This short article was initially included on The Conversation
Animal skin patterns, such as zebra stripes and bright toxin frog color spots, have crucial biological functions. These functions include temperature regulation, camouflage, and warning signals. The colors that make up these patterns need to be distinct and well-separated to be effective. To serve as a warning signal, unique colors make them clearly noticeable to other animals. As camouflage, well-separated colors allow animals to better blend into their surroundings.
In our recent research released in Science Advances, my student Ben Alessio and I propose a potential mechanism explaining how these distinct patterns form, which could possibly be used for medical diagnostics and synthetic materials.
Think about the complex task of achieving unique color patterns. Imagine carefully adding a drop of blue and red dye to a cup of water. The drops will gradually spread throughout the water due to the process of diffusion. Ultimately, the water will have an even concentration of blue and red dyes, resulting in a purple hue. Diffusion tends to create color uniformity.
How then can distinct color patterns form in the presence of diffusion?
Motion and borders
Mathematician Alan Turing first addressed this question in his seminal 1952 paper, “The Chemical Basis of Morphogenesis“. Turing showed that under certain conditions, the chemical reactions involved in producing color can interact in a way that counteracts diffusion. This allows colors to self-organize and create interconnected regions with different colors, forming what are now called Turing patterns.
In mathematical models, the boundaries between color regions are fuzzy due to diffusion. This contrasts with nature, where boundaries are typically sharp and colors are well-separated.
Our team hypothesized that insights into how animals produce distinct color patterns might be found in laboratory 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 laboratories revealed that micron-sized particles form banded structures when placed between an area with a high concentration of other dissolved solutes and an area 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 cause other particles in the liquid to move in specific directions. As the red dye moves into an area where it is at a lower concentration, neighboring particles will be drawn along with it. This phenomenon is called diffusiophoresis.
You take advantage of diffusiophoresis whenever you do your laundry: Dirt particles move away from your clothes as soap particles diffuse out from your shirt and into the water.
