Multiresponsive Cylindrically Symmetric Cholesteric Liquid Crystal Elastomer Fibers Templated by Tubular Confinement

in Advanced Science (2023)

Cylindrically symmetric cholesteric liquid crystal elastomer (CLCE) fibers templated by tubular confinement are reported, displaying mechanochromic, thermochromic, and thermomechanical responses. The ... [more ▼]

Cylindrically symmetric cholesteric liquid crystal elastomer (CLCE) fibers templated by tubular confinement are reported, displaying mechanochromic, thermochromic, and thermomechanical responses. The synthesis inside a sacrificial tube secures radial orientation of the cholesteric helix, and the ground state retroreflection wavelength is easily tuned throughout the visible spectrum or into the near-infrared by varying the concentration of a chiral dopant. The fibers display continuous, repeatable, and quantitatively predictable mechanochromic response, reaching a blue shift of more than −220 nm for 180% elongation. The cylindrical symmetry renders the response identical in all directions perpendicular to the fiber axis, making them exceptionally useful for monitoring complex strains, as demonstrated in revealing local strain during tying of different knots. The CLCE reflection color can be revealed with high contrast against any background by taking advantage of the circularly polarized reflection. Upon heating, the fibers respond—fully reversibly—with red shift and radial expansion/axial contraction. However, there is no transition to an isotropic state, confirming a largely forgotten theoretical prediction by de Gennes. These fibers and the easy way of making them may open new windows for large-scale application in advanced wearable technology and beyond. [less ▲]

Pixelating Structural Color with Cholesteric Spherical Reflectors

in Advanced Photonics Research (2023), 4(4), 2200363

While structural color is a powerful means of obtaining saturated and durable pigments that minimize absorption, scattering, and negative environmental impact, appearing naturally in animals and plants as ... [more ▼]

While structural color is a powerful means of obtaining saturated and durable pigments that minimize absorption, scattering, and negative environmental impact, appearing naturally in animals and plants as well as in carefully designed artificial composites, it is fundamentally limited to spectral colors, leaving white and other mixed colors elusive. It also normally suffers from a strong viewing angle dependence, making color definition difficult. Herein, it is demonstrated that these challenges can be overcome by using cholesteric spherical reflectors (CSRs), spheres of polymerized cholesteric liquid crystal with radial alignment of the self-assembled helical structure. Exhibiting omnidirectional selective retroreflectivity of well-defined color, CSRs are discrete “packages” of structural color. This allows them to be used as pixels for generating nonspectral colors, following the principle of digital displays. A method of creating densely packed monolayers of CSRs with red (R), green (G), and blue (B) retroreflection is developed. Mixing them in equal proportions gives a white surface. By embedding the CSRs in an index matching transparent medium, nonselective specular reflections and scattering are avoided. The approach can be used to create arbitrary colors, including nonspectral ones, without any absorption or nonselective scattering, opening doors to decorating surfaces as desired while minimizing light loss. [less ▲]

Unclonable human-invisible machine vision markers leveraging the omnidirectional chiral Bragg diffraction of cholesteric spherical reflectors

in Light: Science and Applications (2022), 11(309), 10103841377-022-01002-4

The seemingly simple step of molding a cholesteric liquid crystal into spherical shape, yielding a Cholesteric Spherical Reflector (CSR), has profound optical consequences that open a range of ... [more ▼]

The seemingly simple step of molding a cholesteric liquid crystal into spherical shape, yielding a Cholesteric Spherical Reflector (CSR), has profound optical consequences that open a range of opportunities for potentially transformative technologies. The chiral Bragg diffraction resulting from the helical self-assembly of cholesterics becomes omnidirectional in CSRs. This turns them into selective retroreflectors that are exceptionally easy to distinguish— regardless of background—by simple and low-cost machine vision, while at the same time they can be made largely imperceptible to human vision. This allows them to be distributed in human-populated environments, laid out in the form of QR-code-like markers that help robots and Augmented Reality (AR) devices to operate reliably, and to identify items in their surroundings. At the scale of individual CSRs, unpredictable features within each marker turn them into Physical Unclonable Functions (PUFs), of great value for secure authentication. Via the machines reading them, CSR markers can thus act as trustworthy yet unobtrusive links between the physical world (buildings, vehicles, packaging,...) and its digital twin computer representation. This opens opportunities to address pressing challenges in logistics and supply chain management, recycling and the circular economy, sustainable construction of the built environment, and many other fields of individual, societal and commercial importance. [less ▲]

Robust cholesteric liquid crystal elastomer fibres for mechanochromic textiles

in Nature Materials (2022), 21(12), 14411447

Mechanically responsive textiles have transformative potential in many areas from fashion to healthcare. Cholesteric liquid crystal elastomers have strong mechanochromic responses that offer attractive ... [more ▼]

