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).
If you close the other end, pressure will prevent the ink from flowing out the tip.
or even pitch: https://en.wikipedia.org/wiki/Pitch_drop_experiment
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.
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.
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.
German here. WTF? (Not that I don't believe it, but I couldn't find sources for that one)
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.
In California, and probably other states, a verbal contract is binding (though proving what was said can be tough).
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).
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.
> 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.
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.
Are you not following proper protocol of swapping out for new pens between space missions?
Of course, the other solution to this problem is just as in the apocryphal story  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.
(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.
> 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 
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).
The aims of the arts are to make the ink follower follow the ink smoothly.
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.