References of "Lagerwall, Jan 50002154"
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See detailMicrofluidic Tensiometry Technique for the Characterization of the Interfacial Tension between Immiscible Liquids.
Honaker, Lawrence William UL; Lagerwall, Jan UL; Jampani, Venkata UL

in Langmuir (2018)

The interfacial tension between two immiscible fluids is of critical importance for understanding many natural phenomena as well as in industrial production processes; however, it can be challenging to ... [more ▼]

The interfacial tension between two immiscible fluids is of critical importance for understanding many natural phenomena as well as in industrial production processes; however, it can be challenging to measure this parameter with high accuracy. Most commonly used techniques have significant shortcomings because of their reliance on other data such as density or viscosity. To overcome these issues, we devise a technique that works with very small sample quantities and does not require any data about either fluid, based on micropipette aspiration techniques. The method facilitates the generation of a droplet of one fluid inside of the other, followed by immediate in situ aspiration of the droplet into a constricted channel. A modified Young-Laplace equation is then used to relate the pressure needed to produce a given deformation of the droplet's radius to the interfacial tension. We demonstrate this technique on different systems with interfacial tensions ranging from sub-millinewton per meter to several hundred millinewton per meter, thus over 4 orders of magnitude, obtaining precise results in agreement with the literature solely from experimental observations of the droplet deformation. [less ▲]

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See detailThrough the Spherical Looking-Glass: Asymmetry Enables Multicolored Internal Reflection in Cholesteric Liquid Crystal Shells
Geng, Yong UL; Jang, Ju-Hyun; Noh, Kyung-Gyu et al

in Advanced Optical Materials (2017), 6(1), 1700923

Spheres of cholesteric liquid crystal generate dynamic patterns due to selec- tive reflection from a helical structure subject to continuously curved bounda- ries. So far the patterns are investigated ... [more ▼]

Spheres of cholesteric liquid crystal generate dynamic patterns due to selec- tive reflection from a helical structure subject to continuously curved bounda- ries. So far the patterns are investigated exclusively as function of reflections at the sphere exterior. Here it is shown that the cholesteric shells in a microfluidics produced double emulsion enable also a sequence of internal reflections if the shells have sufficiently thin top and thick bottom. While such asymmetry is promoted by buoyancy when the internal droplet has lower density than the liquid crystal, the elasticity of the cholesteric helix prefers a symmetric shell geometry, acting against gravity. This subtle balance can hide the internal reflections for long time. Eventually, however, the asymmetry is established, revealing a new class of photonic patterns characterized by colored sharp concentric rings. With the complete knowledge of the diverse light-reflecting behavior of cholesteric liquid crystal shells, and utilizing the tunability of the structure period by, e.g., temperature, electric field, or expo- sure to various chemical species as well as polymer stabilization for making the shells long-term stable, they may be developed into remarkable new optical elements for photonics, sensing, or security pattern generation. [less ▲]

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See detailElucidating the fine details of cholesteric liquid crystal shell reflection patterns
Geng, Yong; Noh, Junghyun UL; Drevensek-Olenik, Irena et al

in Liquid Crystals (2017), 44(12-13),

Clusters of planar-aligned short-pitch cholesteric liquid crystal spheres generate dynamic colourful patterns due to multiple selective reflections from the radially oriented cholesteric helices in ... [more ▼]

Clusters of planar-aligned short-pitch cholesteric liquid crystal spheres generate dynamic colourful patterns due to multiple selective reflections from the radially oriented cholesteric helices in neighbour shells at varying distances. These photonic communication patterns were widely investigated for the cases of both droplets and shells, demonstrating not only intriguing optical phenomena but also potential for applications as new optical elements for photonics, sensing or security pattern generation. However, the optics of these clusters is truly complex and until now only the strongest and most fundamental reflections have been analysed and explained. In this report, we elucidate the origin of a number of more subtle reflections and we explain the extension in space of various spots as well as their internal colour variations. [less ▲]

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See detailSecurity in the Shell : An Optical Physical Unclonable Function made of Shells of Cholesteric Liquid Crystals
Lenzini, Gabriele UL; Samir, Ouchani; Roenne, Peter UL et al

in Proc. of the 9th IEEE Workshop on Information Forensics and Security (2017, October 02)

We describe the application in security of shells of Cholesteric Liquid Crystals (ChLCs). Such shells have a diameter in the microns range and can be gathered in hundreds in a surface area as small as a ... [more ▼]

