Self-shaping liquid crystal droplets by balancing bulk elasticity and interfacial tension

Morphogenesis of a chiral liquid crystalline droplet with topological reconnection and Lehmann rotation

Morphogenesis is a hierarchical phenomenon that produces various macroscopic structures in living organisms, with high reproducibility. This study demonstrates that such structural formation can also be observed in a chiral liquid crystalline droplet under a temperature gradient. Through specific control of the temperature change process, we were able to switch the final structure obtained as a result of the formation via the appearance and reconnection of loop defects in the transient state during structure formation. Simultaneously, the existence of the gradient resulted in a characteristic rotational phenomenon called Lehmann rotation, which was prominently induced in the transient state. By demonstrating three-dimensional measurements of the flow field, we revealed the existence of Marangoni convection in the state. Consequently, it is indicated that the convection results in high-speed Lehmann rotation and large structural deformation with topological changes, thereby playing a significant role in the structure formation.

Exploiting Molecular Orders at the Interface of Microdroplets for Intelligent Materials

ConspectusThe intrinsic molecular order of liquid crystals (LCs) and liquid crystalline elastomers (LCEs) is the origin of their stimuli-responsive properties. The programmable responsiveness and functionality, such as shape morphing and color change under external stimuli, are the key features that attract interest in designing LC- and LCE-based intelligent material platforms. Methods such as mechanical stretching and shearing, surface alignment, and field-assisted alignment have been exploited to program the order of LC molecules for the desired responsiveness. However, the huge size mismatch between the nanometer-sized LC mesogens and the targeted macroscopic objects calls for questions about how to delicately control molecular order for desired performance. Microparticles that can be synthesized with intrinsic molecular order precisely controlled to micrometer size can be used as building blocks for bulk materials, thus offering opportunities to bridge the gap and transcend molecular orders across scales. By taking advantage of the interfacial anchoring effects, we can control and engineer the molecular orders inside the microdroplets, allowing for the realization of various responsive behaviors. Furthermore, designer LC microparticles with multiple responsiveness can be assembled and confined within a matrix, opening a new pathway to engineering LC-enabled intelligent materials.In this Account, we present our recent work on exploiting the molecular order inside microdroplets for the construction of intelligent materials. We briefly introduce the typical chemicals used in the synthesis and the methods developed to control LC molecular alignment within a microdroplets. We then present examples of microparticles synthesized from microdroplets that can transform into complex morphologies upon cooling from the isotropic to nematic phase or due to phase separation within the droplets coupled with the segregation of LC oligomers (LCOs) with polydisperse chain lengths. Furthermore, we show the synthesis of elliptical LCE microparticles and exploit their thermal and magnetic responsiveness to program shape-morphing behaviors and microarrays with switchable optical polarization. By mixing magnetic nanoparticles in cholesteric liquid crystals (CLCs) and silicone oils, we created Janus microparticles capable of color switching for camouflage and information encryption. Moreover, we can engineer complex molecular orders in LCE microparticles by mixing different surfactants, yielding microparticles of diverse anisotropic, temperature-responsive shapes after photopolymerization and extraction of the template LC molecules with different solvents. We conclude the Account with an outlook on the design of intelligent material systems via the design of unprecedented molecular ordering within the microparticles and their coupling with bulk materials.

Dynamic Microparticle Assembly at the Interface of Chemoresponsive Liquid Crystal Droplets

