Leaf boat powered by pen juice

Putting ballpoint pen ink on the tip of a leaf then putting it on water (or milk if you want it to stand out) causes the leave to whizz around leaving trail of ink behind.

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[SteveMould]

The number of takes I had to throw away because I had ink on my face!

[reesemartens]

Hey Steve,

I'm a Nanotechnology Engineer that has a little bit or perspective to share as this kinda sits right on the border between the chemistry and physics that I work with every day.

First and foremost, the propulsion you are seeing is definitely as a result of that difference in surface tension you mentioned. In fact this particular propulsion method is used a lot in various fields and usually the forces involved are referred to as Marangoni forces (because they arise from the Marangoni effect). Second, the way the ink spreads out actually screams that there is some surfactant and some phenoxy ethanol, and this is actually counter to one of the points you mentioned. You suggested that if a surfactant was present, the ink layer would break up into smaller pieces which can actually be seen on the outer edges of the trails in your videos. There often appears to be fractures with no external causal force. The other thing is that the pieces then appear to "bounce" away from one another despite neither piece involved in a collision being moving all that quickly. Altogether though there is some interesting Materials Science and fluid mechanics going on here and I'm excited about any follow up videos you make on the subject
Cheers

[PlasmaChannel]

The fractal nature of the ink was the most shocking part to me. I couldn't help but imagine a blossoming flower every single time that leaf boat started up. Really interesting stuff Steve.

[alfredchiu]

As an ink developer (but not the kind of a writing pen), I'd say this is perfectly normal and we see it everyday in lab. First of all, solvents used in inks generally have low surface tension to facilitate the wetting of ink on substrates and might help the transfer of ink from the tip of pen onto paper though I'm not sure in that. The low surface tension first causes the spreading of the ink on water, which is a high surface tension substrate. Most solvents are also immiscible with water and have lower density, which makes them stay separated from water and float on top. On the second fact, the pigments usually require anchoring agents/dispersant/surfactants or whatever you call it, to help them disperse in solvents with high stability and long lasting manner. Pigments and dyes used in inks may also poorly disperse in water which prevents them from "dissolving into the water" as in the video. The last fact is that we use various kinds of polymers to give functions to the inks, such as the shininess, water resistance or other desirable mechanical properties. These polymers generally don't dissolve well in water and forms a rigid solid film when the solvent carrier dries away.

In sum, the solvent properties (low surface tension, viscosity, immiscibility with water) causes the spreading of the ink on top, and the pigment or dye used in the ink are chemically modified to improve dispersibility in the solvent and prevents them from into water. Lastly as the solvents evaporates, the polymers in the ink solidifies and what's left behind is a film of water-insoluble polymer, holding pigment or dye molecules that were dispersed in the solvent.

[grandexandi]

What I like about this channel better than other science channels is that he asks us questions and shares his investigation process, instead of just giving us facts and answers.

[LGreenGriffin]

I sincerely love hearing, "I don't really know why it does that, " in this video. It sort of emphasizes that you can pursue a general understanding of a scientific phenomenon to a satisfactory degree without necessarily ruining the mystery of it.

[richierescue]

Many pens are intended to be 'waterfast'.
Once a mark is made on paper it is desirable to fix it there so presumably Bic will have some proprietary chemistry that bonds to the pigment/dye more aggressively than plain water and helps it resist spills and moisture once applied.

[idahogie]

Gonna hold off until I hear from ElectroBoom.

[alexsignell]

A similar thing happens if you get some tree sap on the end of a small stick and place it in water. It will move around and the left over sap will form a similarly brittle layer on the surface of the water.

[zloth]

I have actually done this experiment before by accident when i broke my ball pen and tried to clean it off in the sink with the water not drained, there i saw these shapes coming out from the drops of ink that fell on the water. I then decided to break another pen and played with it for an hour.

[Rubrickety]

The fractal boundary on the wriggling blob around 4:28 is magnificent; it could just about be a Julia set illustration. I’d love to see some high-def magnified images of that.

[davincidamaster]

Interesting observation + good videography and demonstration + clear and easy to follow explanation = Great Video!

[Veeno]

That footage of phenoxyethanol on water at 3:54 immediately reminded me of sped up and zoomed out footage of Conway's Game of Life.

[Lu_Woods]

Very fun !!
As a marbler, I'm wondering if skimming the surface would reduce the surface tension for smoother
trails.
Surfactants used in marbling inks are Oxgall, Photo-Flo, Spirits of soap...
We use aluminum sulphate as a mordant for the paper that captures the pattern.
In case you want a hard copy of your experiments.

[cirkmannzirkel]

Really good teachers let us know when they have no idea what's actually going on, in contrast to all those snobs that make us all think we're dumb while they don't really know it better. Thanks Steve, very refreshing, propelled by curiosity, great!

[piranha]

7:55 : "Maybe [...] something reacts with the water and becomes a hard surface that traps the dye".

I believe that's what a polymer would do, which is likely added to the ink to give it the right viscosity. The phenoxyethanol acts as a plasticizer, but once it dissolves in the water, the polymer becomes hard.

(And by the way, phenoxyethanol itself is somewhat of a surfactant. The hydroxyl moiety is somewhat hydrophilic, whereas the phenyl end is hydrophobic. So that would explain why it first rapidly spreads out, before it dissolves in the water.)

[hermansimonsen]

when the phenoxyethanol dissolves in the water, it looks like fractals. Pretty cool, math and nature go hand in hand.

[mersilvaureus]

Might have to do with the pigment/dye particles being encapsulated by some of the other ingredients? It's apparently a common practice in the paint industry if I remember correctly.

[jacobdavidson]

I used to love doing this as a kid, I would watch the little blob of ink fly around the surface and then place paper on top of the water once there was a cool pattern, made tons of them.

[JulienCope]

As a kid, we used pine needles that we would poke in sap bubbles trapped underneath the bark of pines trees to make needle race boats. The movements and iridescent trail left behind was extremely similar.