In our newly published research in Science Advances, my student Ben Alessio and I propose a potential mechanism explaining how these distinctive patterns form – that could potentially be applied to medical diagnostics and synthetic materials.
Mixing our base colors
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(gentle music) – [Instructor] I often find it’s handy to work with subtle colors to start with rather than really intense ones. What I’ve got here is some burnt sienna, some cadmium orange, and burnt umber. The cadmium orange has got this really lovely intensity to it. With this orange family, there’s a range here between the most intense to a medium orange in the burnt sienna, and then a muted orange in the burnt umber. Equally, when we look at the violet blue tones, I’ve got a violet iron oxide which in its raw state has got a nice, subtle tone to it. I’ve also got some more intense cobalt violet hue. I might not need this for these first mixes, but I just put a bit out anyway. The pigments I’ll be using for the violet blue family are titanium white, violet iron oxide, and ultramarine blue. So when we take our value, this is an N8, and just hold it against the image, you can see for the sky that N8 is pretty much spot on. That’s a really good value for that sky, so that’s going to…
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Movement and boundaries
Mathematician Alan Turing first addressed this question in his seminal 1952 paper, “The Chemical Basis of Morphogenesis.” Turing showed that under appropriate conditions, the chemical reactions involved in producing color can interact with each other in a way that counteracts diffusion. This makes it possible for colors to self-organize and create interconnected regions with different colors, forming what are now called Turing patterns.
However, in mathematical models, the boundaries between color regions are fuzzy due to diffusion. This is unlike in nature, where boundaries are often sharp and colors are well separated.
Our team thought a clue to figuring out how animals create distinctive color patterns could be found in lab experiments on micron-sized particles, such as the cells involved in producing the colors of an animal’s skin. My work and work from other labs found that micron-sized particles form banded structures when placed between a region with a high concentration of other dissolved solutes and a region with a low concentration of other dissolved solutes.
In the context of our thought experiment, changes in the concentration of blue and red dyes in water can propel other particles in the liquid to move in certain directions. As the red dye moves into an area where it is at a lower concentration, nearby particles will be carried along with it. This phenomenon is called diffusiophoresis.
You benefit from diffusiophoresis whenever you do your laundry: Dirt particles move away from your clothing as soap molecules diffuse out from your shirt and into the water.
Drawing sharp boundaries
We wondered whether Turing patterns composed of regions of concentration differences could also move micron-sized particles. If so, would the resulting patterns from these particles be sharp and not fuzzy?
To answer this question, we conducted computer simulations of Turing patterns – including hexagons, stripes and double spots – and found that diffusiophoresis makes the resulting patterns significantly more distinctive in all cases. These diffusiophoresis simulations were able to replicate the intricate patterns on the skin of the ornate boxfish and jewel moray eel, which isn’t possible through Turing’s theory alone.
This video shows small particles moving due to a related phenomenon called diffusioosmosis.
Further supporting our hypothesis, our model was able to reproduce the findings of a lab study on how the bacterium E. coli moves molecular cargo within themselves. Diffusiophoresis resulted in sharper movement patterns, confirming its role as a physical mechanism behind biological pattern formation.
Because the cells that produce the pigments that make up the colors of an animal’s skin are also micron-sized, our findings suggest that diffusiophoresis may play a key role in creating distinctive color patterns more broadly in nature.
Learning nature’s trick
Understanding how nature programs specific functions can help researchers design synthetic systems that perform similar tasks.
Lab experiments have shown that scientists can use diffusiophoresis to create membraneless water filters and low-cost drug development tools.
Our work suggests that combining the conditions that form Turing patterns with diffusiophoresis could also form the basis of artificial skin patches. Just like adaptive skin patterns in animals, when Turing patterns change – say from hexagons to stripes – this indicates underlying differences in chemical concentrations inside or outside the body.
Skin patches that can sense these changes could diagnose medical conditions and monitor a patient’s health by detecting changes in biochemical markers. These skin patches could also sense changes in the concentration of harmful chemicals in the environment.
