Friday 22 December 2023

Chapter 12: Surface Tension

In 2021, after about five years of various related experiments, I started a group of work I now call γ, or gamma. This name came from the symbol physicists use for surface tension.
These works are very small watercolour paintings, of about one to three millimetres in size, framed in white frames with a white matte that has an appropriately sized cut-out at the position of the painting on the leaf.

Although these paintings are very small, the fact that they are small is by itself not what's interesting about them. What is interesting is that they are a very specific kind of small that emphasizes attributes that will only be significant at this scale.
Over the years I've developed a way of painting that gives these painted shapes a very sharp outline of about 20 to 70 micrometers thick. This is about half the width of a human hair and right on the limit of what the human eye can see. Although it is almost imperceptibly thin, this line is what gives a well defined shape and outline to what otherwise would be vague blobs that are barely distinguishable from the paper they rest on.

I had a hypothesis about how this edge is formed which involves cohesion, or the tendency of similar molecules to 'stick together'. As paint is introduced into a water droplet, the water molecules want to stay together and so they 'push' the paint downward, where the effect of the water 'pushing' the paint down is greater in the centre than it is at the edges, resulting in more paint accumulating towards the edges of the droplet.

When I work on these paintings, I only observe them from above through a small loupe, giving me a limited view and thus limited information about what is actually happening inside the droplet. In order to test my hypothesis, I therefore decided to record the process under about 10x magnification. For reference, the brushtip in the following pictures is about 0,4 mm in diameter, making the droplets about 1,5 mm wide. In this process I observed two major things that happened that seemed to be in opposition with each other:

The first thing I observed is what I naively expected to happen. When watercolour paint, itself a suspension of pigments in a solution of water and gum arabic, enters into an existing droplet of water it kind of sinks to the bottom and spreads. This leaves a uniform covering of pigment on the paper that takes the shape of the droplet after it dries.
However, as can be seen in the above photographs, this mechanism also seems to leave a thin barrier of clear water at the bottom edges of the droplet. If that area contains little, if any, paint, then how can that be the part that contains a much stronger concentration of pigments after it has dried?
Clearly something else is also at work here, and one thing I noticed is that on the right side a little blob of paint doesn't seem to enter into the water droplet, but instead kind of slides down the surface of the droplet. I thought this would be caused by that part of the paint not breaking the surface tension of the droplet and thus never entering into it.

I tested this by creating another droplet with a higher contact angle, which from what I understand should increase the surface tension. While in the previous example the contact angle was approximately 60º, in this droplet it is roughly 90º.
As you can see, the behaviour of the added watercolour paint changes in this situation. The paint indeed 'sticks' more to the edges of the droplet, instead of 'falling' straight down.

A third water droplet has a contact angle of about 120º. In this larger droplet, clearly under more tension, the paint seems to stay almost exclusively at the surface of the droplet, with only a small strand of paint passing the barrier. What's interesting to note is that this strand seems to reach the centre of the droplet almost instantly and it stays connected to the rest of the paint as it glides down the side. It also does this much more quickly than the larger blob in the first example.

These are the second and seventh frame from the first and third examples. The time between these two points was 3,13 seconds in the top example and 1,77 seconds in the second. Dissolution takes time, and on such a small scale these differences can have a significant effect.

A difficulty related to documenting these phenomena is that although the previous images give a reasonable idea about the paint being on the side or on the centre of the droplet, it doesn't show what is happening at the bottom at all sides. As understanding what happens there is vital to the impact the final paintings may have, a different experimental set-up is required in future research.

Yet another aspect that has to be kept in mind with these paintings is that different pigments will have different molecular composition, which will dissolve differently in both pure water and a solution of water and gum arabic.

This is an orange pigment, which produces much more coarse flakes of pigment. It quickly sinks to the surface of the paper and doesn't move much from its inital position. What is interesting to note is that the small amount of pigment that does dissolve in the water ultimately ends up at the edge of the droplet, as can be seen in the final frame.
This effect of pigment clustering at the edges can also be seen in a different test, that had a minimal droplet volume:

After drying, this gave a nearly closed outline of the droplet, with little to no pigment in the centre:

Again, please remember that the diameter of this circle is only about 1,5 mm.
Although these observations have given me some insights into the behaviour of the paint when introduced to a droplet of water, the system is in constant motion and dependent on many variables. It takes anywhere from a few minutes to an hour for a droplet to fully dry, so there is plenty of opportunity for the paint to dissolve and move about.

This sequence shows the full drying process. The first six images span the first few seconds and the last six images comprise about a minute. The three middle images on the other hand are about fifteen minutes apart.
The last few images also clearly show a strong interaction with the fibres of the paper, something that hasn't yet been considered in the previous images.

Since making these images I have continued to create some paintings, with some experiments using varying temperatures, separating differently coloured pigments inside one droplet and controlling the exact shape and intensity of the 'ring' at the edge of the droplets.
Yet further research is still warranted. As it is a system that is constantly in motion, there are many variables and the accompanying theory is complex, with observations from one instance not necessarily carrying over to another. Important variables like pigment and paint composition, paper structure, temperature, drying time and methods for introducing the paint are not yet mapped.
All of this also doesn't say anthing yet about the artistic effects that could be obtained after the mechanisms are properly understood and can be predictably deployed.
Even though I have been working on it on some way for the past two years, I thus feel like my knowledge about the subject is still in its infancy.