Mohamed Amine Gharbi

Exploiting imperfections in the bulk to direct assembly of surface colloids

We exploit the long-ranged elastic fields inherent to confined nematic liquid crystals to assemble colloidal particles trapped at the liquid crystal interface into reconfigurable structures with complex symmetries and packings. Spherical colloids with homeotropic anchoring trapped at the interface between air and the nematic liquid crystal 5CB create quadrupolar distortions in the director field causing particles to repel and consequently form close-packed assemblies with a triangular habit. Here we report on complex, open structures organized via interactions with defects in the bulk. Specifically, by confining the nematic liquid crystal in an array of microposts with homeotropic anchoring conditions, we cause defect rings to form at well-defined locations in the bulk of the sample. These defects source elastic deformations that direct the assembly of the interfacially-trapped colloids into ring-like assemblies, which recapitulate the defect geometry even when the microposts are completely immersed in the nematic. When the surface density of the colloids is high, they form a ring near the defect and a hexagonal lattice far from it. Since topographically complex substrates are easily fabricated and liquid crystal defects are readily reconfigured, this work lays the foundation for a new, robust mechanism to dynamically direct assembly over large areas by controlling surface anchoring and associated bulk defect structure.Significance In this research, we develop new means of directing colloids at an interface to assemble into complex configurations by exploiting defects in a liquid crystal (LC). Through confinement of a nematic LC over a topographically patterned surface, we demonstrate the formation of defects at precise locations in the LC bulk. These defects source elastic distortion fields that guide the assembly of colloids constrained to the LC–air interface. This work significantly extends prior work in which LCs confined in film or droplet geometries guide colloidal assembly beyond simple triangular lattices and chains. Here, we demonstrate colloidal assembly at precise locations, with particle-rich and -poor regions, determined remotely by defects deliberately seeded in the LC bulk. Experimental results are supported by numerical and analytical investigation. We exploit the long-ranged elastic fields inherent to confined nematic liquid crystals (LCs) to assemble colloidal particles trapped at the LC interface into reconfigurable structures with complex symmetries and packings. Spherical colloids with homeotropic anchoring trapped at the interface between air and the nematic LC 4-cyano-4′-pentylbiphenyl create quadrupolar distortions in the director field causing particles to repel and consequently form close-packed assemblies with a triangular habit. Here, we report on complex open structures organized via interactions with defects in the bulk. Specifically, by confining the nematic LC in an array of microposts with homeotropic anchoring conditions, we cause defect rings to form at well-defined locations in the bulk of the sample. These defects source elastic deformations that direct the assembly of the interfacially trapped colloids into ring-like assemblies, which recapitulate the defect geometry even when the microposts are completely immersed in the nematic. When the surface density of the colloids is high, they form a ring near the defect and a hexagonal lattice far from it. Because topographically complex substrates are easily fabricated and LC defects are readily reconfigured, this work lays the foundation for a versatile, robust mechanism to direct assembly dynamically over large areas by controlling surface anchoring and associated bulk defect structure.

Behavior of colloidal particles at a nematic liquid crystal interface

We examine the behavior of spherical silica particles trapped at an air–nematic liquid crystal interface. When a strong normal anchoring is imposed, the beads spontaneously form various structures depending on their area density and the nematic thickness. Using optical tweezers, we determine the pair potential and explain the formation of these patterns. The energy profile is discussed in terms of capillary and elastic interactions. Finally, we detail the mechanisms that control the formation of a hexagonal lattice and analyze the role of gravity for curved interfaces.

Microparticles confined to a nematic liquid crystal shell

A seminal paper [D. R. Nelson, Nano Lett., 2002, 2, 1125.] has proposed that a nematic coating could be used to create a valency for spherical colloidal particles through the functionalization of nematic topological defects. Experimental realizations however question the complex behaviour of solid particles and defects embedded in such a nematic spherical shell. In order to address the related topological and geometrical issues, we have studied micrometer-sized silica beads trapped in nematic shells. We underline the mechanisms that strongly modify the texture of the simple (particle-free) shells when colloidal particles are embedded. Finally, we show how the coupling between capillarity and nematic elasticity offers new ways to control the valence and directionality of shells.

Reversible Nanoparticle Cubic Lattices in Blue Phase Liquid Crystals

Blue phases (BPs), a distinct class of liquid crystals (LCs) with 3D periodic ordering of double twist cylinders involving orthogonal helical director twists, have been theoretically studied as potential templates for tunable colloidal crystals. Here, we report the spontaneous formation of thermally reversible, cubic crystal nanoparticle (NP) assemblies in BPs. Gold NPs, functionalized to be highly miscible in cyanobiphenyl-based LCs, were dispersed in BP mixtures and characterized by polarized optical microscopy and synchrotron small-angle X-ray scattering (SAXS). The NPs assemble by selectively migrating to periodic strong trapping sites in the BP disclination lines. The NP lattice, remarkably robust given the small particle size (4.5 nm diameter), is commensurate with that of the BP matrix. At the BP I to BP II phase transition, the NP lattice reversibly switches between two different cubic structures. The simultaneous presence of two different symmetries in a single material presents an interesting opportunity to develop novel dynamic optical materials.

Use of topological defects as templates to direct assembly of colloidal particles at nematic interfaces

In this work, we experimentally investigate the ability of topological defects to guide interfacial assembly of spherical particles with homeotropic anchoring confined to nematic interfaces. We propose two different systems: In the first one, particles are trapped at an air/nematic interface where they spontaneously form various 2D patterns. We demonstrate that the phase transition between these patterns can be controlled by defects formed in the nematic bulk. In the second system, we explore the behavior of particles at the surface of bipolar nematic drops. We found that particles assemble into linear chains and interact with surface defects at the North and South poles of the drop, giving rise to the formation of star structures in a self-assembly process. We detail the mechanism that guides the behavior of particles and discuss the role of defects in the formation of the observed patterns.

