Structural Colour – may explains why ink sheen

[AceNinja]

So I was just watching the video from Doodlebud titled “Sheening Ink Comparison”. Inside the video he writes with a bunch of high sheening inks, and at one point he used a digital microscope to look at the sheen. What shows up in the sheening part of the ink is – dyes that has the water evaporated (due to not absorbed by the paper) that has become crystalized form.

 

 

 

When I saw that, I somehow remember another youtube explainer video I saw a few years back. The video explains the phenomena of “Structural Colour”. In the video, it is explained using a blue butterfly. The blue butterfly actually doesn’t has any blue pigment, it is actually a dull brownish colour.  The surface of the wings has this microscopic structure that are able to interfere wavelength, refraction, etc etc, and at certain angle, the lights reaches our eyes as blue colour.

 

 

 

you can see that the butterfly’s blue is also kinda shiny and “sheeny” almost just like fountain pen inks are. They have the iridescent feel.

 

So, it strikes me that this may have been the phenomenal of why ink sheens.  To get sheen out of ink, the general rule is:

1. use the wetter pen, and use a less absorbent paper. This means the ink will be left pooled on the paper, and the water evaporates away slowly, leaving crustation / crystalized dye on surface.
2. some crystalized dye, just so happens has the correct microscopic structure to interfere the wavelength which reflects colour that is different from its base dye colour.  And hence we see it as sheen at a certain angle.   
3. maybe that’s why most of the high sheening inks are very very saturated ink. Because then there’ll be higher chances to crystalize the dye on paper surface.
4. not all crystalized dye will sheen. Remember it has to have to correct microscopic structure.

 

What do y'all think?  Plausible?

[InesF]

Thank you @AceNinja for the interesting information and for the references!

Quite often colour in nature is built from structures - insects are the masters of this effect.

 

However, in case of sheening ink, it's rarely structure. The light reflection on a crystal surface has a combined effect of absorption and diffraction. It is similar to the effect some mirrored sunglasses have, for example. The light passing through has a different colour tone than the reflected.

(btw.: that's the difference between a mirror and any other reflecting surface; the mirror has no specific absorption in the visible light spectrum, the reflecting surfaces of a dye or some metals have)

 

When I once synthesised crystal violet and cleaned it from impurities, crystals were formed. These were deep purple but their surface reflected golden-yellow light (the opposite colour). You can have this with many dyes in solid (crystalised) form. A sheening ink, therefore, requires a thin solid layer on top of the paper, not necessarily a crystallisation in a certain shape or structure. And that's the exact reason why a non-absorbent paper lets our inks sheen more than an absorbent where the dye is sucked in between the fibres.

[yazeh]

@InesF  You have the gift of making science fun, simple and understandable ! I wish I had a teacher like you at school. Physics would have been much more fun

 

[InesF]

18 hours ago, yazeh said:

@InesF  You have the gift of making science fun, simple and understandable ! I wish I had a teacher like you at school. Physics would have been much more fun

 

Thank you @yazeh, you are very welcome! 😊

[txomsy]

Not that I have any idea.

 

Disclaimer forward, I think the layout of ink on paper might possibly make it unlikely that all crystals are oriented in the same way. In that case, the diffraction/color effect would be apparent from any direction and in principle one would expect to see a more or less homogeneous color result of both components.

 

With a more uniform layer, the reflection/refraction/diffraction, whatever, would tend to happen more uniformly making the distinction with direction more clear. Problem then is rough surfaces would have less uniformity and one would see less sheen and more of the  mix of colors while "flatter" surfaces like coated paper would allow a more uniform layer and a more uniform effect and distinction of colors depending on direction.

 

Whether that requires crystallization, I do not know.

 

A mirror is a good example: a well polished metal surface will act as a better mirror the better polished it is as all the surface behaves more similarly depending on direction; whereas a matte or irregular metallic surface will result in a more homogeneous color resulting from the mix of wavelengths in all directions from any point of view.

 

It seems to me that if ink crystallized on paper it might tend to form crystals in various orientations, but I am no crystallographer, so that is a totally unscientific assumption on my part.

 

Again, I do not really know, so I am likely totally off, but to me it seems that formation of many small crystals would explain it worse than light effects from a relatively uniform surface.

 

[AceNinja]

When I first thought / linking ink sheening to structural colour, I really thought I cracked the mystery.. haha.  But then I'm no expert, so it just remains a possibility/plausible theory only.  I don't think i've seen much (or at all) people explaining why ink sheen may (or may not) related to structural colour, so I just thought I would share my thoughts.

 

On 3/10/2023 at 6:01 PM, InesF said:

Thank you @AceNinja for the interesting information and for the references!

Quite often colour in nature is built from structures - insects are the masters of this effect.

 

However, in case of sheening ink, it's rarely structure. The light reflection on a crystal surface has a combined effect of absorption and diffraction. It is similar to the effect some mirrored sunglasses have, for example. The light passing through has a different colour tone than the reflected.

(btw.: that's the difference between a mirror and any other reflecting surface; the mirror has no specific absorption in the visible light spectrum, the reflecting surfaces of a dye or some metals have)

 

When I once synthesised crystal violet and cleaned it from impurities, crystals were formed. These were deep purple but their surface reflected golden-yellow light (the opposite colour). You can have this with many dyes in solid (crystalised) form. A sheening ink, therefore, requires a thin solid layer on top of the paper, not necessarily a crystallisation in a certain shape or structure. And that's the exact reason why a non-absorbent paper lets our inks sheen more than an absorbent where the dye is sucked in between the fibres.

