A scientific team from the SAS has clarified the behavior of liquid crystals
Liquid crystals are an important component of many modern technologies, ranging from smartphones displays and smart windows to sensitive sensors. In a new study published in the prestigious journal Scientific Reports, a team of scientist from the Institute of Experimental Physics of the Slovak Academy of Sciences in Košice, in cooperation with international partners, demonstrated how these materials behave in electric and magnetic fields and how their properties can be significantly influenced by even very small changes in composition. The research focused on cholesteric liquid crystals, which are unique because their molecules naturally form a helical structure, and it gives the material its unique optical properties.
The scientists observed what happens when a special additive, a chiral dopant, is added to the material to promote the twisting of molecules into a helix. They found that a low concentration of this substance, the helical structure does not form at all in thin layers. A crucial role played here the specially treated surface of the experimental cell, which forces the molecules to align perpendicularly to the cell surface. This leads to a conflict between material´s natural tendency to form a helical arrangement and the influence of the cell surface. Only after exceeding a critical concentration of the chiral dopant is it possible to create a stable helical structure.
The researchers investigated how these structures respond to external stimuli. It turned out that a stronger electric or magnetic field can unwind the tightly wound helix. However, at higher concentrations of the additive, this unwinding did not occur smoothly, but rather in discrete steps.
The most interesting finding was the behaviour of the material when the field was reduced again. The system does not immediately return to its original state, but exhibits so-called hysteresis- a memory effect. This is a state when two different stable optical arrangements can coexist within the material. This phenomenon is crucial for technologies that require energy- efficient switching between multiple states.
„Our results show that a properly chosen concentration of the chiral additive, combined with the geometry of the cell, determined whether the cholesteric structure unwinds smoothly or abruptly, and what field strength is required for this transition,“ explains the study´s lead author, Veronika Lacková. „The findings are important for designing materials for future electro- and magneto- optical applications, where a precise and repeatable change of the optical state is desired at the lowest possible energy consumption.“
The study also lays the foundation for the future research into more complex systems, in which the Košice- based scientists plan to instigate liquid crystals in combination with magnetic nanoparticles. Such hybrid systems could respond more sensitivity to external stimuli and fins application in future display and photonic technologies.
Prepared by: Veronika Lacková, Institute of Experimental Physics SAS