researchers at the korea advanced institute of science and technology (kaist) in daejon, south korea have observed the phase transition of topological defects formed by liquid crystals for the first time, according to a report on the kaist website.

polarizing optical microscopy images of topological defects depending on the strength
of the director field. (a,b,e) convergent director field arrangements of lc molecules
and corresponding schematic images; (c,d,f) divergent director field arrangements of lc
molecules and corresponding schematic images. (kaist)

the researchers were able to study liquid crystals because they are observable with an optical microscope and also because the phase transition of a defect occurs in a few seconds and extends to a few minutes. the defects formed by liquid crystals have radial, circular, or spiral shapes centered on a defect core (which the article equates to the singularity introduced in the recent hollywood movie, interstellar).

liquid crystals are typically used for lcd screens and for optical sensors because of their specific orentations and fast response characteristics.

the article explained, “it is advantageous in terms of the performance of lcds that the defects of the lc materials are minimized. the research team led by professor dong ki yoon in the graduate school of nanoscience and technology did not simply minimize such defects but actively tried to use the lc defects as building blocks to make micro- and nanostructures for the patterning applications. during these efforts, they found the way to directly study the phase transition of topological defects under in-situ conditions.”

to study the liquid crystals, researchers designed a platform in which the molecules were allowed to move freely. the liquid crystals were placed on the surface of water and spread to form a thin film.

“the topological defects formed under this circumstance could show the thermal phase transition when the temperature was changed,” the article noted. “in addition, this approach can trace back the morphology of the original defect structure from the sequential changes during the temperature changes, which can give hints to the study of the formation of topological defects in the cosmos or skyrmions.”

the research was recently published in nature communications. the abstract read:

“the liquid crystalline phases of matter each possess distinct types of defects that have drawn great interest in areas such as topology, self-assembly and material micropatterning. however, relatively little is known about how defects in one liquid crystalline phase arise from defects or deformations in another phase upon crossing a phase transition.

“here, we directly examine defects in the in situ thermal phase transition from nematic to smectic a in hybrid-aligned liquid crystal droplets on water substrates, using experimental, theoretical and numerical analyses.

“the hybrid-aligned nematic droplet spontaneously generates boojum defects. during cooling, toric focal conic domains arise through a sequence of morphological transformations involving nematic stripes and locally aligned focal conic domains.

“this simple experiment reveals a surprisingly complex pathway by which very different types of defects may be related across the nematic–smectic a phase transition, and presents new possibilities for controlled deformation and patterning of liquid crystals.”