References of "Noh, Junghyun 50002766"
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See detailLiquid crystals in micron-scale droplets, shells and fibers
Urbanski, Martin UL; Reyes, Catherine UL; Noh, Junghyun UL et al

in Journal of Physics : Condensed Matter (2017), 29

The extraordinary responsiveness and large diversity of self-assembled structures of liquid crystals are well documented and they have been extensively used in devices like displays. For long, this ... [more ▼]

The extraordinary responsiveness and large diversity of self-assembled structures of liquid crystals are well documented and they have been extensively used in devices like displays. For long, this application route strongly influenced academic research, which frequently focused on the performance of liquid crystals in display-like geometries, typically between flat, rigid substrates of glass or similar solids. Today a new trend is clearly visible, where liquid crystals confined within curved, often soft and flexible, interfaces are in focus. Innovation in microfluidic technology has opened for high-throughput production of liquid crystal droplets or shells with exquisite monodispersity, and modern characterization methods allow detailed analysis of complex director arrangements. The introduction of electrospinning in liquid crystal research has enabled encapsulation in optically transparent polymeric cylinders with very small radius, allowing studies of confinement effects that were not easily accessible before. It also opened the prospect of functionalizing textile fibers with liquid crystals in the core, triggering activities that target wearable devices with true textile form factor for seamless integration in clothing. Together, these developments have brought issues center stage that might previously have been considered esoteric, like the interaction of topological defects on spherical surfaces, saddle-splay curvature-induced spontaneous chiral symmetry breaking, or the non-trivial shape changes of curved liquid crystal elastomers with non-uniform director fields that undergo a phase transition to an isotropic state. The new research thrusts are motivated equally by the intriguing soft matter physics showcased by liquid crystals in these unconventional geometries, and by the many novel application opportunities that arise when we can reproducibly manufacture these systems on a commercial scale. This review attempts to summarize the current understanding of liquid crystals in spherical and cylindrical geometry, the state of the art of producing such samples, as well as the perspectives for innovative applications that have been put forward. [less ▲]

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See detailTaming Liquid Crystal Self-Assembly: The Multifaceted Response of Nematic and Smectic Shells to Polymerization.
Noh, Junghyun UL; henx, Benjamin; Lagerwall, Jan UL

in Advanced Materials (2016)

By photopolymerizing liquid crystal shells, their rich variety of self-assembled structures can be rendered permanent and the lifetime extended from days to months, without removing the characteristic ... [more ▼]

By photopolymerizing liquid crystal shells, their rich variety of self-assembled structures can be rendered permanent and the lifetime extended from days to months, without removing the characteristic responsiveness. If polymerization is carried out close to either boundary of the nematic phase, the process triggers the transition into the adjacent phase, to higher or to lower degree of order. [less ▲]

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See detailCorrelation between structural properties and iridescent colors of cellulose nanocrystalline films
Licen, M. V.; Majaron, B.; Noh, Junghyun UL et al

in Cellulose (2016)

We investigate the effect of shear flow applied during the drying of aqueous suspension of cellulose nanocrystals on optical reflective properties and structural characteristics of the resulting ... [more ▼]

We investigate the effect of shear flow applied during the drying of aqueous suspension of cellulose nanocrystals on optical reflective properties and structural characteristics of the resulting solidified films. Shear flow can significantly improve internal structural homogeneity of the films, while its effect on optical reflective properties is relatively minor. The measured width of the selective reflection peak is an order of magnitude larger than expected for an ideal helically modulated structure, which reflects a distribution of pitch values and possibly also of regimes of distorted helical modulation. We attribute these imperfections to the broad size distribution of the cellulose nanocrystals. [less ▲]

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See detailHigh-fidelity spherical cholesteric liquid crystal Bragg reflectors generating unclonable patterns for secure authentication
Geng, Yong UL; Noh, Junghyun UL; Drevensek-Olenik, Irena et al

in Scientific Reports (2016), 6(26840), 1-8

Monodisperse cholesteric liquid crystal microspheres exhibit spherically symmetric Bragg reflection, generating, via photonic cross communication, dynamically tuneable multi-coloured patterns. These ... [more ▼]

Monodisperse cholesteric liquid crystal microspheres exhibit spherically symmetric Bragg reflection, generating, via photonic cross communication, dynamically tuneable multi-coloured patterns. These patterns, uniquely defined by the particular sphere arrangement, could render cholesteric microspheres very useful in countless security applications, as tags to identify and authenticate their carriers, mainly physical objects or persons. However, the optical quality of the cholesteric droplets studied so far is unsatisfactory, especially after polymerisation, a step required for obtaining durable samples that can be used for object identification. We show that a transition from droplets to shells solves all key problems, giving rise to sharp patterns and excellent optical quality even after polymerisation, the polymerised shells sustaining considerable mechanical deformation. Moreover, we demonstrate that, counter to prior expectation, cross communication takes place even between non-identical shells. This opens additional communication channels that add significantly to the complexity and unique character of the generated patterns. [less ▲]

