Transport of particles in liquid crystals

Hybrid photoluminescent materials containing a benzobisthiazole core for liquid crystal and gel applications

Tetra- and hexacatenar amide compounds containing a linear centrosymmetric benzobisthiazole core were synthesized with good yields. These compounds were characterized and their structures confirmed by elemental analysis, and FT-IR, Maldi mass and NMR spectroscopy. All compounds exhibited excellent thermal stability up to 330 °C. The tetracatenar series containing a double substitution in the meta positions did not show mesomorphic behaviour, whereas the hexacatenar and tetracatenar series having a double substitution in the meta and para positions showed liquid crystal properties with optical textures typical of columnar mesophases corroborated by POM analysis. The mesomorphic properties were dependent on the length, number and position of alkoxy chains attached at the end of the rigid core. XRD studies of the hexacatenar series showed the hexagonal columnar structure of the mesophases. Photoluminescence properties in solution were observed in the visible region, with good quantum yields. In the solid state, these compounds behave as blue emitters and they are able to change colour with acid or base addition. The hexacatenar benzobisthiazole compound with an alkoxy chain of 14 carbons presented properties of a supergelator in chloroform, leading to the formation of a fluorescent organogel material with fluorescence emission in the blue region.

Propelling and spinning of microsheets in nematic liquid crystals driven by ac electric field

Dynamics of microparticles in isotropic liquids by transducing the energy of an applied electric field have been studied for decades. Recently, such studies in anisotropic media like liquid crystals have opened up new perspectives in colloid science. Here, we report studies on ac-electric-field-driven dynamics of microsheets in nematic liquid crystals. In planar aligned liquid crystals, with negative dielectric anisotropy, the microsheets are propelled parallel to the director. A steady spinning of the microsheets is observed in homeotropic cells with positive dielectric anisotropy liquid crystals. The velocity of propelling and the angular frequency of spinning depends on the amplitude and the frequency of the applied electric field. The electrokinetic studies of anisotropic microparticles are important as they are potential for applications in microfluidics and in areas where the controlled transport or rotation is required.

Light-induced displacement of a microbead through the thermal expansion of liquid crystals

We found the displacement of a microbead by the oblique irradiation of UV light in pure 4-cyano-4'-pentylbiphenyl (5CB) without photo-responsive molecules. This system is very simple and a complicated experimental setup is not necessary. The displacement is observed in the nematic state of 5CB and not in the isotropic state. The distance of the displacement was proportional to the intensity of the UV light. After the irradiation was stopped, the microbead started to return to the original position. These phenomena are attributed to the thermal expansion of 5CB induced by the photo-thermal effect of polyimide coated on the glass substrate.

Electric-field-induced transport of microspheres in the isotropic and chiral nematic phase of liquid crystals

The application of an electric field to microspheres suspended in a liquid crystal causes particle translation in a plane perpendicular to the applied field direction. Depending on applied electric field amplitude and frequency, a wealth of different motion modes may be observed above a threshold, which can lead to linear, circular, or random particle trajectories. We present the stability diagram for these different translational modes of particles suspended in the isotropic and the chiral nematic phase of a liquid crystal and investigate the angular velocity, circular diameter, and linear velocity as a function of electric field amplitude and frequency. In the isotropic phase a narrow field amplitude-frequency regime is observed to exhibit circular particle motion whose angular velocity increases with applied electric field amplitude but is independent of applied frequency. The diameter of the circular trajectory decreases with field amplitude as well as frequency. In the cholesteric phase linear as well as circular particle motion is observed. The former exhibits an increasing velocity with field amplitude, while decreasing with frequency. For the latter, the angular velocity exhibits an increase with field amplitude and frequency. The rotational sense of the particles on a circular trajectory in the chiral nematic phase is independent of the helicity of the liquid crystalline structure, as is demonstrated by employing a cholesteric twist inversion compound.

