Colloidal shape controlled by molecular adsorption at liquid crystal interfaces

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.

Self-Folding Liquid Crystal Network Filaments Patterned with Vertically Aligned Mesogens

Fibrous soft actuators with high molecular anisotropy are of interest for shape morphing from 1D to 2D and 3D in response to external stimuli with high actuation efficiency. Nevertheless, few have fabricated fibrous actuators with controlled molecular orientations and stiffness. Here, we fabricate filaments from liquid crystal networks (LCNs) with segmental crosslinking density and gradient porosity from a mixture of di-acrylate mesogenic monomers and small-molecule nematic or smectic liquid crystals (LCs) filled in a capillary. During photopolymerization, phase separation between the small-molecule LCs and LCN occurs, making one side of the filament considerably denser than the other side. To direct its folding mode (bending or twisting), we control the alignment of LC molecules within the capillary, either along or perpendicular to the filament long axis. We show that the direction of UV exposure can determine the direction of phase separation, which in turn direct the deformation of the filament after removal of the small-molecule LCs. We find that the vertical alignment of LCs within the filament is essential to efficiently direct bending deformation. By photopatterning the filament with segmental crosslinking density, we can induce a reversible folding/unfolding into 2D and 3D geometries triggered by deswelling/swelling in an organic solvent. Moreover, by taking advantage of the large elastic modulus of LCNs and large contrast of the modulus before and after swelling, we show that the self-folded LCP filament could act as a strong gripper.

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.

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Molecular photoswitches mediate the motility of droplets

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Photoactive micelles formed by molecular switches fuel droplet motion

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Photoactive micelles gate motility in response to light

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Droplets evolve motile patterns such as run-and-halt and photokinetic motion

From the swim of bacteria to the beat of a heart, macroscopic movement is a hallmark of life and is ultimately driven by molecular machines. Artificial molecular machines display sophisticated motion with the potential to be harnessed into the purposeful movement of compartments. However, our perception of macroscopic movement differs from the rules that govern movement at the molecular scale. Large-scale functional movement can only emerge when molecular chemistry is coupled to physical processes that operate at larger length scales. We show that the geometry of the amphiphilic switches (molecular level) determines the geometry of micelles (supramolecular level), which in turn determines whether droplet movement can emerge (ensemble level). Eventually, the droplets display motile patterns reminiscent of those of swimming cells. We conclude that molecular behavior can be related to droplet motility rationally, which is a prerequisite for the design of functional motile systems.

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.

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.

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

Liquid crystal (LC) research is rapidly expanding to include studies of curved and topologically nontrivial structures. Here, we explore the role of the bulk LC elasticity and interfacial free energy under weak thermal stimuli to achieve structural transformations in LC emulsions using two different surfactants. Our method is universal and could be used for any LC material or phase. A theoretical model for transforming LC emulsions into uniform fibers and vice versa is presented. We also show the self-shaping of smectic vesicle structures and monodispersed droplet formation at the nematic–smectic transition, utilizing the LC bulk elasticity. This work shows the potential to obtain the controllable shape of complex curved structures for a constant volume of different LCs and other soft materials.

The shape diversity and controlled reconfigurability of closed surfaces and filamentous structures, universally found in cellular colonies and living tissues, are challenging to reproduce. Here, we demonstrate a method for the self-shaping of liquid crystal (LC) droplets into anisotropic and three-dimensional superstructures, such as LC fibers, LC helices, and differently shaped LC vesicles. The method is based on two surfactants: one dissolved in the LC dispersed phase and the other in the aqueous continuous phase. We use thermal stimuli to tune the bulk LC elasticity and interfacial energy, thereby transforming an emulsion of polydispersed, spherical nematic droplets into numerous, uniform-diameter fibers with multiple branches and vice versa. Furthermore, when the nematic LC is cooled to the smectic-A LC phase, we produce monodispersed microdroplets with a tunable diameter dictated by the cooling rate. Utilizing this temperature-controlled self-shaping of LCs, we demonstrate life-like smectic LC vesicle structures analogous to the biomembranes in living systems. Our experimental findings are supported by a theoretical model of equilibrium interface shapes. The shape transformation is induced by negative interfacial energy, which promotes a spontaneous increase of the interfacial area at a fixed LC volume. The method was successfully applied to many different LC materials and phases, demonstrating a universal mechanism for shape transformation in complex fluids.

Nematic Liquid-Crystal Necklace Structure Made by Microfluidics System

We report a necklace structure made of liquid crystal dispersed in poly(vinyl alcohol) (PVA) aqueous solution, which is fabricated by a microfluidic device. In the necklace structure, liquid crystal droplets that are tens of micrometers in diameter are connected by microtethers, which are birefringent, are not penetrating the droplets, and can be elastically stretched by applying external force. The necklace structure was analyzed by fluorescent confocal microscopy, and the tethers were made of liquid crystal and PVA composite. The elastic constant of the tether was determined by using laser tweezers to stretch the tether. The Whispering Gallery Modes circulating inside individual droplets in the necklace structure were also observed.