Ring around the colloid

In this work, we show that Janus washers, genus-one colloids with hybrid anchoring conditions, form topologically required defects in nematic liquid crystals. Experiments under crossed polarizers reveal the defect structure to be a rigid disclination loop confined within the colloid, with an accompanying defect in the liquid crystal. When confined to a homeotropic cell, the resulting colloid-defect ring pair tilts relative to the far field director, in contrast to the behavior of toroidal colloids with purely homeotropic anchoring. We show that this tilting behavior can be reversibly suppressed by the introduction of a spherical colloid into the center of the toroid, creating a new kind of multi-shape colloidal assemblage.

Microbullet assembly: interactions of oriented dipoles in confined nematic liquid crystal

Microbullet particles, cylinders with one blunt end and one hemispherical end, offer a novel platform to study the effects of anisotropy and curvature on colloidal assembly in complex fluids. Here, we disperse microbullets in 4-cyano-4′-pentylbiphenyl nematic liquid crystal (NLC) cells and form oriented elastic dipoles with a nematic point defect located near the curved end. This feature allows us to study particle interactions as a function of dipole alignment. By careful control of the surface anchoring at the particle surface and the confining boundaries, we study the interactions and assembly of microbullets under various conditions. When microbullets with homeotropic surface anchoring are dispersed in a planar cell, parallel dipoles form linear chains parallel to the director, similar to those observed with spherical particles in a planar cell, while antiparallel dipoles orient side-to-side. In a homeotropic cell, however, particles rotate to orient their long axis parallel to the director. When so aligned, parallel dipoles repel and form 2D ordered assemblies with hexagonal symmetry that ripen over time owing to attraction between antiparallel neighbours. Further, we show that the director orientation inside the cell can be altered by application of an electrical field, allowing us to flip microbullets to orient parallel to the director, an effect driven by an elastic torque. Finally, we detail the mechanisms that control the formation of 1D chains and hexagonal lattices with respect to the elasticity of the NLC.

Fine Golden Rings: Tunable Surface Plasmon Resonance from Assembled Nanorods in Topological Defects of Liquid Crystals.

Unprecedented, reversible, and dynamic control over an assembly of gold nanorods dispersed in liquid crystals (LC) is demonstrated. The LC director field is dynamically tuned at the nanoscale using microscale ring confinement through the interplay of elastic energy at different temperatures, thus fine-tuning its core replacement energy to reversibly sequester nanoscale inclusions at the microscale. This leads to shifts of 100 nm or more in the surface plasmon resonance peak, an order of magnitude greater than any previous work with AuNR composites.

Elastocapillary interactions on nematic films

Significance In this research we demonstrate complementary roles for elastic and capillary interactions for particles on thin nematic liquid crystal films. A priori estimates would suggest that surface tension effects would dominate over those associated with bend, twist, or splay of nematic director fields. This work is the first to our knowledge to clearly demonstrate complementary roles for both effects and extends significantly the means for developing oriented anisotropic assemblies. Anisotropic particles create strong capillary interactions that can dictate assembly. These assemblies obey orientations dictated by director fields in oriented planar nematic films. Curved interfaces add another degree of freedom, because curvature capillary-driven alignment competes with elastic-driven orientations. Experimental results are supported by analysis. Rod-like colloids distort fluid interfaces and interact by capillarity. We explore this interaction at the free surface of aligned nematic liquid crystal films. Naive comparison of capillary and elastic energies suggests that particle assembly would be determined solely by surface tension. Here, we demonstrate that, under certain circumstances, the capillary and elastic effects are complementary and each plays an important role. Particles assemble end-to-end, as dictated by capillarity, and align along the easy axis of the director field, as dictated by elasticity. On curved fluid interfaces, however, curvature capillary energies can overcome the elastic orientations and drive particle migration along curvature gradients. Domains of dominant interaction and their transition are investigated.

Focal Conic Flower Textures at Curved Interfaces

Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA(Received 23 October 2013; published 10 December 2013)Focal conic domains (FCDs) in smectic-A liquid crystals have drawn much attention, both for theirexquisitely structured internal form and for their ability to direct the assembly of micromaterials andnanomaterials in a variety of patterns. A key to directing FCD assembly is control over the eccentricity ofthe domain. Here, we demonstrate a new paradigm for creating spatially varying FCD eccentricity byconfining a hybrid-aligned smectic with curved interfaces. In particular, we manipulate interface behaviorwith colloidal particles in order to experimentally produce two examples of what has recently beendubbed the flower texture [C. Meyer et al., Focal Conic Stacking in Smectic A Liquid Crystals: SmecticFlower and Apollonius Tiling, Materials 2, 499, 2009], where the focal hyperbolae diverge radiallyoutward from the center of the texture, rather than inward as in the canonical e´ventail or fan texture. Weexplain how this unconventional assembly can arise from appropriately curved interfaces. Finally, wepresent a model for this system that applies the law of corresponding cones, showing how FCDs may beembedded smoothly within a ‘‘background texture’’ of large FCDs and concentric spherical layers, in amannerconsistentwiththequalitativefeaturesofthesmecticflower.Suchunderstandingcouldpotentiallylead to disruptive liquid-crystal technologies beyond displays, including patterning, smart surfaces,microlens arrays, sensors, and nanomanufacturing.