Controlling Smectic Liquid Crystal Defect Patterns by Physical Stamping-Assisted Domain Separation and Their Use as Templates for Quantum Dot Cluster Arrays

Controlling the organization of self-assembling building blocks over a large area is crucial for lithographic tools based on the bottom-up approach. However, the fabrication of liquid crystal (LC) defect patterns with a particular ordering still remains a challenge because of the limited close-packed morphologies of LC defects. Here, we introduce a multiple-stamping domain separation method for the control of the dimensions and organization of LC defect structures. Prepatterns with various grid shapes on planar polyimide (PI) surfaces were fabricated by pressing a line-shaped stamp into the PI surfaces in two different directions, and then these surfaces were used to prepare LC defect structures confined to these grid domains. The dimensions of the LC defect structures, namely, the equilibrium diameter and the center to center spacing, are controlled by varying the line spacing of the stamps and the film thickness. A variety of arrangements of LC defects, including square, rhombic, hexagonal, and other oblique lattices, can be obtained by simply varying the stamping angle (Ω) between the first and second stamping directions. Furthermore, we demonstrate that the resulting controllable LC defect arrays can be used as templates for generating various patterns of nanoparticle clusters by trapping quantum dots (QDs) within the cores of the LC defects.

Domed Silica Microcylinders Coated with Oleophilic Polypeptides and Their Behavior in Lyotropic Cholesteric Liquid Crystals of the Same Polypeptide

Liquid crystals can organize dispersed particles into useful and exotic structures. In the case of lyotropic cholesteric polypeptide liquid crystals, polypeptide-coated particles are appealing because the surface chemistry matches that of the polymeric mesogen, which permits a tighter focus on factors such as extended particle shape. The colloidal particles developed here consist of a magnetic and fluorescent cylindrically symmetric silica core with one rounded, almost hemispherical end. Functionalized with helical poly(γ-stearyl-l-glutamate) (PSLG), the particles were dispersed at different concentrations in cholesteric liquid crystals (ChLC) of the same polymer in tetrahydrofuran (THF). Defects introduced by the particles to the director field of the bulk PSLG/THF host led to a variety of phases. In fresh mixtures, the cholesteric mesophase of the PSLG matrix was distorted, as reflected in the absence of the characteristic fingerprint pattern. Over time, the fingerprint pattern returned, more quickly when the concentration of the PSLG-coated particles was low. At low particle concentration the particles were "guided" by the PSLG liquid crystal to organize into patterns similar to that of the re-formed bulk chiral nematic phase. When their concentration increased, the well-dispersed PSLG-coated particles seemed to map onto the distortions in the bulk host's local director field. The particles located near the glass vial-ChLC interfaces were stacked lengthwise into architectures with apparent two-dimensional hexagonal symmetry. The size of these "crystalline" structures increased with particle concentration. They displayed remarkable stability toward an external magnetic field; hydrophobic interactions between the PSLG polymers in the shell and those in the bulk LC matrix may be responsible. The results show that bio-inspired LCs can assemble suitable colloidal particles into soft crystalline structures.

Microfluidic flow of cholesteric liquid crystals

We explore the rheology and flow-induced morphological changes of cholesteric liquid crystal patterns subject to Poiseuille flow within a slab geometry, and under different anchoring conditions at the wall. Our focus is particularly on the behaviour of "Cholesteric Fingers of the first kind" and of Blue Phase II. Depending on the applied pressure gradient, we observe a number of dynamic regimes with different rheological properties. Our results provide the first insight into the flow response of cholesteric phases with fully two- or three-dimensional director field patterns and normal and planar degenerate anchoring conditions as commonly realised in experiments. They are also of high relevance for a fundamental understanding of complex liquid crystals in confinement and an important step towards future microfluidic applications that are based on cholesteric liquid crystals.

Dynamics of electro-orientation of birefringent microsheets in isotropic and nematic liquid crystals

We study the dynamics of electric field driven multiaxis electro-orientation of birefringent microsheets in both the isotropic and nematic phases of a liquid crystal. For a fixed direction of applied field in the isotropic phase, there are two critical fields above which the microsheets show two orientations. In the nematic phase, it shows three rotations in both planar and homeotropic cells. These orientations are observed at varying voltages and wide time scales and are explained based on the competing effect of the electric, elastic, and viscous torques. The control of the orientation of anisotropic microparticles (both optically and geometrically) by transducing external energy may be useful in electro-optics and photonics.

Two-dimensional assemblies of nematic colloids in homeotropic cells and their response to electric fields

Micrometer-sized colloidal particles dispersed in nematic liquid crystals interact with each other through anisotropic interactions induced by orientational deformation of the nematic field. In the case of so-called dipole nematic colloids, their interaction is of the dipole-dipole type. Two-dimensional, non-close-packed colloidal assemblies having various characteristics were fabricated using optical tweezers by exploiting the attraction between anti-parallel dipole nematic colloids in homeotropically aligned nematic cells. Structures comprising polygons, squares, and tetrahedra were built using equal-sized particles, and hexagonal structures were built using particles of two sizes. As the nematic field is sensitive to electric fields, the response of the fabricated assemblies toward an alternating electric field was also studied. All assemblies exhibited homogeneous reversible shrinkage, and their shrinkage rates were dependent on the structure. The maximum shrinkage rate in the linear dimension of the assemblies was over 20% at 5 Vrms for a hexagon comprising tetrahedral units.