Mechanically responsive textiles have transformative potential in many areas from fashion to healthcare. Cholesteric liquid crystal elastomers have strong mechanochromic responses that offer attractive opportunities for such applications. Nonetheless, making liquid crystalline elastomer fibres suitable for textiles is challenging since the Plateau–Rayleigh instability tends to break up precursor solutions into droplets. Here, we report a simple approach that balances the viscoelastic properties of the precursor solution to avoid this outcome and achieve long and mechanically robust cholesteric liquid crystal elastomer filaments. These filaments have fast, progressive and reversible mechanochromic responses, from red to blue (wavelength shift of 155 nm), when stretched up to 200%. Moreover, the fibres can be sewed into garments and withstand repeated stretching and regular machine washing. This approach and resulting fibres may be useful for applications in wearable technology and other areas benefiting from autonomous strain sensing or detection of critically strong deformations. [less ▲]

Lipid islands on liquid crystal shells

in Physical Review Research (2022), 4

By inducing phase separation in lipid monolayers on liquid crystal (LC) shells—thin hollow spheres of LC with water inside and outside—we reveal a rich set of coupled two- and three-dimensional (2D and 3D ... [more ▼]

By inducing phase separation in lipid monolayers on liquid crystal (LC) shells—thin hollow spheres of LC with water inside and outside—we reveal a rich set of coupled two- and three-dimensional (2D and 3D) self- organization phenomena enabled by the dual closely spaced internal and external spherical LC-water interfaces. Spindle-shaped 2D islands of condensed lipid monolayer first form at the primary interface where lipids are deposited, later also at the initially unexposed secondary interface, because lipids transfer through the LC. The LCs’ elastic response to the 3D deformation caused by islands moves them from thin to thick regions on the shell and creates an attraction between opposite-side islands, topologically separated by the LCs, until they stack in a sandwich-like manner. We propose that the phase separation may be used for studying liposome adsorption on soft hydrophobic substrates, and to create unconventional colloidal particles with programmed interactions. [less ▲]

Combining responsiveness and durability in liquid crystal-functionalised electrospun fibres with crosslinked sheath

in Liquid Crystals (2021)

Responsive functional composite fibre mats that are mechanically stable and impervious to water exposure are produced by coaxial electrospinning of thermotropic liquid crystal (LC) core inside a water ... [more ▼]

Responsive functional composite fibre mats that are mechanically stable and impervious to water exposure are produced by coaxial electrospinning of thermotropic liquid crystal (LC) core inside a water-based solution of poly(vinyl alcohol) (PVA) and poly(acrylic acid) (PAA) forming the sheath. Because thermotropic LCs usually cannot be spun inside water-based solutions due to excessive interfacial tension γ, a n enabling step is the addition of ethanol or dioxane to the LC as a co-solvent compatible with both core and sheath fluids. This reduces γ sufficiently that coaxial jet spinning is possible. After spinning, thermal cross-linking of the PVA+PAA sheath yields LC-functionalised fibres that can be manipulated by hand and remain intact even upon full immersion in water. The LC core retains its behaviour, nematics showing well-aligned birefringence and transitioning to isotropic upon heating above the clearing point, and cholesterics showing selective reflection which is even enhanced upon water immersion due to the removal of sheath scattering. Our results pave the way to producing LC-functionalised responsive fibre mats using durable polymer sheaths, thereby enabling numerous innovative applications in wearable technology, and they also open new opportunities to study LCs in confinement, without visible impact of the container walls. [less ▲]

Stable Electrospinning of Core-Functionalized Coaxial Fibers Enabled by the Minimum-Energy Interface Given by Partial Core− Sheath Miscibility

in Langmuir (2021), 37(45), 1326513277

Core−sheath electrospinning is a powerful tool for producing composite fibers with one or multiple encapsulated functional materials, but many material combinations are difficult or even impossible to ... [more ▼]

Core−sheath electrospinning is a powerful tool for producing composite fibers with one or multiple encapsulated functional materials, but many material combinations are difficult or even impossible to spin together. We show that the key to success is to ensure a well-defined core−sheath interface while also maintaining a constant and minimal interfacial energy across this interface. Using a thermotropic liquid crystal as a model functional core and polyacrylic acid or styrene-butadiene-styrene block copolymer as a sheath polymer, we study the effects of using water, ethanol, or tetrahydrofuran as polymer solvent. We find that the ideal core and sheath materials are partially miscible, with their phase diagram exhibiting an inner miscibility gap. Complete immiscibility yields a relatively high interfacial tension that causes core breakup, even preventing the core from entering the fiber- producing jet, whereas the lack of a well-defined interface in the case of complete miscibility eliminates the core−sheath morphology, and it turns the core into a coagulation bath for the sheath solution, causing premature gelation in the Taylor cone. Moreover, to minimize Marangoni flows in the Taylor cone due to local interfacial tension variations, a small amount of the sheath solvent should be added to the core prior to spinning. Our findings resolve a long-standing confusion regarding guidelines for selecting core and sheath fluids in core−sheath electrospinning. These discoveries can be applied to many other material combinations than those studied here, enabling new functional composites of large interest and application potential. [less ▲]

