Direct Observation of the Phase Transition of Topological Defects

Professor Dong Ki Yoon’s research team at KAIST observed the phase transition of topological defects formed by liquid crystal (LC) materials (Nature Communications, 2017, 8, 15453). The phase transition of topological defects, which was also the theme of the Nobel Prize for Physics in 2016, can be difficult to understand for a layperson, but it needs to be studied to understand the mysteries of the universe or the underlying physics of skyrmions, which have intrinsic topological defects.

If the galaxy is taken as an example in the universe, it is difficult to observe topological defects because the system is too large to observe some changes over a limited period of time. In the case of defect structures formed by LC molecules, they are not only a suitable size to observe with an optical microscope, but also the time period in which the phase transition of a defect occurs can be directly observed over a few seconds, which can be extended to a few minutes. The defect structures formed by LC material have radial, circular, or spiral shapes centering on a singularity (defect core), like the singularity that was already introduced in the famous movie “Interstellar,” which is the center point of a black hole.

In general, LC materials are mainly used in liquid crystal displays (LCDs) and optical sensors because it is easy to control their specific orientation and they have fast response characteristics and significant anisotropic optical properties. For the performance of LCDs, it is advantageous for the defects of LC materials to be 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 LC defects as building blocks to make micro- and nanostructures for patterning applications. During these efforts, they found a way to directly study the phase transition of topological defects under in-situ conditions.

Considering LC materials from the viewpoint of a device like an LCD, robustness is important. Therefore, the LC material is injected through the capillary phenomenon between a rigid two-glass plate, and the orientation of the LCs can be followed by the surface anchoring condition of the glass substrate. However, in this conventional case, it is difficult to observe the phase transition of LC defects due to this strong surface anchoring force induced by the solid substrate.

To solve this problem, the research team designed a platform, in which the movement of LC molecules was not restricted, by forming a thin film of LC material which, like oil, floats on water. For this, a droplet of LC material was dripped onto 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. In addition, this approach can trace the morphology of the original defect structure from the sequential changes during the temperature changes, which can provide insights for the study of the formation of topological defects in the cosmos or skyrmions.

Prof. Yoon said, “LC crystal defects themselves have been extensively studied by physicists and mathematicians for about 100 years. However, this is the first time that we have observed the phase transition of LC defects directly.”