Experimental realization of the "lock-and-key" mechanism in liquid crystals

The ability to control the movement and assembly of particles in liquid crystals is not only an important route to design functional materials, but also sheds light on the mechanisms of colloidal interactions. In this study we place micron-sized particles with "Saturn ring" defects near a wall with hills and dales that impose perpendicular (homeotropic) molecular anchoring. The strong splay distortion at the wall interacts with the distortion around the particles in the near field and favors their migration towards the dales via the so-called "lock-and-key" mechanism. We demonstrate experimentally that the lock-and-key mechanism can robustly localize a particle at specific topographical features. We observe the complex trajectories traced by the particles as they dock on the dales, estimate the binding energy, and explore a range of parameters that favor or disfavor the docking event, thus exploiting the capabilities of our experimental system. We extend the study to colloids with homeotropic anchoring but with an associated point defect instead of a Saturn ring and show that they find a different preferred location, i.e. we can place otherwise identical particles at well defined sites according to their topological defect structure. Finally, for deep enough wells, confinement drives topological transitions of Saturn rings to dipoles. This ability to tailor wall geometry to guide colloids to well defined sites within nematic liquid crystals represents an important new tool in directed assembly.

Around the corner: Colloidal assembly and wiring in groovy nematic cells

We study colloids suspended in nematic liquid crystal in grooves with homeotropic anchoring. We observe "eyelashes", topological dipole chains that follow the local, curved director field. These beget wires that connect the groove corners to topographical features on the cell lid to yield oriented, curvilinear colloidal wires spanning the cell, formed in a nonsingular director field. As the groove aspect ratio changes, we find different ground states and corroborate our observation with numerics. Our results rely upon on the scale of topographical features, the sharpness of edges, and the colloid-sourced distortions; all these elements can be exploited to guide the formation of reconfigurable structures in nematics.

Design of Responsive and Active (Soft) Materials Using Liquid Crystals

Liquid crystals (LCs) are widely known for their use in liquid crystal displays (LCDs). Indeed, LCDs represent one of the most successful technologies developed to date using a responsive soft material: An electric field is used to induce a change in ordering of the LC and thus a change in optical appearance. Over the past decade, however, research has revealed the fundamental underpinnings of potentially far broader and more pervasive uses of LCs for the design of responsive soft material systems. These systems involve a delicate interplay of the effects of surface-induced ordering, elastic strain of LCs, and formation of topological defects and are characterized by a chemical complexity and diversity of nano- and micrometer-scale geometry that goes well beyond that previously investigated. As a reflection of this evolution, the community investigating LC-based materials now relies heavily on concepts from colloid and interface science. In this context, this review describes recent advances in colloidal and interfacial phenomena involving LCs that are enabling the design of new classes of soft matter that respond to stimuli as broad as light, airborne pollutants, bacterial toxins in water, mechanical interactions with living cells, molecular chirality, and more. Ongoing efforts hint also that the collective properties of LCs (e.g., LC-dispersed colloids) will, over the coming decade, yield exciting new classes of driven or active soft material systems in which organization (and useful properties) emerges during the dissipation of energy.

Nematic droplets on fibers

The emergence of new techniques for the fabrication of nematic droplets with nontrivial topology provides new routes for the assembly of responsive devices. Here we explore some of the properties of nematic droplets on fibers, which constitute the basic units of a type of device that is able to respond to external stimuli, including the detection of gases. We perform a numerical study of spherical nematic droplets on fibers. We analyze the equilibrium textures for homogeneous and hybrid boundary conditions and find that in some cases the nematic avoids the nucleation of topological defects, which would provide a different optical response. We consider in detail a homeotropic nematic droplet wrapped around a fiber with planar anchoring. We investigate the effect of an electric field on the texture of this droplet. In the presence of a dc field, the system undergoes an orientational transition above a given threshold E(c), for which a ring defect is transformed into a figure-eight defect. We also consider ac fields, at high and low frequencies, and find that the textures are similar to those observed for static fields, in contrast with recently reported experiments.