The confinement of liquid crystals (LCs) in spherical microdroplets results in exotic internal configurations and topological defects in response to physical and chemical stimuli. Recent exploration into the placement of colloids on the surface of LC microdroplets has led to the design of a new class of functional materials with patterned surface properties. It is established that the placement of a colloid on a LC droplet surface can pin the topological defect at the interface, thereby restricting changes in the LC configuration. Herein, we build upon the handful of reports published to provide a fundamental understanding of the colloid positioning in response to external stimuli. Using polystyrene (PS) colloids, we explored the dynamics of particle self-assembly in response to an interfacial enzymatic breakdown of poly-l-lysine by trypsin. We found that for a significant population of droplets, the positioning of the colloid is unaffected by the changes in the internal ordering of LC. Inspired by the new observations, we delved deeper to understand the role of interfacial stabilizers in modulating the preferential alignment of LC and the placement of colloidal microparticles. We also demonstrated that for a certain population of droplets, the positioning of the colloids remains unperturbed in response to multistep reversible adsorption of interfacial amphiphiles. Our findings reveal interesting possibilities of correlating the stimuli-responsive switching of internal configurations of LC with colloid placement on the particle-decorated LC droplets.

Photonic features of blue phase liquid crystals under curved confinement

Blue phase (BP) liquid crystals represent a fascinating state of soft matter that showcases unique optical and electro-optical properties. Existing between chiral nematic and isotropic phases, BPs are characterized by a three-dimensional cubic lattice structure resulting in selective Bragg reflections of light and consequent vivid structural colors. However, the practical realization of these material systems is hampered by their narrow thermal stability and multi-domain crystalline nature. This feature article provides an overview of the efforts devoted to stabilizing these phases and creating monodomain structures. In particular, it delves into the complex relationship between geometrical confinement, induced curvature, and the structural stability and photonic features of BPs. Understanding the interaction of curved confinement and structural stability of BPs proves crucially important for the integration of these materials into flexible and miniaturized devices. By shedding light on these critical aspects, this feature review aims to highlight the significance of understanding the coupling effects of physical and mechanical forces on the structural stability of these systems, which can pave the way for the development of efficient and practical devices based on BP liquid crystals.

Curvature and confinement effects on chiral liquid crystal morphologies

Chiral liquid crystals (ChLCs) exhibit an inherent twist that originates at the molecular scale and can extend over multiple length scales when unconstrained. Under confinement, the twist is thwarted, leading to formation of defects in the molecular order that offer distinct optical responses and opportunities for colloidal driven assembly. Past studies have explored spheroidal confinement down to the nanoscopic regime, where curved boundaries produce surface defects to accommodate topological constraints and restrict the propagation of cuboidal defect networks. Similarly, strict confinement in channels and shells has been shown to give rise to escaped configurations and skyrmions. However, little is known about the role of extrinsic curvature in the development of cholesteric textures and Blue Phases (BP). In this paper, we examine the palette of morphologies that arises when ChLCs are confined in toroidal and cylindrical cavities. The equilibrium morphologies are obtained following an annealing strategy of a Landau-de Gennes free energy functional. Three dimensionless groups are identified to build phase diagrams: the natural twist, the ratio of elastic energies, and the circumscription of a BP cell. Curvature is shown to introduce helical features that are first observed as a Double Twist, and progress to Chiral Ribbons and, ultimately, Helical BP and BP. Chiral ribbons are examined as useful candidates for driven assembly given their tunability and robustness.

Phase transitions and topological defects in discotic liquid crystal droplets with planar anchoring: a Monte Carlo simulation study

In this work we present the results of Monte Carlo (MC) simulations at the isothermal-isobaric ensemble for a discotic liquid crystal (DLC) droplet whose surface promotes edge-on (planar) anchoring. For a given pressure, we simulate an annealing process that enables observation of phase transitions within the spherical droplet. In particular, we report a first order isotropic-nematic transition as well as a nematic-columnar transition at the center of the droplet. We found the appearance of topological defects consisting of two disclination lines with ends at the surface of the sphere. We also observed that both transitions, isotropic-nematic and nematic-columnar, occur at lower temperatures as compared to the unconfined system.

Topological transformations of a nematic drop

Morphogenesis of living systems involves topological shape transformations which are highly unusual in the inanimate world. Here, we demonstrate that a droplet of a nematic liquid crystal changes its equilibrium shape from a simply connected tactoid, which is topologically equivalent to a sphere, to a torus, which is not simply connected. The topological shape transformation is caused by the interplay of nematic elastic constants, which facilitates splay and bend of molecular orientations in tactoids but hinders splay in the toroids. The elastic anisotropy mechanism might be helpful in understanding topology transformations in morphogenesis and paves the way to control and transform shapes of droplets of liquid crystals and related soft materials.

