material At The End Of A Gel Pen Refill to close the plastic pipe and prevent exposure to air to prevent evaporation!

There is gas pressure pens that have a small amount of pressurize inert gas at the end instead, which means that those are in fact closed on the end and operate much longer due to no evaporation issues.

My favorite least exciting experiment!

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.

Good points - but here the ink is still trapped in its enclosure and we don't want it to spread at all when we're not writing.

Speak for yourself - I use more ink for art than writing :)

Realistically, though, this still happens when writing. The thinner ink puts out a thin layer of ink when you write: The gel ink builds up a little bit. You can make little lines in a pool of gel ink in ways you can't do with a regular ink pen. Different ink moves differently, even when it is in a pen.

Haha, fair enough. But what I mean is that the whole discussion in the article is about ink that still in its compartment inside the pen, prior to writing/drawing/etc. It has nowhere to go and cannot spread.

Oh wow. Did not know that there was an ISO standard for pen refills: https://unsharpen.com/pen-refill-guide/

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.

> 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.

German here. WTF? (Not that I don't believe it, but I couldn't find sources for that one)

I don't know about any special rules for marriage licenses, but for inks there's DIN/ISO 12757-2. Search for "Dokumentenechtheit". It's not that exotic, though.

It gets a bit weird for printers. Laser and dot-matrix printers generally are okay, but you need a certified ink jet. Problem is that by the time they're certified, they might not be available. And then you only get the certification on proper paper, with the default inks, so don't cheap out on your old HP...

As this is a ISO norm, too, I don't think this is something peculiar to Germany, after all, document storage isn't just a teutonic issue.

I imagine there must be other countries with similar rules, just as the opposite is true. For example in the US, in general whatever two parties freely agree to is litigatable, even if scribbled with a pencil on the back of a shopping bag. The units or language used are not constrained.

In California, and probably other states, a verbal contract is binding (though proving what was said can be tough).

This is about archival, not contract law. You can seal contracts with handshakes in Germany, too.

But for certain types of records, you need to ensure longevity, and there are norms for that regarding paper, ink or even digital storage. I would assume the NARA has some thoughts about that, but on the local/state level things will probably be a lot more mixed than in most European nations. Both for different attitudes towards independence (I regret ever mentioning the advantages of a federal ID in Ohio), and experience (outside of universities, Europe is littered with more old documents).

I'm reminded of checks. They use (used? not sure if it's still a thing) MICR, or Magnetic Ink Character Recognition for the code at the bottom of the check.

Back in the days before any bespoke product was a search away, tracking done printer ink and toner to print checks for a small business was a surprising chore. Heck, just finding the fonts was itself a bit hard.

This is Iron Gall Ink I think - we have registrar ink here in the UK too. https://en.wikipedia.org/wiki/Iron_gall_ink

Iron gall ink is a poor choice for documents that should last a long time. It's an acidic ink, which damages paper. The iron rusts over time. The ink blooms. These things are manageable if the documents in question are stored in a cool, dry place. But why bother when there are better inks today?

The last wedding I attended had it on a list of notices in the Rathaus.

Looks like the patents outline it pretty well for the Pilot series: Polybutene 3SH, LUCANT HC-150, Fatty acid amide. Also a mention of mineral oil.

> Follower materials are highly patented, which means brands have protected their mixtures with patent protection.

Highly patented!

That was interesting but

> 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.

Nothing beats a Fisher Space Pen!

I can attest to that.

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.

> The problem is they last only a couple of years before they leak or dry out

Are you not following proper protocol of swapping out for new pens between space missions?

That's a pretty awesome story to share. I may have to go find the one I was given many years ago too.

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

Of course, the other solution to this problem is just as in the apocryphal story [1] about the origins of the space pen... use a pencil instead.

I keep a couple pencils and a $2 sharpener in the car. Of course, there are some situations where a pencil is not ideal (like writing a check), but if you really need to write something, it's always going to work.

[1] https://www.scientificamerican.com/article/fact-or-fiction-n...


Yeah, but... space pen!

I just use my pocket knife to sharpen pencils, but I live where it is legal to carry one.

Aren't pocket knifes legal in most places?

(I assume you mean some swiss army style folding knife?)

Most because that kind of folding knife that doesn't lock in place would be less useful in a fight than leaving the knife folded and wrapping your hand around it as a weight.

Other than a place like an airplane, where is it illegal to carry a pocket knife?

Australia more than 2.5 cm pretty much anywhere built up. New York City.

So a leatherman would be illegal in Australia? Wait, I always associate large knives with Crocodile Dundee. I know it is a fictional character, but he wouldn't have quite the same effect saying "That's not a knife, this is a knife." and pulls out a knife with 2.5 cm blade.

Parts of Australia.


UK, I imagine.

Penknives are allowed to be carried without needing a justification in the UK.

> It’s illegal to [...] carry a knife in public without good reason, unless it has a folding blade with a cutting edge 3 inches long or less [0]

It also cannot be locking to not need a good reason to carry it. This is set in case law not the text of the relevant laws (and could could also probably be overturned if anyone had the time, money and inclination to challenge it).

[0] https://www.gov.uk/buying-carrying-knives

I believe it's legal to carry a pocket knife in the UK. Just not one with a lockable blade.

Well, there go the majority of my knives (if I lived in the UK). Even my LeatherMan has a locking blade.

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

Thanks for this tip. I used to carry a space pen regularly but its small size and shape would slip out of my pockets and I would routinely find myself having to replace them.

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.

[1] https://patents.google.com/patent/EP1002663A1/en

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.

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