A drop of paint drops on the wet pottery embryo, as if it had gained life, growing a twig-like pattern on the surface of the pottery:

I recently saw this video from ceramic blogger Kevin Kowalski. The automatic pattern of growth and flowering looks very comfortable, and what I want to know more is, how on earth did these patterns grow? So, I did a little search and try with this question, and I will share it with you below.

first of all, it is about this pottery decoration technology, which actually has a long history and can be traced back to the 1780s. The process is called "Mocha diffusion", and Mocha refers to the port city of Mocha by the Red Sea, which is located in present-day Yemen, where Mocha Coffee comes from.

(a finished product using this process)

to make this pattern decoration, you should first cover the pottery embryo with a layer of flowable clay mud, and then drop a specially formulated pigment on it. The earliest formula added liquid made from tobacco leaves, but this is not a key ingredient, and many additives can achieve this dendritic diffusion effect, such as nail polish remover or alcohol.

interestingly, making this pattern is not a patent for pottery, and it can also be achieved with acrylic paint. Pour some flowable acrylic pigments with the right thickness on the surface, and then drop other liquid pigments mixed with alcohol (or isopropanol). As long as the ratio is right, the droplets will spread outward while forming a dendritic pattern.

the following is the result of my attempt with acrylic pigments. The white background pigments and the pigments dripped on them are diluted with some acrylic blending solution. At the same time, the blue part is also diluted with medical alcohol:

although it is not so beautiful (and later the pattern pasted off, it feels that the pigment is a little too thin), but the recurrence of the phenomenon can be said to be successful.

so, what exactly is the principle of this pattern formation? In view of the fact that the process itself has not found research and quantitative analysis, I would like to briefly summarize the discussions I have seen + my personal conjecture.

first of all, the pigment droplets spread outward on the wet surface, and the driving force for this outward flow should be surface tension. The common characteristic of additives such as nail polish remover, isopropanol and alcohol is that the surface tension is lower than that of water, while both acrylic pigments and clay mud can be seen as dispersed in water. As a result, when the pigment with low surface tension drips on it, the imbalance of surface tension pulls the surface liquid outward.

this is similar to the feeling of adding detergent to milk pigment:

the next focus is why the pigment flows outward and twigs appear at the same time. I think this phenomenon may be similar to sticky fingering (viscous fingering). When a low-viscosity fluid flows into another high-viscosity fluid, there are many finger-like, dendritic structures on their interface.

below is a viscosity pointer produced by the injection of air (low viscosity fluid) into a thin layer of oil (high viscosity fluid):

I feel that the pigment is more like a viscosity finger powered by surface tension. but this is really uncertain because of the lack of research at present. To be sure, the viscosity of mud and pigment liquid is indeed an important factor affecting the effect of pattern.

other articles about the Mocha diffusion process will say that this process involves acid-base reactions, but I don't agree with that. The reason for disagreement is that the pattern can be achieved with materials with very different chemical compositions (ceramic mud and acrylic pigments have nothing in common except that they both contain water). If it involves a chemical reaction, it is a bit inconceivable that such a big difference in composition does not affect the effect. And although the original formula may contain acid, but such as alcohol, nail polish remover, this formula does not have any acid or alkali.

to sum up, I think the pigment growing branches should be the result of the joint action of surface tension and fluid viscosity. If you are interested, you can also find some paint to actually test the effect.

reference to this article: https://www.physics.utoronto.ca/~smorris/edl/mochaware/mochaware.html

https://www.yadawei.net/single-post/2016/07/18/Mocha-Diffusion-Explained

https://physics.stackexchange.com/questions/264013/what-is-the-physics-behind-mocha-diffusion

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https://en.wikipedia.org/wiki/Viscous_fingering