Currently, very few materials can achieve the level of whiteness required for practical white pigments. This is because highly efficient light scattering is necessary to maintain sufficient whiteness even when applied as a thin coating. For example, chalk (calcium carbonate) is a white powder when solidified, but produces only a translucent white when used as paint. At present, only materials such as white lead and titanium dioxide, both of which have raised toxicity concerns, can produce sufficiently bright white.
A research group led by Professor Easan Sivaniah, Program-Specific Assistant Professor Detao Qin of the Institute for Integrated Cell-Material Sciences (iCeMS), Kyoto University, and Associate Professor Taiki Yanagishima of Tokyo Metropolitan University has established an innovative polymer-processing technology called Deep Foam Photolithography (DFP). The team succeeded in generating world-leading levels of whiteness without using pigments or paints. The findings were published in Nature.
The research group discovered a phenomenon in which microscopic foam structures form within common polymers. These foam structures possess highly efficient light-scattering capabilities and can be applied to high-resolution white printing. The researchers named this method Deep Foam Photolithography (DFP).
DFP is a technique for generating and printing foam structures in polymer films or fibers. DFP consists of two steps: UV exposure and development. When UV light is applied to a polymer containing a photocrosslinking agent, radicals are generated, and polymer chain scission and crosslinking proceed simultaneously. Chain scission produces low-molecular-weight fragments, while crosslinking forms a polymer network.
Next, when the exposed polymer is immersed in a solvent that interacts weakly with the polymer, the solvent penetrates from the polymer surface through osmotic pressure. At that point, the low-molecular-weight products generated through chain scission dissolve more readily in the solvent than the original polymer and therefore act as solutes, selectively attracting surrounding solvent molecules.
At the solvent penetration front, solvent molecules selectively gather around the cleavage products, causing localized phase separation within the polymer. This nucleation-and-growth-type phase separation leads to the formation of foam-like structures. Eventually, when osmotic pressure reaches equilibrium and swelling stops, coalescence and Ostwald ripening of the foam structures are thought to become dominant. As a result, the number of foam structures decreases, while the size of individual foam structures increases.
By selecting appropriate UV exposure conditions, an optimal balance between polymer chain scission and crosslinking can be achieved, enabling the formation of foam structures. Although these structures form in a solvent, they can be removed into air and dried without being damaged. As coalescence of the foam structures progresses more rapidly near the surface, a structure is formed in which the foams become progressively smaller from the surface toward the interior.
The researchers found that this foam structure efficiently scatters light of all wavelengths multiple times. As the original polymer is transparent, there is no significant light absorption, resulting in a nearly perfect white appearance. It has also been confirmed that the DFP process can be applied to any polymer that can be crosslinked using a crosslinking agent.
Specifically, the researchers succeeded in generating foam structures and the accompanying whiteness in a wide range of polymers, including polystyrene, polycarbonate, polysulfone, cellulose acetate, PMMA (acrylic resin), and PET. The technique can also be applied to printing.
Because foam structures form only in UV-exposed areas, white patterns can be generated selectively using simple UV exposure methods such as stencils and UV projectors. Advanced UV exposure systems used for photolithography in the semiconductor industry can also be employed, enabling white printing at a resolution of 20,000 dpi. As a secondary phenomenon, the researchers discovered that the DFP process can also generate superhydrophobic surfaces.
When foam structures repeatedly grow and coalesce in organic solvents under conditions that promote extreme swelling, the foam walls eventually become so thin that they rupture. At that point, solvent is squeezed out from within the foam cells, causing local polymer concentration. This occurs because the foam walls act as semipermeable membranes, allowing only small solvent molecules to pass through while preventing larger polymer molecules from doing so.
In the case of amorphous polymers, polymers that can no longer remain dissolved as a result of this concentration precipitate to form spherical colloids. In contrast, in crystalline and semicrystalline polymers, spherical structures form while crystals simultaneously develop, producing structures that resemble blooming flowers. These flower-like structures exhibit lotus leaf-like superhydrophobicity. Like the white foam structures, the flower-like structures form only in UV-exposed regions. In other words, regions exhibiting superhydrophobicity can be controlled precisely.
Furthermore, flower-like structures can be fabricated not only in polymer films but also in polymer fibers, producing superhydrophobic fibers with fine spine-like surface textures. Although DFP is based on a conventional photolithography process, it can be regarded as a negative-positive hybrid that fits neither of the two classical categories of photolithography: positive-type and negative-type photolithography.
Photolithography is generally considered a mature nanomaterials fabrication technique, but the discovery of unique phenomena such as these under slightly modified conditions provides a new perspective for nanoscience. In addition, sheets produced using this technique can be finely ground into powder and used as a new type of white pigment with broad potential applications. The flower-like structures can likewise be converted into powder, opening up possibilities for their use as superhydrophobic materials that could serve as alternatives to PFAS-based technologies.
Journal Information
Publication: Nature
Title: Foaming photopolymers as a high-resolution biomimetic printing platform
DOI: 10.1038/s41586-026-10968-9
This article has been translated by JST with permission from The Science News Ltd. (https://sci-news.co.jp/). Unauthorized reproduction of the article and photographs is prohibited.