Effects of confinement, surface-induced orientations and strain on dynamical behaviors of bacteria in thin liquid crystalline films

We report on the organization and dynamics of bacteria (Proteus mirabilis) dispersed within lyotropic liquid crystal (LC) films confined by pairs of surfaces that induce homeotropic (perpendicular) or hybrid (homeotropic and parallel orientations at each surface) anchoring of the LC. By using motile vegetative bacteria (3 µm in length) and homeotropically aligned LC films with thicknesses that exceed the length of the rod-shaped cells, a key finding reported in this paper is that elastic torques generated by the LC are sufficiently large to overcome wall-induced hydrodynamic torques acting on the cells, thus leading to LC-guided bacterial motion near surfaces that orient LCs. This result extends to bacteria within LC films with hybrid anchoring, and leads to the observation that asymmetric strain within a hybrid aligned LC rectifies motions of motile cells. In contrast, when the LC film thickness is sufficiently small that confinement prevents alignment of the bacteria cells along a homeotropically aligned LC director (achieved using swarm cells of length 10-60 µm), the bacterial cells propel in directions orthogonal to the director, generating transient distortions in the LC that have striking "comet-like" optical signatures. In this limit, for hybrid LC films, we find LC elastic stresses deform the bodies of swarm cells into bent configurations that follow the LC director, thus unmasking a coupling between bacterial shape and LC strain. Overall, these results provide new insight into the influence of surface-oriented LCs on dynamical bacterial behaviors and hint at novel ways to manipulate bacteria using confined LC phases that are not possible in isotropic solutions.

Anisotropic colloidal transport and periodic stick-slip motion in cholesteric finger textures

We have investigated the mobility of discoidal colloidal particles sedimenting within cholesteric finger textures formed by mixtures of the nematic liquid crystal 4-cyano-4'-pentylbiphenyl (5CB) and the chiral dopant 4-(2-methylbutyl)-4'-cyanobiphenyl (CB15) with cholesteric pitch p between 24 and 114 μm. The nickel disks, with radius 17 μm and thickness 300 nm, displayed varied transport behavior that depended on the size of the pitch and the orientation of the gravitational force with respect to the cholesteric axis. In textures with small pitch (p < 40 μm), the disks moved perpendicular to the axis irrespective of the orientation of gravity as a result of an elastic retarding force that prevented motion along the axis. In textures with larger pitch, the disks similarly moved perpendicular to the axis when the angle between the force and axis was large. When the angle was small, the disks displayed stick-slip motion caused by periodic yielding of the finger texture. A model considering viscous drag on the particles and the elastic energy cost of deforming the finger texture describes the stick-slip motion accurately. The effective drag viscosities obtained from the disk motion are anomalously large compared with those of pure nematic 5CB indicating a large contribution to the dissipation from the motion of disclinations in the texture in the vicinity of the translating disks.

Elastocapillary interactions on nematic films

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.

Electric field controlled columnar and planar patterning of cholesteric colloids

We study how dispersions of colloidal particles in a cholesteric liquid crystal behave under a time-dependent electric field. By controlling the amplitude and shape of the applied field wave, we show that the system can be reproducibly driven out of equilibrium through different kinetic pathways and navigated through a glassylike free energy landscape encompassing many competing metastable equilibria. Such states range from simple Saturn rings to complex structures featuring amorphous defect networks, or stacks of disclination loops. A nonequilibrium electric field can also trigger the alignment of particles into columnar arrays, through defect-mediated force impulses, or their repositioning within a plane. Our results are promising in terms of providing new avenues towards controlled patterning and self-assembly of soft colloid-liquid crystal composite materials.

Nematic caps on colloidal particles in a nematogenic liquid under an electric field

We examine the localized nematic regions (caps) on spherical colloidal particles suspended in a nematogenic liquid in the isotropic phase in the bulk by solving the Poisson equation with an orientation-dependent dielectric tensor. These caps appear and grow with an increasing applied electric field. We assume positive dielectric anisotropy of the nematogenic liquid and a high dielectric constant of the particles. Then, the electric field becomes the strongest near the poles of each particle along the field direction, leading to nematic caps. This cap formation occurs continuously for homeotropic anchoring, but is a discontinuous transition otherwise. We also discuss how the nematic caps can be observed in dielectric response, birefringence, and depolarized light scattering.

