A thin slice of eucalyptus under a polarized light microscope lights up like a stained-glass window: purple, yellow, and blue patches. These colors are not just decoration. They show the direction of the tiny cellulose fibers within the wood's cell walls. Previously, seeing this arrangement required destroying the sample, but now polarized light reads it like a map, without damaging the wood.
The secret is that cellulose refracts light differently depending on the direction of the fibers. When light passes through two polarizing filters with the sample in between, the tilt of the fibers is translated into color. But there was a problem: fibers tilted at 45° and 135° produced the same color, making them indistinguishable. The researchers added a compensator, a plate that shifts the light wave by 530 nanometers. This shift separates the two directions and turns them into different colors, like an interpreter who distinguishes between similar words.
The method was tested on slices from a eucalyptus trunk and branch. It turned out that in the trunk, 67% of cells have a twisted spiral arrangement, while in the branch bark, cells with a crossed spiral were found that are indistinguishable from linear ones under a regular microscope, but under polarized light they appear purple instead of blue. Three different identification methods gave the same result, confirming the accuracy of the color markers. Now this color map can be used for automated analysis with machine vision, speeding up wood quality control and the development of new materials.
Cellulose microfibrils in a tree's cell wall are thinner than the wavelength of visible light, but their collective arrangement creates colors visible to the eye. In the eucalyptus trunk, 67% of cells have a twisted spiral structure that resembles the cholesteric liquid crystal phase.
The idea of reading the 'memory of a tree' through color patterns echoes Ursula K. Le Guin's novel 'The Word for World Is Forest', where the forest stores information in its structure. Here, polarized light acts as a translator from the language of cellulose spirals to the language of color.