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See detailTransmission polarized optical microscopy of short-pitch cholesteric liquid crystal shells
Geng, Yong UL; Noh, Junghyun UL; Lagerwall, Jan UL

in Proceedings of SPIE - The International Society for Optical Engineering (2016, March 07), 9769

We recently demonstrated that colloidal crystal arrangements of monodisperse droplets or shells of planar-aligned cholesteric liquid crystal exhibit intricate patterns of circularly polarized reflection ... [more ▼]

We recently demonstrated that colloidal crystal arrangements of monodisperse droplets or shells of planar-aligned cholesteric liquid crystal exhibit intricate patterns of circularly polarized reflection spots of different colors. The spots appear as a result of photonic cross communication between droplets, hence the patterns reflect the macroscopic arrangement of droplets or shells. Apart from being an interesting optical phenomenon, it offers attractive application opportunities in photonics and beyond, due to the unique characteristics of the patterns. It turns out that the optical quality of shells is much enhanced compared to that of droplets, hence we focus our attention primarily on shells, of varying thickness. Here we analyze and explain the intriguing textures arising when studying planar-aligned short-pitch cholesteric shells in transmission polarizing optical microscopy. In this case, the texture reflects the properties of each individual shell, without any sign of cross communication, yet also this pattern holds some fascinating mysteries. These can only be elucidated by considering all the peculiar optical properties of cholesterics together, as well as the unusual situation given by the spherical shell geometry. [less ▲]

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See detailInfluence of interface stabilisers and surrounding aqueous phases on nematic liquid crystal shells
Noh, Junghyun UL; Reguengo De Sousa, Kevin; Lagerwall, Jan UL

in Soft Matter (2015), in press

We investigate the nematic–isotropic (N–I) transition in shells of the liquid crystal 5CB, surrounded by aqueous phases that conven- tionally are considered to be immiscible with 5CB. The aqueous phases ... [more ▼]

We investigate the nematic–isotropic (N–I) transition in shells of the liquid crystal 5CB, surrounded by aqueous phases that conven- tionally are considered to be immiscible with 5CB. The aqueous phases contain either sodium dodecyl sulfate (SDS) or polyvinyl alcohol (PVA) as stabiliser, the former additionally promoting homeotropic director alignment. For all shell configurations we find a depression of the clearing point compared to pure 5CB, indicating that a non-negligible fraction of the constituents of the surrounding phases enter the shell, predominantly water. In hybrid- aligned shells, with planar outer and homeotropic inner boundary (or vice versa), the N–I transition splits into two steps, with a consequent three-step textural transformation. We explain this as a result of the order-enhancing effect of a monolayer of radially aligned SDS molecules adsorbed at the homeotropic interface. [less ▲]

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See detailDynamic and complex optical patterns from colloids of cholesteric liquid crystal droplets
Noh, Junghyun UL; Drevensek-Olenik, Irena; Yamamoto, Jun et al

in Proceedings of the SPIE (2015), 9384

Drops or shells of a planar-aligned short-pitch cholesteric liquid crystal exhibit unique optical properties due to the combination of Bragg reflection in the cholesteric helix and a radial orientation of ... [more ▼]

Drops or shells of a planar-aligned short-pitch cholesteric liquid crystal exhibit unique optical properties due to the combination of Bragg reflection in the cholesteric helix and a radial orientation of the helix axis. If such a droplet is illuminated from above, light is reflected into a continuous set of cones, the opening angles of which depend on where on the droplet the light hits its surface. For the wavelength that fulfills the Bragg condition the reflection is dramatically enhanced, yielding the light cones colored. A photonic cross communication scheme arises for certain angles, reflecting light back to the observer from a different droplet than the one originally illuminated. This gives rise to an intricate pattern of colored and circularly polarized spots. A number of interesting applications may be developed based on this pattern, e.g. in identification and authentication devices. We have carried out a detailed spectrophotometric analysis of the patterns, localized to individual spot maxima. A quantitative comparison between the measured spectra and the reflection wavelength expected from a model for the pattern generation allows us to conclude that the droplets are in fact not spherical but slightly ellipsoidal. [less ▲]

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See detailMacroscopic control of helix orientation in films dried from cholesteric liquid crystalline cellulose nanocrystal suspensions

Park, Ji Hyun; Noh, Junghyun UL; Schütz, Christina et al

in Chemphyschem : A European Journal of Chemical Physics and Physical Chemistry (2014), 15(7), 1477-1484

The intrinsic ability of cellulose nanocrystals (CNCs) to self-organize into films and bulk materials with helical order in a cholesteric liquid crystal is scientifically intriguing and potentially ... [more ▼]