Direct visualization of spatiotemporal structure of self-assembled colloidal particles in electrohydrodynamic flow of a nematic liquid crystal

Characterization of spatiotemporal dynamics is of vital importance to soft matter systems far from equilibrium. Using a confocal laser scanning microscopy, we directly reveal three-dimensional motion of surface-modified particles in the electrohydrodynamic convection of a nematic liquid crystal. Particularly, visualizing a caterpillar-like motion of a self-assembled colloidal chain demonstrates the mechanism of the persistent transport enabled by the elastic, electric, and hydrodynamic contributions. We also precisely show how the particles' trajectory is spatially modified by simply changing the surface boundary condition.

Surface charge and interactions of 20-nm nanocolloids in a nematic liquid crystal

We studied real-time motion of individual 20-nm silica nanoparticles in a thin layer of a nematic liquid crystal using a dark-field optical videomicroscopy. By tracking the positions of individual nanoparticles we observed that particle pair interactions are not only mediated by strong thermal fluctuations of the nematic liquid crystal, but also with a repulsive force of electric origin. We determined the total electric charge of silanated silica particles in the nematic liquid crystal 5CB by observing the electric-force-driven drift. Surprisingly, the surface electric charge density depends on colloidal size and is ∼4.5×10(-3)C/m(2) for 20-nm nanocolloids, and two orders of magnitude lower, i.e., ∼2.3×10(-5)C/m(2), for 1-μm colloids. We conclude that electrostatic repulsion between like-charged particles prevents the formation of permanent colloidal assemblies of nanometer size. We also observed strong attraction of 20-nm silica nanoparticles to confining polyimide surfaces and larger clusters, which gradually results in complete expulsion of nanoparticles from the nematic liquid crystal to the surfaces of the confining cell.

Optically driven translational and rotational motions of microrod particles in a nematic liquid crystal

A small amount of azo-dendrimer molecules dissolved in a liquid crystal enables translational and rotational motions of microrods in a liquid crystal matrix under unpolarized UV light irradiation. This motion is initiated by a light-induced trans-to-cis conformational change of the dendrimer adsorbed at the rod surface and the associated director reorientation. The bending direction of the cis conformers is not random but is selectively chosen due to the curved local director field in the vicinity of the dendrimer-coated surface. Different types of director distortions occur around the rods, depending on their orientations with respect to the nematic director field. This leads to different types of motions driven by the torques exerted on the particles by the director reorientations.

Elastic multipoles in the field of the nematic director distortions

Theory of the interaction between all types of elastic dipoles and quadrupoles and distortions of the nematic director is presented. If a particle is small relative to the characteristic distortion length, the interaction is determined by the director derivatives at the particle location. We consider a spherical particle since, even under the standard assumptions of the multipole theory (weak deformations, one constant approximation), the problem can be solved analytically only in this case. Different dipoles interact with different distortion modes (e.g., isotropic dipole interacts with the splay, chiral dipole with the twist, and so on). In the main order, the interaction of a dipole is linear in the director derivatives, and the interaction of a quadrupole is linear in the second-order director derivatives. The theory goes beyond the main-order terms and predicts an effective distortion-induced dipolar component on a particle. This effect is described by the free energy term quadratic in the director derivatives and has contributions both of a bulk and surface origin. The bulk effect takes place even if the director at the particle surface is fixed, whereas the surface effect appears if the surface director is perturbed by the distortions due to a weak surface anchoring. The theory is illustrated by simple examples of the interaction of elastic dipoles with a disclination line, with cholesteric spiral, and with the director distortions in a hybrid cell.

Colloidal caterpillars for cargo transportation

Tunable transport of tiny objects in fluid systems is demanding in diverse fields of science such as drug delivery, active matter far from equilibrium, and lab-on-a-chip applications. Here, we report the directed motion of colloidal particles and self-assembled colloidal chains in a nematic liquid crystal matrix using electrohydrodynamic convection (EHC) rolls. The asymmetric distortion of the molecular orientation around the particles results - for single particles - in a hopping motion from one EHC roll to the next and - for colloidal chains - in a caterpillar-like motion in the direction perpendicular to the roll axes. We demonstrate the use of colloidal chains as microtraction engines for the transport of various types of microcargo.

