Chemical reactions with Liesegang rings: generation of non-permanent thermal patterns†

Patterns are encountered and employed in nature, such as in the communication or growth of organisms and sophisticated behaviors such as camouflage. Artificial patterns are not rare, either. They can also be used in sensing, recording information, and manipulating material properties. Natural or artificial, most patterns have the colors of visible light. Patterns of infrared radiation are rare, even though some organisms and artificial sensors can detect infrared. In this work, we display a formation of non-permanent infrared Liesegang patterns in different gel media. We used the exothermic neutralization of the solid Mg(OH)₂ formed as Liesegang patterns (LPs) to obtain these patterns. The LPs of infrared radiation (IR patterns) appear due to a temperature increase of up to 4 °C on the gel surface. We also show that it is possible to ‘read’ the information with an IR camera when the patterns are concealed and invisible. The idea can be expanded to other chemical systems and used to communicate with IR. On the other hand, the chemical reactions on the patterned solids can also be used to pattern other artificial material patterns directly inaccessible with the existing reaction–diffusion systems.