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Why we always require a surface to write something? Why we can't write in air?

I think that it is due to friction but I am not sure about it.

Edit

As per helpful comments and responses, I think there is a confusion about this question here. By the term "write'' I meant writing something on the surface of air with a pen.

It is quite non intuitive to write with a pen in air but my main motive to ask this question was to know the reason of non capability of words being unprinted on the surface of air when we sway our pen/pencil in air.

Sorry for grammatical errors because I belong to a non English speaking nation.By surface of air I meant a small section of air.

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    $\begingroup$ How do you imagine "writing in air" working? Moving a pen through the air and leaving a trail of ink that just hovers there? I can't quite tell what you actually want to know because every realization of "writing in air" (aside from sky-writing) that I can imagine is patently absurd and would require the absence of gravity, among other things. $\endgroup$
    – ACuriousMind ♦
    Apr 28, 2019 at 12:14
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    $\begingroup$ As I read it, the question is about writing with ink on paper, but without holding the paper against a solid surface. $\endgroup$
    – svavil
    Apr 28, 2019 at 14:15
  • $\begingroup$ @Infinitesimal I suggest you update your question post to make it more verbose and understandable. $\endgroup$
    – svavil
    Apr 28, 2019 at 14:25
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    $\begingroup$ It is still not clear to me. What do you mean by the "surface" of a gas? When you move an object through air, you're moving it through the air, not across any "surface". I still cannot fathom what process you're imagining here that is not completely absurd. $\endgroup$
    – ACuriousMind ♦
    Apr 28, 2019 at 14:56
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    $\begingroup$ So you're imagining a situation where shaking the nib of a pen in the air releases a stream of ink which stays rigid, thus tracing the path of the nib through the air, and asking why that doesn't take place? $\endgroup$
    – user191954
    Apr 28, 2019 at 14:59

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This is a bizarre question, but I think I see your point.

We always require for our writing products to be durable: we want to be able to observe them again sometime later.

If you write on a gas using a spray even if you are very precise and you remove gravity in order not make your "ink" fall on the ground, your writing will dissolve in really few seconds: the molecules of the air(or gas) will hit frenetically your ink, which will undergo a diffusion around the surrounding gas.

If you are at zero temperature (-273 °C) this will not happen, but you cannot really talk about air and ideal gases in that case.

If you use fluid (like water, liquid ink or even honey) and you remove gravity then your writing will last longer. Of course you will need something different from the usual pens because you need to eject the substance you are using as ink since no capillarity helps it out of your writing device.

The problem in this case will be that whatever you use as ink is not fixed on a solid background and you can cause it to change shape if you touch it. Note that if you make your ink become solid after writing (like molten metal that solidifies or water that freezes) you will be fine, but maybe far from your idea of writing.

If your question was about how the usual pens work, the answer is capillarity: the material in contact with the tip of the pen attracts the ink out of the pen (try to keep the tip still on a point of a sheet and you will see the ink being "sucked out" in the paper). At the end of the day this is a result of surface tension and the behavior of liquids at an interface.

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  • $\begingroup$ Hi friend would you please have a look at the edited version of this question? $\endgroup$
    – user230246
    Apr 28, 2019 at 14:36
  • $\begingroup$ @user230246 the last piece of my answer should address what you brought up in the edit: the capillarity of the material you write on is fundamental. A further desirable feature is that the material needs to fix the pigments of the ink and to let dry the purely liquid component of the ink. In this way the pigments are trapped in the material and you can store them in the position you chose (letter shaped for instance). $\endgroup$
    – AoZora
    Apr 28, 2019 at 16:04
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We can write in air see Wikipedia page on skywriting.

In a different way, you could use also a 3d printing pen.

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    $\begingroup$ I don't understand why this is so highly upvoted. It doesn't answer the question at all. $\endgroup$
    – Moyli
    Apr 28, 2019 at 13:48
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Writing is nothing more than sticking a bunch of atoms in a particular shape either on top of other atoms or placing one kind of atoms in the voids created by others. For them to stay longer at a fixed place, we need a strong affinity between the said two kinds of atoms. Also, we need to differentiate them, so we need a sharp contrast in color.

We write usually on a sheet of paper using ink because the ink sticks to paper. It has a high affinity for the medium it's being applied to. Since the ink is wet it will also be absorbed by the paper. Usage of graphite pencil or other such substrates is primarily due to similar reasons.

Can the same be done with air molecules? Are the interacting forces strong enough to hold them for long periods of time?

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  • $\begingroup$ It was very helpful.Thanks! I have edited the question which makes it more clear. $\endgroup$
    – user230246
    Apr 28, 2019 at 14:38