We describe the application in security of shells of Cholesteric Liquid Crystals (ChLCs). Such shells have a diameter in the microns range and can be gathered in hundreds in a surface area as small as a nail’s head. Because of their structural properties, a bundle of them reflects light, creating colorful patterns that we argue to be unique and computationally hard to predict. We argue also that the bundle itself is unclonable. These are typical properties of Physically Unclonable Functions, a family to which shells of ChLCs belong too. Herein we discuss their physical and security properties and their potential use in object authentication. [less ▲]

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See detailWhy organically functionalized nanoparticles increase the electrical conductivity of nematic liquid crystal dispersions
Urbanski, Martin; Lagerwall, Jan UL

in Journal of Materials Chemistry C (2017), 5(34), 8802-8809

Doping liquid crystals with gold nanoparticles increases the conductivity by up to three orders of magnitude, an increase even stronger than expected for equimolar amounts of organic electrolytes. Despite ... [more ▼]

Doping liquid crystals with gold nanoparticles increases the conductivity by up to three orders of magnitude, an increase even stronger than expected for equimolar amounts of organic electrolytes. Despite recent high activity in the field of liquid crystalline nanocomposites, the origin of this increase has rarely been addressed and is not well understood. In this dielectric spectroscopy study we discuss the origin of the increased conductivity and identify its source. We demonstrate that the hydrodynamic radius of the mobile charge carrier species in nanoparticle dispersions is significantly smaller than the 3–5 nm gold core, which rules out the particles themselves to be the source of conductivity. Likewise, also the ligand molecules from the organic capping layer do not themselves add to the conductivity of the dispersions, but affect the electrical properties by acting as a trap for ionic impurities. We suggest that the partial release of these impurities upon interactions of the ligand shell with the uniaxial nematic host phase is the most likely source for the increased conductivity. Our study opens a new perspective on synthesis strategies for functionalized nanoparticles and will help to overcome the current issues preventing high-performing liquid crystal nanodispersions. [less ▲]

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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 detailEnhancing Self-Assembly in Cellulose Nanocrystal Suspensions Using High-Permittivity Solvents
Bruckner, Johanna; Kuhnhold, Anja UL; Honorato Rios, Camila UL et al

in Langmuir (2016)

Helical liquid crystal self-assembly in suspensions of cellulose nanocrystals (CNCs), bioderived nanorods exhibiting excellent mechanical and optical properties, opens attractive routes to sustainable ... [more ▼]

Helical liquid crystal self-assembly in suspensions of cellulose nanocrystals (CNCs), bioderived nanorods exhibiting excellent mechanical and optical properties, opens attractive routes to sustainable production of advanced functional materials. For convenience, in most studies until now, the CNCs were suspended in water, leaving a knowledge gap concerning the influence of the solvent. Using a novel approach for aggregation-free solvent exchange in CNC suspensions, here we show that protic solvents with a high dielectric permittivity εr significantly speed up self-assembly (from days to hours) at high CNC mass fraction and reduce the concentration dependence of the helix period (variation reducing from more than 30 μm to less than 1 μm). Moreover, our computer simulations indicate that the degree of order at constant CNC content increases with increasing εr, leading to a shorter pitch and a reduced threshold for liquid crystallinity. In low-εr solvents, the onset of long-range orientational order is coupled to kinetic arrest, preventing the formation of a helical superstructure. Our results show that the choice of solvent is a powerful parameter for tuning the behavior of CNC suspensions, enhancing our ability to control the self-assembly and thereby harvesting valuable novel cellulose-based materials. [less ▲]

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See detailNon-electronic gas sensors from electrospun mats of liquid crystal core fibers for detecting volatile organic compounds at room temperature
Reyes, Catherine UL; Sharma, Anshul UL; Lagerwall, Jan UL

in Liquid Crystals (2016)

Non-woven mats comprised of liquid crystal-functionalised fibres are coaxially electrospun to create soft gas sensors that function non-electronically, thus requiring no power supply, detect- ing organic ... [more ▼]

Non-woven mats comprised of liquid crystal-functionalised fibres are coaxially electrospun to create soft gas sensors that function non-electronically, thus requiring no power supply, detect- ing organic vapours at room temperature. The fibres consist of a poly(vinylpyrrolidone) (PVP) sheath surrounding a core of nematic 4-cyano-4ʹpentylbiphenyl (5CB) liquid crystal. Several types of mats, containing uniformly cylindrical or irregular beaded fibres, in uniform or random orientations, are exposed to toluene vapour as a representative volatile organic compound. Between crossed polarisers all mats respond with a fast (response time on the order of a second or faster) reduction in brightness during gas exposure, and they return to the original state upon removal of the gas almost as quickly. With beaded fibres, the response of the mats is visible even without polarisers. We discuss how variations in fibre spinning conditions such as humidity level and the ratio of core-sheath fluid flow rates can be used to tune fibre morphology and thereby the response. Considering future development perspectives, we argue that fibres turned respon- sive through the incorporation of a liquid crystal core show promise as a new generation of sensors with textile form factor, ideal for wearable technology applications. [less ▲]

