Light Controls Liquid Crystals express
Molecules in a solution behave like a crowd in a square: they can jostle randomly or line up in neat rows. An aqueous solution of disodium cromoglycate is a well-known model for chromonic liquid crystals, where such 'rows' form spontaneously. But if you shine ultraviolet light on the solution, order collapses: molecules start breaking apart.
These fragments, photodegradation products, wedge themselves between intact molecules and prevent them from assembling into tidy columns. As a result, the disordered region in the phase diagram expands, and the ordered phase retreats. Scientists confirmed this by combining X-ray scattering and polarized optical microscopy: the scattering pattern reveals how regular structures disappear.
This is not just a lab curiosity. It turns out that light can finely tune the properties of soft materials: irradiate, and the liquid becomes less ordered; wait, and it partially recovers. Such a 'light switch' could be useful in creating smart coatings, sensors, or even in biomedicine, where a reversible response to an external stimulus is important.
Original abstract
as written by the authors arXiv:2603.14725
Aqueous solutions of disodium cromoglycate (DSCG), a representative model system for chromonic liquid crystals, exhibit temperature- and concentration-dependent phase behaviors spanning isotropic, nematic, and columnar phases, as well as their coexistence regions. Nastishin et al. (2018) reported that UV irradiation can alter the phase diagram, transforming a nematic phase into a nematic-isotropic biphasic state due to weakened molecular attractions, accompanied by a slow post-irradiation relaxation. Here, we revisit this phenomenon and elucidate the molecular origin of this phase diagram shift: the UV-induced photodegradation of DSCG into specific photodegradation products, which we identify using liquid chromatography-mass spectrometry. Through an integrated approach combining in situ X-ray scattering and polarized optical microscopy, we demonstrate that these degradation products disrupt the self-assembly of DSCG aggregates, thereby expanding the isotropic and biphasic regions in the phase diagram. These findings demonstrate that chromonic assemblies and their phase behaviors are highly sensitive to minor chemical alterations, providing a potential route toward light-controlled self assembly of soft matter.
Chromonic liquid crystals occur in nature: for example, some dyes and even the molecules that make up viruses form such columnar structures.