Coupling of nematic in-plane orientational ordering and equilibrium shapes of closed flexible nematic shells

The impact of the intrinsic curvature of in-plane orientationally ordered curved flexible nematic molecules attached to closed 3D flexible shells was studied numerically. A Helfrich-Landau-de Gennes-type mesoscopic approach was adopted where the flexible shell's curvature field and in-plane nematic field are coupled and concomitantly determined in the process of free energy minimisation. We demonstrate that this coupling has the potential to generate a rich diversity of qualitatively new shapes of closed 3D nematic shells and the corresponding specific in-plane orientational ordering textures, which strongly depend on the shell's volume-to-surface area ratio, so far not predicted in mesoscopic-type numerical studies of 3D shapes of closed flexible nematic shells.

Flattened and Wrinkled Encapsulated Droplets: Shape Morphing Induced by Gravity and Evaporation

We report surprising morphological changes of suspension droplets (containing class II hydrophobin protein HFBI from Trichoderma reesei in water) as they evaporate with a contact line pinned on a rigid solid substrate. Both pendant and sessile droplets display the formation of an encapsulating elastic film as the bulk concentration of solute reaches a critical value during evaporation, but the morphology of the droplet varies significantly: for sessile droplets, the elastic film ultimately crumples in a nearly flattened area close to the apex while in pendant droplets, circumferential wrinkling occurs close to the contact line. These different morphologies are understood through a gravito-elastocapillary model that predicts the droplet morphology and the onset of shape changes, as well as showing that the influence of the direction of gravity remains crucial even for very small droplets (where the effect of gravity can normally be neglected). The results pave the way to control droplet shape in several engineering and biomedical applications.

Shaping Liquid Droplets on an Active Air-Ferrofluid Interface

An air-liquid interface is important in many biological and industrial applications, where the manipulation of liquids on the air-liquid interface can have a significant impact. However, current manipulation techniques on the interface are mostly limited to transportation and trapping. Here, we report a magnetic liquid shaping method that can squeeze, rotate, and shape nonmagnetic liquids on an air-ferrofluid interface with programmable deformation. We can control the aspect ratio of the ellipse and generate repeatable quasi-static shapes of a hexadecane oil droplet. We can rotate droplets and stir liquids into spiral-like structures. We can also shape phase-changing liquids and fabricate shape-programmed thin films at the air-ferrofluid interface. The proposed method may potentially open up new possibilities for film fabrication, tissue engineering, and biological experiments that can be carried out at an air-liquid interface.

Toroidal nuclei of columnar lyotropic chromonic liquid crystals coexisting with an isotropic phase

Nuclei of ordered materials emerging from the isotropic state usually show a shape topologically equivalent to a sphere; the well-known examples are crystals and nematic liquid crystal droplets. In this work, we explore experimentally and theoretically the toroidal in shape nuclei of columnar lyotropic chromonic liquid crystals coexisting with the isotropic phase. The geometry of these toroids depends strongly on concentrations of the disodium cromoglycate (DSCG) and the crowding agent, polyethylene glycol (PEG). High concentrations of DSCG and PEG result in thick toroids with small central holes, while low concentrations yield thin toroids with wide holes. The multitude of the observed shapes is explained by the balance of bending elasticity and anisotropic interfacial tension.