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See detailEquilibrium Liquid Crystal Phase Diagrams and Detection of Kinetic Arrest in Cellulose Nanocrystal Suspensions
Honorato Rios, Camila UL; Kuhnhold, Anja UL; Bruckner, Johanna et al

in Frontiers in Materials (2016), 3

The cholesteric liquid crystal self-assembly of water-suspended cellulose nanocrystal (CNC) into a helical arrangement was observed already more than 20 years ago, and the phenomenon was used to produce ... [more ▼]

The cholesteric liquid crystal self-assembly of water-suspended cellulose nanocrystal (CNC) into a helical arrangement was observed already more than 20 years ago, and the phenomenon was used to produce iridescent solid films by evaporating the solvent or via sol–gel processing. Yet, it remains challenging to produce optically uniform films and to control the pitch reproducibly, reflecting the complexity of the three-stage drying process that is followed in preparing the films. An equilibrium liquid crystal phase formation stage is followed by a non-equilibrium kinetic arrest, which in turn is followed by structural collapse as the remaining solvent is evaporated. Here, we focus on the first of these stages, combining a set of systematic rheology and polarizing optics experiments with computer simulations to establish a detailed phase diagram of aqueous CNC suspensions with two different values of the surface charge, up to the concentration where kinetic arrest sets in. We also study the effect of varying ionic strength of the solvent. Within the cholesteric phase regime, we measure the equilibrium helical pitch as a function of the same parameters. We report a hitherto unnoticed change in character of the isotropic–cholesteric transition at increasing ionic strength, with a continuous weakening of the first-order character up to the point where phase coexistence is difficult to detect macroscopically due to substantial critical fluctuations. [less ▲]

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See detailNanoparticles dispersed in liquid crystals: impact on conductivity, low-frequency relaxation and electro-optical performance
Urbanski, Martin UL; Lagerwall, Jan UL

in Journal of Materials Chemistry C (2016), 4(16), 3485-3491

We study the impact of functionalized gold nanoparticles on the impedance response of nematic nanoparticle/liquid crystal dispersions in the frequency range of 0.1 Hz–100 kHz. By fitting a suitable ... [more ▼]

We study the impact of functionalized gold nanoparticles on the impedance response of nematic nanoparticle/liquid crystal dispersions in the frequency range of 0.1 Hz–100 kHz. By fitting a suitable equivalent electric circuit model to the experimental data we show that nanoparticle doping does not affect the permittivity of the nematic host, but significantly increases its conductivity. This causes a Debye-type relaxation process in the Hz and low kHz regime, which originates from mobile charge carriers accumulating near the electrodes of the test cell. The effect of this electrode polarization on the electro-optical response of the nanocomposites is discussed with respect to threshold voltage and dielectric permittivity. We demonstrate that nanoparticle doping does not alter the electro-optic response at frequencies above the occurrence of electrode polarization, while it strongly deteriorates the performance in the low frequency regime. [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 detailRod packing in chiral nematic cellulose nanocrystal dispersions studied by small angle X-ray scattering and laser diffraction
Schütz, Christina; Agthe, Michael; Fall, Andreas et al

in Langmuir (2015), 31(23), 6507-6513

The packing of cellulose nanocrystals (CNC) in the anisotropic chiral nematic phase has been investigated over a wide concentration range by small angle X-ray scattering (SAXS) and laser diffraction. The ... [more ▼]

The packing of cellulose nanocrystals (CNC) in the anisotropic chiral nematic phase has been investigated over a wide concentration range by small angle X-ray scattering (SAXS) and laser diffraction. The average separation distance between the CNCs and the average pitch of the chiral nematic phase have been determined over the entire isotropic-anisotropic biphasic region. The average separation distances range from 51 nm, at the onset of the anisotropic phase formation, to 25 nm above 6 vol% (fully liquid crystalline phase) whereas the average pitch varies from 15.5 μm down to ≈2 μm as φ increases from 2.5 up to 6.5 vol%. Using the cholesteric order, we determine that the twist angle between neighboring CNCs increases from about 1 ° up to 4 ° as φ increases from 2.5 up to 6.5 vol%. The dependence of the twisting on the volume fraction was related to the increase in the magnitude of the repulsive interactions between the charged rods as the average separation distance decrease. [less ▲]