Locomotion and transport in a hexatic liquid crystal

The swimming behavior of bacteria and other microorganisms is sensitive to the physical properties of the fluid in which they swim. Mucus, biofilms, and artificial liquid-crystalline solutions are all examples of fluids with some degree of anisotropy that are also commonly encountered by bacteria. In this article, we study how liquid-crystalline order affects the swimming behavior of a model swimmer. The swimmer is a one-dimensional version of G. I. Taylor's swimming sheet: an infinite line undulating with small-amplitude transverse or longitudinal traveling waves. The fluid is a two-dimensional hexatic liquid-crystalline film. We calculate the power dissipated, swimming speed, and flux of fluid entrained as a function of the swimmer's wave form as well as properties of the hexatic film, such as the rotational and shear viscosity, the Frank elastic constant, and the anchoring strength. The departure from isotropic behavior is greatest for large rotational viscosity and weak anchoring boundary conditions on the orientational order at the swimmer surface. We even find that if the rotational viscosity is large enough, the transverse-wave swimmer moves in the opposite direction relative to a swimmer in an isotropic fluid.

Magnetically responsive gourd-shaped colloidal particles in cholesteric liquid crystals

Particle shape and medium chirality are two key features recently used to control anisotropic colloidal self-assembly and dynamics in liquid crystals. Here, we study magnetically responsive gourd-shaped colloidal particles dispersed in cholesteric liquid crystals with periodicity comparable or smaller than the particle's dimensions. Using magnetic manipulation and optical tweezers, which allow one to position colloids near the confining walls, we measured the elastic repulsive interactions of these particles with confining surfaces and found that separation-dependent particle-wall interaction force is a non-monotonic function of separation and shows oscillatory behavior. We show that gourd-shaped particles in cholesterics reside not on a single sedimentation level, but on multiple long-lived metastable levels separated by a distance comparable to cholesteric periodicity. Finally, we demonstrate three-dimensional laser tweezers assisted assembly of gourd-shaped particles taking advantage of both orientational order and twist periodicity of cholesterics, potentially allowing new forms of orientationally and positionally ordered colloidal organization in these media.

Optical manipulation of the nematic director field around microspheres covered with an azo-dendrimer monolayer

We report here the optical manipulation of the director and topological defect structures of nematic liquid crystals around a silica microparticle with azobenzene-containing dendrimers (azo-dendrimers) on its surface. We successfully demonstrate the successive switching processes from hedgehog, to boojum, and further to Saturn ring configurations by ultraviolet (UV) light irradiation and termination. The switching time between these defect structures depends on the UV light intensity and attains about 50 ms. Since the pretreatment of microparticles is not necessary and the surface modification is spontaneously performed just by dissolving the azo-dendrimers in liquid crystals, this dendrimer supplies us with a variety of possible applications.

Reconfigurable swarms of nematic colloids controlled by photoactivated surface patterns

Different phoretic driving mechanisms have been proposed for the transport of solid or liquid microscopic inclusions in integrated chemical processes. It is now shown that a substrate that was chemically modified with photosensitive self-assembled monolayers enables the direct control of the assembly and transport of large ensembles of micrometer-sized particles and drops that were dispersed in a thin layer of anisotropic fluid. This strategy separates particle driving, which was realized by AC electrophoresis, and steering, which was achieved by elastic modulation of the nematic host fluid. Inclusions respond individually or in collective modes following arbitrary reconfigurable paths that were imprinted by irradiation with UV or blue light. Relying solely on generic material properties, the proposed procedure is versatile enough for the development of applications that involve either inanimate or living materials.

Chiral bipolar colloids from nonchiral chromonic liquid crystals

We demonstrate that high anisotropy of elastic constants of chromonic liquid crystals leads to a number of spontaneously twisted nematic director fields around colloidal particles in these non-chiral fluids. For spherical colloidal particles with surface inducing degenerate planar nematic ordering we observe that boojum defects at the particles' poles acquire twisted internal structure, extending up to three particle diameters along the rubbing direction of the cell. The twist handedness of the two boojum defects at the poles of the particle can be either the same or opposite, and we can switch the defects handedness by localized thermal microquenching. Numerical simulations confirm that the transitions into the distorted states are induced by lowering of the twist elastic constant, which results in two (meta)stable chiral configurations of the boojums, separated by an energy barrier much higher than the thermal energy. We show that boojum handedness can change the pairwise elastic interaction between the two particles positioned along the rubbing direction from repulsive to attractive.

Peculiarity of the interaction of small particles in smectic liquid crystals

We investigate the peculiarity of the interaction between particles immersed into a smectic liquid crystal with a layered structure. Such a structure of a liquid crystal imposes restrictions on possible deformations of the layer displacement field. Previous studies neglect this fact and give improper results for the interaction potential within one molecular layer. The present paper shows that such restrictions yield an interaction potential substantially different from those of previous studies. Oscillatory behavior, which was not present in the potentials of previous studies, might give rise to superstructures of immersed particles with finite interparticle distance.