Run-and-halt motility of droplets in response to light

Microscopic motility is a property that emerges from systems of interacting molecules. Unraveling the mechanisms underlying such motion requires coupling the chemistry of molecules with physical processes that operate at larger length scales. Here, we show that photoactive micelles composed of molecular switches gate the autonomous motion of oil droplets in water. These micelles switch from large trans-micelles to smaller cis-micelles in response to light, and only the trans-micelles are effective fuel for the motion. Ultimately, it is this light that controls the movement of the droplets via the photochemistry of the molecules composing the micelles used as fuel. Notably, the droplets evolve positive photokinetic movement, and in patchy light environments, they preferentially move toward peripheral areas as a result of the difference in illumination conditions at the periphery. Our findings demonstrate that engineering the interplay between molecular photo-chemistry and microscopic motility allows designing motile systems rationally.

Focal conic flowers, dislocation rings, and undulation textures in smectic liquid crystal Janus droplets

Liquid crystalline phases of matter often exhibit visually stunning patterns or textures. Mostly, these liquid crystal (LC) configurations are uniquely determined by bulk LC elasticity, surface anchoring conditions, and confinement geometry. Here, we experimentally explore defect textures of the smectic LC phase in unique confining geometries with variable curvature. We show that a complex range of director configurations can arise from a single system, depending on sample processing procedures. Specifically, we report on LC textures in Janus drops comprised of silicone oil and 8CB in its smectic-A LC phase. The Janus droplets were made in aqueous suspension using solvent-induced phase separation. After drop creation, smectic layers form in the LC compartment, but their self-assembly is frustrated by the need to accommodate both the bowl-shaped cavity geometry and homeotropic (perpendicular) anchoring conditions at boundaries. A variety of stable and metastable smectic textures arise, including focal conic domains, dislocation rings, and undulations. We experimentally characterize their stabilities and follow their spatiotemporal evolution. Overall, a range of fabrication kinetics produce very different intermediate and final states. The observations elucidate assembly mechanisms and suggest new routes for fabrication of complex soft material structures in Janus drops and other confinement geometries.

Emulsion Gels as Precursors for Porous Silicones and All-Polymer Composites-A Proof of Concept Based on Siloxane Stabilizers

In spite of its versatility, the emulsion templating method is rather uncommon for the preparation of porous silicones. In this contribution, two siloxane-containing stabilizers, designed to be soluble in polar (water) and non-polar (toluene) solvents, respectively, were used in low concentrations to produce stable emulsions, wherein polysiloxane gels were obtained by UV-photoinitiated thiol-ene click cross-linking. The stabilizers exhibited negative interfacial tension, as measured by Wilhelmy plate tensiometry. The emulsion gels evolved into porous silicones (xerogels), with tunable morphology and properties. According to TEM and SEM investigations, the emulsion template was preserved in the final materials. Several parameters (e.g., the structure of the polysiloxane precursors, composition of the emulsion gels, nature of the continuous phase, cross-linking conditions, or additives) can be varied in order to obtain porous elastic materials with desired properties, such as Janus membranes, absorbent monoliths, all-polymer porous composites, or silicone-swollen gels. The feasibility of these types of materials was tested, and exemplary porous silicones were briefly characterized by contact angle measurements, mechanical testing, and absorption tests. The proposed method is simple, fast, and economic, uses very little amounts of stabilizers, and can be adjusted as a green technique. In this contribution, all the silicon-based materials with a convenient design were prepared in house.

Growth of Myelin Figures from Parent Multilamellar Vesicles

We examine the formation and growth of isolated myelin figures and microscale multilamellar tubules from isotropic micellar solutions of an anionic surfactant. Upon cooling, surfactant micelles transform into multilamellar vesicles (MLVs) whose contact is found to trigger the unidirectional growth of myelins. While the MLV diameter grows as d_MLV ∝ t^1/2, myelins grow linearly in time as L_M ∝ t¹, with a fixed diameter. Combining time-resolved small-angle neutron scattering (SANS) and optical microscopy, we demonstrate that the microscopic growth of spherical MLVs and cylindrical myelins stems from the same nanoscale molecular mechanism, namely, the surfactant exchange from micelles into curved lamellar structures at a constant volumetric rate. This mechanism successfully describes the growth rate of (nonequilibrium) myelin figures based on a population balance at thermodynamic equilibrium.