Molecular heterogeneity drives reconfigurable nematic liquid crystal drops

With few exceptions1-3, polydispersity or molecular heterogeneity in matter tends to impede self-assembly and state transformation. For example, shape transformations of liquid droplets with monodisperse ingredients have been reported in equilibrium4-7 and non-equilibrium studies8,9, and these transition phenomena were understood on the basis of homogeneous material responses. Here, by contrast, we study equilibrium suspensions of drops composed of polydisperse nematic liquid crystal oligomers (NLCOs). Surprisingly, molecular heterogeneity in the polydisperse drops promotes reversible shape transitions to a rich variety of non-spherical morphologies with unique internal structure. We find that variation of oligomer chain length distribution, temperature, and surfactant concentration alters the balance between NLCO elastic energy and interfacial energy, and drives formation of nematic structures that range from roughened spheres to 'flower' shapes to branched filamentous networks with controllable diameters. The branched structures with confined liquid crystal director fields can be produced reversibly over areas of at least one square centimetre and can be converted into liquid crystal elastomers by ultraviolet curing. Observations and modelling reveal that chain length polydispersity plays a crucial role in driving these morphogenic phenomena, via spatial segregation. This insight suggests new routes for encoding network structure and function in soft materials.

Self-Regulated Smectic Emulsion with Switchable Lasing Application

A structurally reversible smectic liquid crystal (LC) emulsion made of semifluorinated rod-type molecules in silicon oil, which is controlled by simple heating and cooling, is presented. Without adding any kind of additives, such as surfactants, polymers or emulsifiers, and without using any special tools, such as microfluidics or gas bubbling, the LC molecules spontaneously form monodisperse spherical and myelin-like structures upon cooling from the isotropic temperature. The LC emulsion can easily trap guest materials, providing a platform for repeatable and reliable switchable emulsification. For example, this interesting system enables the realization of an on-off lasing system by confining fluorescent dyes in the LC droplets.

Myelin structures formed by thermotropic smectic liquid crystals

We report on transient structures, formed by thermotropic smectic-A liquid crystals, resembling the myelin figures of lyotropic lamellar liquid crystals. The thermotropic myelin structures form during the solubilization of a smectic-A droplet in an aqueous phase containing a cationic surfactant at concentrations above the critical micelle concentration. Similar to the lyotropic myelin figures, the thermotropic myelins appear in an optical microscope as flexible tubelike structures growing at the smectic/aqueous interface. Polarizing microscopy and confocal fluorescence microscopy show that the smectic layers are parallel to the tube surface and form a cylindrically bent arrangement around a central line defect in the tube. We study the growth behavior of this new type of myelins and discuss similarities to and differences from the classical lyotropic myelin figures.

Solubilization of thermotropic liquid crystal compounds in aqueous surfactant solutions

We study the micellar solubilization of three thermotropic liquid crystal compounds by immersing single drops in aqueous solutions of the ionic surfactant tetradecyltrimethylammonium bromide. For both nematic and isotropic drops, we observe a linear decrease of the drop size with time as well as convective flows and self-propelled motions. The solubilization is accompanied by the appearance of small aqueous droplets within the nematic or isotropic drop. At low temperatures, nematic drops expell small nematic droplets into the aqueous environment. Smectic drops show the spontaneous formation of filament-like structures which resemble the myelin figures observed in lyotropic lamellar systems. In all cases, the liquid crystal drops become completely solubilized, provided the weight fraction of the liquid crystal in the system is not larger than a few percent. The solubilization of the liquid crystal drops is compared with earlier studies of the solubilization of alkanes in ionic surfactant solutions.

Director configuration of liquid-crystal droplets encapsulated by polyelectrolytes

Liquid-crystal 4-n-pentyl-4'-cyanobiphenyl (5CB) droplets dispersed in aqueous solution are prepared by the assembly of poly(styrenesulfonic acid) (PSSH) and poly(styrenesulfonate sodium) (PSSNa) at the 5CB/water interface. The micrometer sized PSSH-coated 5CB droplets in the space confinement formed by two parallel glass slides break up into submicrometer sized droplets under evaporation-induced flow. We find that the size reduction of the PSSH-coated droplets is accompanied by the bipolar-to-radial configuration transition of the 5CB in the droplets, while the PSSNa-coated 5CB droplets show no size-dependent configuration transition in the same size range. Our results suggest that the size-dependent director configuration of liquid-crystal droplets encapsulated by polyelectrolytes can be modulated by changing the interface conditions, which is important in designing liquid-crystal droplets for optical and biological applications.