JungHyun Noh, featured ILCC liquid crystal artist, March 2015

JungHyun Noh was born in Seoul, Korea. She obtained her B.Sc. degree in Biomedical Engineering at Gachon University (2012) and her M.Sc. degree (2014) at the Graduate School of Convergence Science and Technology (Program in Nano Science and Technology) at Seoul National University, Korea. During her master study, the research topic was mainly focused on cholesteric liquid crystal systems, in particular optical phenomena appearing in cholesteric liquid crystal droplets and self-assembled cellulose nanocrystal films. In 2014, she started her Ph.D. study as AFR scholarship holder at the Physics and Materials Science Research Unit at the University of Luxembourg, advised by Prof. Jan Lagerwall. Her current research interest is on phase transitions and topological defects in liquid crystal shells, produced by a microfluidic system.

Email: junghyun.noh@uni.lu or njjungh@gmail.com
Cano preparation of red-reflecting cholesteric liquid crystal. The concentric texture is obtained by Cano preparation when a red reflecting cholesteric liquid crystal is filled between a flat substrate and a lens. When the liquid crystal is confined between two substrates, imposing the uniform planar surface anchoring, the helix forms from substrate to substrate since the helix axis is perpendicular to the orientation of the director n. Due to the curvature of the lens the distance to the flat substrate increases from the centre to the edge, inducing a spatial modulation of the helix pitch. For parallel anchoring directions at the two substrates the liquid crystal must form an integer number of half turns of the helix between the substrates. In other words, although the substrate distance increases continuously, the helix pitch variation is quantized into half-pitch units. When the substrate distance differs from an integer number m of half-pitch lengths (mp/2), the helix adapts by either stretching out somewhat to fill a gap slightly greater than mp/2, or by compressing a bit in order to fit in a gap that is slightly smaller than mp/2. The distortion gets substantial if m is small, hence the reflection color ranges from green-yellow to red from the inner to the outer side of the innermost regions with constant m, in the extreme case of m=1 even reaching the blue range of selective reflection as seen in the innermost colored circle. At the very center the thickness of the liquid crystal layer fitting between substrate and lens is too small to yield any visible selective reflection. Whenever the radially increasing substrate separation makes the distortion of a compressed (m+1)p/2 helix smaller than the corresponding expanded mp/2 helix, an additional half-turn of the helix is added, giving rise to the set of circular defect lines. Their distance decreases outwards, reflecting the accelerating substrate separation due to the curvature of the lens. With larger m the required distortion of the helix decreases, explaining why the reflection color approaches the natural red color towards the periphery. The image was taken by polarizing optical microscope in reflection mode, using a 20X objective. The width of the image is about 430 μm.

Jury comment: A beautifully centered Cano preparation using a circular convex lens - with its iris-like appearance the image almost seems to look back at the observer.