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See detailMultifunctional responsive fibers produced by dual liquid crystal core electrospinning
Kye, Yoomee; Kim, Changsoon; Lagerwall, Jan UL

in Journal of Materials Chemistry C (2015), 3

We demonstrate that coaxial electrospinning with more than one core channel, each containing a different type of liquid crystal, can be used to produce multifunctional fibers in a one-step process. They ... [more ▼]

We demonstrate that coaxial electrospinning with more than one core channel, each containing a different type of liquid crystal, can be used to produce multifunctional fibers in a one-step process. They respond to more than one stimulus or with multiple threshold values, and the individual cores may feature different physical properties such as iridescent reflection in one core and birefringence in another. In order to ensure good fiber morphology and intact, unmixed and well separated cores, two important precautions must be taken. First, the fibers should not be collected on a hydrophilic substrate, as this will lead to severe fiber deformation and core mixing after collection, as a result of capillary forces from the water that condenses on the fiber during spinning. Second, the addition of surfactants to the polymer solution should be avoided, although it may appear beneficial for the spinning process as it reduces surface tension and increases conductivity. This is because the surfactant enters the liquid crystal core, possibly together with water in the form of inverse micelles, seriously degrading the performance of the liquid crystal. [less ▲]

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See detailUltra-long ordered nanowires from the concerted self-assembly of discotic liquid crystal and solvent molecules.
Park, J. H.; Kim, K. H.; Park, Y. W. et al

in Langmuir (2015), 31(34), 9432-9440

The realization of long and aligned molecular wires is a great challenge and different approaches have been proposed. Interestingly, hexapentyloxytriphenylene (HAT5) discotic liquid crystal molecules ... [more ▼]

The realization of long and aligned molecular wires is a great challenge and different approaches have been proposed. Interestingly, hexapentyloxytriphenylene (HAT5) discotic liquid crystal molecules, model system of molecules with flat and aromatic cores, can spontaneously form well aligned, micrometer long yet only tens of nanometers thick nanowires on solid surfaces. We have investigated the formation mechanism of these wires by using different solvents with selected characteristics like chemical structure, boiling point, vapor pressure and surface tension. When casting from toluene and benzene solutions, atomic force microscopy reveals that the discotics spontaneously form very long and thin wires, self-aligning along a common orientation. If instead dodecane or heptane are used, different and in general thicker structures are obtained. The chemical structure of the solvent appears to have a key role, coupling to the liquid crystal self-assembly by allowing solvent molecules to enter in the ordered structure if their design matches the core of HAT5 molecules, thereby guiding the assembly. However, also other aspects are relevant in the assembly, like the nature of the substrate or the rate of solvent evaporation, and these can favor or interfere with the self-assembly into long structures. The use of solvents with aromatic structure is advantageous not only because it affects the geometry of the assembly, promoting long wire formation, but it is also compatible with good quality of the intermolecular order, as suggested by a high anisotropy of the Raman spectra of the nanowires formed from these solvents. Finally, the electrical properties of ordered systems show a clearly higher electrical conductivity compared to the disorganized aggregates. [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 detailInfluence of Wetting on Morphology and Core Content in Electrospun Core-Sheath Fibers
Kim, Dae Kyom; Lagerwall, Jan UL

in ACS Applied Materials and Interfaces (2014), 6(18), 16441-16447

Coaxial electrospinning allows easy and cost-effective realization of composite fibers at the nano- and microscales. Different multifunctional materials can be incorporated with distinct localization to ... [more ▼]

Coaxial electrospinning allows easy and cost-effective realization of composite fibers at the nano- and microscales. Different multifunctional materials can be incorporated with distinct localization to specific regimes of the fiber cross section and extended internal interfaces. However, the final composite properties are affected by variations in internal structure, morphology, and material separation, and thus, nanoscale control is mandatory for high-performance application in devices. Here, we present an analysis with unprecedented detail of the cross section of liquid core-functionalized fibers, yielding information that is difficult to reveal. This is based on focused ion beam (FIB) lift-out and allowing HR-TEM imaging of the fibers together with nanoscale resolution chemical analysis using energy dispersive X-ray spectroscopy (EDS). Unexpectedly, core material escapes during spinning and ends up coating the fiber exterior and target substrate. For high core injection rate, a dramatic difference in fiber morphology is found, depending on whether the surface on which the fibers are deposited is hydrophobic or hydrophilic. The latter enhances postspinning extraction of core fluid, resulting in the loss of the functional material and collapsed fiber morphology. Finally, in situ produced TiO2 nanoparticles dispersed in the polymer appear strikingly different when the core fluid is present compared to when the polymer solution is spun on its own. [less ▲]

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