An unusual arrangement of particles has been discovered in the cells of chicken eyes. It's the first time scientists have seen such a system in a biological system — one that allows materials to behave like both a crystal and a liquid.
The unique arrangement is called "disordered hyperuniformity," and it could help researchers design advanced materials, such as optics that can transmit light with the efficiency of a crystal and the flexibility of a liquid.
When matter is organized into states of disordered hyperuniformity, it exhibits order over large distances and disorder over small distances. At one level, it's like a crystal that greatly suppresses differences in the density of particles across large spatial distances. But at another scale, it's liquid-like in that it exhibits similar physical properties in all directions.
Image: Left: visual cell distribution in chickens; right: a computer-simulation model showing pretty much the exact same thing. The colored dots represent the centers of the chicken's eye cells (Courtesy of Salvatore Torquato, Princeton University).
"We've...discovered that such physical systems are endowed with exotic physical properties and therefore have novel capabilities," noted Princeton researcher Salvatore Torquato in a statement. "The more we learn about these special disordered systems, the more we find that they really should be considered a new distinguishable state of matter."
Neat, right? But why would a chicken need such a seemingly advanced biological trait?
It has to do with the way the chicken's four photoreceptors — violet, blue, green, and red — and a fifth light-detecting type are crammed into a single tissue layer on the retina. Finding a way to organize these five elements in an optimal way is a huge technical challenge (which is called a "packing problem) — but evolution found a way.
Image: At left, two cells appear randomly placed. The exclusion regions for both the black (center) and red cells (right) push against nearby cells and give the cell arrangements structure. The black cells fall into a triangle-like pattern in the absence of other cell types. (Courtesy of Salvatore Torquato, Princeton University).
"Because the cones are of different sizes it's not easy for the system to go into a crystal or ordered state," Torquato said. "The system is frustrated from finding what might be the optimal solution, which would be the typical ordered arrangement. While the pattern must be disordered, it must also be as uniform as possible. Thus, disordered hyperuniformity is an excellent solution."
This means that the arrangement of photoreceptors in chickens, although not perfectly regular, are, in fact, as regular as they can be given the packing constraints in the cellular tissue.
Read the entire study at Physical Review E.