Measuring the Anisotropy in Interfacial Tension of Nematic Liquid Crystals

in Crystals (2021), 11(6), 687

<jats:p>Liquid crystal (LC) phases typically show anisotropic alignment-dependent properties, such as viscosity and dielectric permittivity, so it stands to reason that LCs also have anisotropic ... [more ▼]

<jats:p>Liquid crystal (LC) phases typically show anisotropic alignment-dependent properties, such as viscosity and dielectric permittivity, so it stands to reason that LCs also have anisotropic interfacial tensions. Measuring the interfacial tension ? of an LC with conventional methods, such as pendant drops, can be challenging, however, especially when we need to know ? for different LC aligning conditions, as is the case when we seek ??, the interfacial tension anisotropy. Here, we present measurements of ?? of the common synthetic nematic LC compound 5CB against water using a microfluidic droplet aspiration technique. To ensure tangential and normal alignment, respectively, we add poly(vinyl alcohol) (PVA) and sodium dodecylsulfate (SDS), respectively, as a stabilizer and measure ? for different concentrations of stabilizer. By fitting the Szyszkowski equation to the data, we can extrapolate to zero-stabilizer concentration, obtaining the ? of 5CB to pure water for each alignment. For normal alignment, we find ??=31.9?0.8 mN?m-1, on the order of 1 mN?m-1 greater than ?$_$=30.8?5 mN?m-1 for tangential alignment. This resonates with the empirical knowledge that 5CB aligns tangentially to an interface with pure water. The main uncertainty arises from the use of polymeric PVA as tangential-promoting stabilizer. Future improvements in accuracy may be expected if PVA can be replaced by a low molar mass stabilizer that ensures tangential alignment.</jats:p [less ▲]

Liquid crystal elastomer shells with topological defect-defined actuation: Complex shape morphing, opening/closing, and unidirectional rotation

in Journal of Applied Physics (2021), 129(17), 174701

We produce hollow sphere liquid crystal elastomer (LCE) actuators from a nematic precursor mixture, brought into the shape of a self-closing shell with tangential anchoring of the director field n(r ... [more ▼]

We produce hollow sphere liquid crystal elastomer (LCE) actuators from a nematic precursor mixture, brought into the shape of a self-closing shell with tangential anchoring of the director field n(r), using a solvent-assisted microfluidic technique. By separating the shell production from the polymerization and cross-linking, the precursor is allowed to approach its equilibrium n(r) configuration in the shell, spontaneously forming topological defects of total strength +2. However, the photopolymerization into an LCE induces a brief but strong distortion of the overall n(r) and the defect configuration, even changing the ground state shape in the case of thick shells. The resulting LCE shells show a rich capacity for reversible shape morphing upon heating and cooling, the exact actuation mode defined by n(r), and the final defect configuration stabilized at the end of polymerization. In regions with a single +1 defect, a reversal of curvature from concave to convex is found, punctured shells exhibit a strong shape change between a nearly closed sphere at low temperature and an open-ended spherocylinder at high temperature, and all shells rotate upon actuation when suspended in a fluid. As the rotation is stronger during relaxation than during actuation, thus breaking the symmetry, the net rotation is unidirectional. [less ▲]

Embedding Intelligence in Materials for Responsive Built Environment using Liquid Crystal Elastomer Actuators and Sensors

E-print/Working paper (2021)

Liquid Crystal Elastomers (LCEs) are an exciting category of material that has tremendous application potential across a variety of fields, owing to their unique properties that enable both sensing and ... [more ▼]

Liquid Crystal Elastomers (LCEs) are an exciting category of material that has tremendous application potential across a variety of fields, owing to their unique properties that enable both sensing and actuation. To some, LCEs are simply another type of Shape Memory Polymer, while to others they are an interesting on-going scientific experiment. In this visionary article, we bring an interdisciplinary discussion around creative and impactful ways that LCEs can be applied in the Built Environment to support kinematic and kinetic buildings and situational awareness. We focus particularly on the autonomy made possible by using LCEs, potentially removing needs for motors, wiring and tubing, and even enabling fully independent operation in response to natural environment variations, requiring no power sources. To illustrate the potential, we propose a number of concrete application scenarios where LCEs could offer innovative solutions to problems of great societal importance, such as autonomous active ventilation, heliotropic solar panels systems which can also remove snow or sand autonomously, and invisible coatings with strain mapping functionality, alerting residents in case of dangerous (static or dynamic) loads on roofs or windows, as well as assisting building safety inspection teams after earthquakes. [less ▲]