The intrinsic ability of cellulose nanocrystals (CNCs) to self-organize into films and bulk materials with helical order in a cholesteric liquid crystal is scientifically intriguing and potentially important for the production of renewable multifunctional materials with attractive optical properties. A major obstacle, however, has been the lack of control of helix direction, which results in a defect-rich, mosaic-like domain structure. Herein, a method for guiding the helix during film formation is introduced, which yields dramatically improved uniformity, as confirmed by using polarizing optical and scanning electron microscopy. By raising the CNC concentration in the initial suspension to the fully liquid crystalline range, a vertical helix orientation is promoted, as directed by the macroscopic phase boundaries. Further control of the helix orientation is achieved by subjecting the suspension to a circular shear flow during drying. [less ▲]

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See detailTuneable Multicoloured Patterns From Photonic Cross Communication Between Cholesteric Liquid Crystal Droplets
Noh, Junghyun UL; Liang, Hsin-Ling; Drevensek-Olenik, Irena et al

in Journal of Materials Chemistry C (2014), 2(5), 806-810

Monodisperse droplets of planar-aligned cholesteric (N*) liquid crystal exhibit an intriguing capacity for photonic cross-communication, giving rise to colourful patterns that depend sensitively on the N ... [more ▼]

Monodisperse droplets of planar-aligned cholesteric (N*) liquid crystal exhibit an intriguing capacity for photonic cross-communication, giving rise to colourful patterns that depend sensitively on the N* pitch, droplet positions and illuminated area. The phenomenon results from a combination of omnidirectional selective reflection of N* droplets—which thus act as spherically symmetric self-assembled photonic crystals—and total internal reflection at the continuous phase surface. We outline how the unique optical properties can be employed in numerous applications. [less ▲]

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See detailCellulose nanocrystal-based materials: from liquid crystal self-assembly and glass formation to multifunctional thin films
Lagerwall, Jan UL; Schütz, Christina; Salajkova, Michaela et al

in NPG Asia Materials (2014), 6(1), 80

Cellulose nanocrystals (CNCs), produced by the acid hydrolysis of wood, cotton or other cellulose-rich sources, constitute a renewable nanosized raw material with a broad range of envisaged uses: for ... [more ▼]

Cellulose nanocrystals (CNCs), produced by the acid hydrolysis of wood, cotton or other cellulose-rich sources, constitute a renewable nanosized raw material with a broad range of envisaged uses: for example, in composites, cosmetics and medical devices. The intriguing ability of CNCs to self-organize into a chiral nematic (cholesteric) liquid crystal phase with a helical arrangement has attracted significant interest, resulting in much research effort, as this arrangement gives dried CNC films a photonic band gap. The films thus acquire attractive optical properties, creating possibilities for use in applications such as security papers and mirrorless lasing. In this critical review, we discuss the sensitive balance between glass formation and liquid crystal self-assembly that governs the formation of the desired helical structure. We show that several as yet unclarified observations—some constituting severe obstacles for applications of CNCs—may result from competition between the two phenomena. Moreover, by comparison with the corresponding self-assembly processes of other rod-like nanoparticles, for example, carbon nanotubes and fd virus particles, we outline how further liquid crystal ordering phenomena may be expected from CNCs if the suspension parameters can be better controlled. Alternative interpretations of some unexpected phenomena are provided, and topics for future research are identified, as are new potential application strategies. [less ▲]

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See detailTuning the defect configurations in nematic and smectic liquid crystalline shells.
Liang, H. L.; Noh, Junghyun UL; Zentel, R. et al

in Philosophical Transactions of the Royal Society of London. Series A : Mathematical and Physical Sciences (2013), 371(1988), 20120258

Thin liquid crystalline shells surrounding and surrounded by aqueous phases can be conveniently produced using a nested capillary microfluidic system, as was first demonstrated by Fernandez-Nieves et al ... [more ▼]

Thin liquid crystalline shells surrounding and surrounded by aqueous phases can be conveniently produced using a nested capillary microfluidic system, as was first demonstrated by Fernandez-Nieves et al. in 2007. By choosing particular combinations of stabilizers in the internal and external phases, different types of alignment, uniform or hybrid, can be ensured within the shell. Here, we investigate shells in the nematic and smectic phases under varying boundary conditions, focusing in particular on textural transformations during phase transitions, on the interaction between topological defects in the director field and inclusions in the liquid crystal (LC), and on the possibility to relocate defects within the shell by rotating the shell in the gravitational field. We demonstrate that inclusions in a shell can seed defects that cannot form in a pristine shell, adding a further means of tuning the defect configuration, and that shells in which the internal aqueous phase is not density matched with the LC will gently rearrange the internal structure upon a rotation that changes the influence of gravity. Because the defects can act as anchor points for added linker molecules, allowing self-assembly of adjacent shells, the various arrangements of defects developing in these shells and the possibility of tuning the result by modifying boundary conditions, LC phase, thickness and diameter of the shell or applying external forces make this new LC configuration very attractive. [less ▲]

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