I don't quite follow here, are you saying the sheen is a combined effect of absorption and diffraction ?

 

 

On 3/15/2023 at 4:13 AM, txomsy said:

A mirror is a good example: a well polished metal surface will act as a better mirror the better polished it is as all the surface behaves more similarly depending on direction; whereas a matte or irregular metallic surface will result in a more homogeneous color resulting from the mix of wavelengths in all directions from any point of view.

 

It seems to me that if ink crystallized on paper it might tend to form crystals in various orientations, but I am no crystallographer, so that is a totally unscientific assumption on my part

I think structural colour doesn't requires well polished flat surface.  Flat surface means light reflected in the same directions, means a good mirror.  A not so flat surface (for example ink that crystallized in various orientation) 'could' have structural colour if the light reflected has the wavelength shifted or something?  What I meant to say is that flat/polished/uniform surface is not the requirement here.  And again, same as you, this is totally unscientific assumption on my part.

 

Was hoping more people can chime in their thoughts / view~

[txomsy]

Once more: I am no expert! I do not really know what I am talking about! I am just trying to express a most likely wrong mental model.

 

My thoughts were based on the assumption that ink binds to paper and paper has an irregular surface.

 

For structural color to work the structures must have the appropriate orientations to exploit diffraction, refraction, wave interference or whichever other method they rely on to achieve the effect. For a living organism this can be accomplished by defining a basic underlying "algorithm" (a regular combination of gene expression) that ensures all structures grow in the desired orientation (imagine a fractal, or a series of layers).

 

If we assume that paper fibers are more or less randomly oriented or distorted/twisted, when ink is laid down on them, if it forms crystals or any structure, the orientation will be dependent on that of the underlying substrate (the paper fibers) and may result in random, pseudorandom or unpredictable patterns that would interfere with each other, most likely destructively.

 

If we assumed instead that the ink forms a film layer, then the surface would, at most, have a more or less regular wavy disposition and then constructive interference would be more likely.

 

The following is a lame quick and inaccurate attempt at showing it graphically.

 

 

On glossy, more uniform coated or sized paper, behavior might be obviously different, but I would expect it to still be more pronounced with a film-like structure/surface than with less-uniformly oriented crystals.

 

I'm basing my twisted-and-likely-wrong-reasoning on the assumptions that unsized paper offers a more irregular surface, and ink may form crystals with random orientations even on sized/glossy paper. Which are purely hypothetical, mind you.

 

Finally, a plea:  do not let the image mislead your judgement. It is a common mental dissonance to think that just because something is presented with more paraphernalia the underlying reasoning must be sounder. There is no actual connection between presentation and soundness of arguments. Exercise your own reasoning and take it  for what it is worth (i.e. close to nil).

 

[txomsy]

Not to mention it might be a combination of both.

 

In other words, do not take my pitiful reasonings seriously at all.

 

Added:

 

I found the next two threads here on FPN which may shed additional light (bad pun, I know):

 

 

 

So most likely, I was wrong all the way.

And maybe, like in Nature, it may also be that different inks achieve it using different effects or combinations.

[InesF]

Thank you @txomsy for this beautiful drawings.

 

3 hours ago, txomsy said:

I'm basing my twisted-and-likely-wrong-reasoning on the assumptions that unsized paper offers a more irregular surface, and ink may form crystals with random orientations even on sized/glossy paper. ...

Exactly this!

 

The crystals do not need to be in exact the same orientation and therefore the sheen is visible from more than only one narrow angle. If the paper is rough, the crystals appear in a wider span of orientations (= less sheen) or do not appear at all on the surface of absorbent paper (= no sheen).

Sheen monster inks are 1) highly saturated (the more dye the higher the chance of forming a crystal or solid surface on paper) and have 2) some slower evaporating additives, such as polyhydric alcohols that extend the drying time and increase the chance of crystal and solid layer formation for the dye.

 

For the naked eye, paper is a flat surface. That's enough for the sheen effect as we experience it. Perfection in the surface structure is not required.

[amberleadavis]

Wow what a great post. Thank you.

[tim77]

 

Hello everybody,

Here’s a short explanation of the sheening effect of concentrated dyes, from a professional physicist:

 

The more strongly light is absorbed, the more it is reflected. In normal concentrations blue ink scatters blue light and absorbs red and green light. But if the dye is sufficiently concentrated the red and green can start to reflect.

 

It sounds peculiar, but consider a guitar string: if you pinch it strongly over the hard metal of a fret, any string movement at that point is completely absorbed. The result is that the wave just bounces backwards off the fret towards the bridge and you get a standing wave. If you hold the string more loosely the wave is not reflected and the energy dissipates.

 

Metals are reflective because they conduct: the electric field in light waves hitting the metal surface is short-circuited - analogous to pinching the guitar string - so the wave is reflected.

 

Inks tend to sheen if they're allowed to pool on the surface of smooth paper. As they dry out the dye becomes more and more concentrated until (depending on the dye type) it becomes dense enough to reflect. The reflected colour is made up of the light that the ink would normally absorb, so you can't get a blue ink to sheen blue.

 

Footnote:

Certain materials are known for being extremely black. How does that work, if strongly absorbing light causes it to be reflected? They work by having a very rough matte surface, so that any reflected light is likely to be scattered back into the surface for another chance to be absorbed. The description above is for smooth surfaces.

[txomsy]

Most interesting. Thanks.