Anisotropic laser trapping in nematic colloidal dispersion

Thermophoresis of colloids in nematic liquid crystal

Thermophoresis, or the directional motion of colloidal particles in liquids driven by a temperature gradient, is of both fundamental interest and practical use. In this work we explore the thermophoresis of colloids suspended in nematic liquid crystals (LCs). We observe that the motion of these colloids is fundamentally different from that in isotropic systems as a result of elastic distortions in the director fields caused by the colloidal inclusions. In the case of a sufficiently large local temperature and gradient, the elastic energy drives negative thermophoresis of immersed particles, which has a strongly nonlinear dependence on temperature. We develop a theory that incorporates elastic energy minimization into the traditional thermophoretic formulation and demonstrated a good agreement with experimental observations. We also examine the temperature dependence of the effective viscosity of the colloids and highlight the large magnitude of the Soret coefficient (|ST| > 5000), which results from the inherent enhancement in thermophoresis due to elastophoretic considerations and suppression of Brownian diffusion in LC media.

Ground state and peculiarity of particle interactions in liquid crystal colloids

This research proposes a general approach to determine the peculiarity of the interaction between colloidal particles in different liquid crystals. The main idea of this approach is in the definition of the colloidal particle as a source of the possible deformation of the ground state of the director field distribution. The ground state imposes restrictions on possible deformations and determines the peculiarity of the interaction between colloidal particles. Based on this approach, the Coulomb-like interaction between dipole particles in a cholesteric liquid crystal and a crucial change in the character of the interaction in a smectic liquid crystal are predicted.

Annihilation dynamics of topological monopoles on a fiber in nematic liquid crystals

We use the laser tweezers to create isolated pairs of topological point defects in a form of radial and hyperbolic hedgehogs, located close and attracted to a thin fiber with perpendicular surface orientation of nematic liquid crystal molecules in a thin planar nematic cell. We study the time evolution of the interaction between the two monopoles by monitoring their movement and reconstructing their trajectories and velocities. We find that there is a crossover in the pair interaction force between the radial and hyperbolic hedgehog. At small separation d, the elastic force between the opposite monopoles results in an increase of the attractive force with respect to the far field, and their relative velocity v scales as a v(d)∝d^{-2±0.2} power law. At large separations, the two oppositely charged monopoles can either attract or repel with constant interaction force. We explain this strange far-field behavior by the experimental inaccuracy in setting the fiber exactly perpendicular to the cell director.

Transport of particles in liquid crystals

Colloidal particles in a liquid crystal (LC) behave very differently from their counterparts in isotropic fluids. Elastic nature of the orientational order and surface anchoring of the director cause long-range anisotropic interactions and lead to the phenomenon of levitation. The LC environment enables new mechanisms of particle transport that are reviewed in this work. Among them the motion of particles caused by gradients of the director, and effects in the electric field: backflow powered by director reorientations, dielectrophoresis in LC with varying dielectric permittivity and LC-enabled nonlinear electrophoresis with velocity that depends on the square of the applied electric field and can be directed differently from the field direction.

Transport of particles by a thermally induced gradient of the order parameter in nematic liquid crystals

We demonstrate manipulation and transport of microparticles and even fluorescent molecules by the thermally induced gradient of the order parameter in the nematic liquid crystal. We use IR light absorption of the tightly focused beam of laser tweezers to heat locally a thin layer of the nematic liquid crystal by several degrees. This creates a spatial gradient of temperature of the nematic liquid crystal over separations of several tens of micrometers. We show that a dipolar colloidal particle is attracted into the hot spot of the laser tweezers. The depth of the trapping potential scales linearly with particle radius, indicating that the trapping mechanism is due to elastic self-energy of the distorted nematic liquid crystal around the particle and softening of the elasticity with increased temperature of the liquid crystal. We also demonstrate that this thermal trapping mechanism is efficient down to the nanoscale, as fluorescent molecules are also transported into hotter regions of the liquid crystal. This effect is absent in the isotropic phase, which calls into question particle transport due to the Soret effect.

Optical trapping induced by reorientational nonlocal effects in nematic liquid crystals

We report a detailed analysis of optical trapping of low index particles in liquid crystals under experimental conditions that prevent the effect of conventional trapping originated by optical gradient forces. The observation of stable, long-range trapping shows that this phenomenon in liquid crystals is regulated by a completely different mechanism than in isotropic media. In particular, the role of the nonlocality of optical reorientation is highlighted by showing the dependence of the trapping force on the size of the reoriented area. A model based on the actual form of the Gaussian focused beam impinging on the liquid-crystalline medium in the trapping experiment is also reported, with good agreement with experimental data.

Instantaneous axial force of a high-order Bessel vortex beam of acoustic waves incident upon a rigid movable sphere

The present investigation examines the instantaneous force resulting from the interaction of an acoustical high-order Bessel vortex beam (HOBVB) with a rigid sphere. The rigid sphere case is important in fluid dynamics applications because it perfectly simulates the interaction of instantaneous sound waves in a reduced gravity environment with a levitated spherical liquid soft drop in air. Here, a closed-form solution for the instantaneous force involving the total pressure field as well as the Bessel beam parameters is obtained for the case of progressive, stationary and quasi-stationary waves. Instantaneous force examples for progressive waves are computed for both a fixed and a movable rigid sphere. The results show how the instantaneous force per unit cross-sectional surface and unit pressure varies versus the dimensionless frequency ka (k is the wave number in the fluid medium and a is the sphere's radius), the half-cone angle β and the order m of the HOBVB. It is demonstrated here that the instantaneous force is determined only for (m,n) = (0,1) (where n is the partial-wave number), and vanishes for m>0 because of symmetry. In addition, the instantaneous force and normalized amplitude velocity results are computed and compared with those of a rigid immovable (fixed) sphere. It is shown that they differ significantly for ka values below 5. The proposed analysis may be of interest in the analysis of instantaneous forces on spherical particles for particle manipulation, filtering, trapping and drug delivery. The presented solutions may also serve as a method for comparison to other solutions obtained by strictly numerical or asymptotic approaches.

Acoustic radiation force on an air bubble and soft fluid spheres in ideal liquids: example of a high-order Bessel beam of quasi-standing waves

The partial wave series for the scattering of a high-order Bessel beam (HOBB) of acoustic quasi-standing waves by an air bubble and fluid spheres immersed in water and centered on the axis of the beam is applied to the calculation of the acoustic radiation force. A HOBB refers to a type of beam having an axial amplitude null and an azimuthal phase gradient. Radiation force examples obtained through numerical evaluation of the radiation force function are computed for an air bubble, a hexane, a red blood and mercury fluid spheres in water. The examples were selected to illustrate conditions having progressive, standing and quasi-standing waves with appropriate selection of the waves' amplitude ratio. An especially noteworthy result is the lack of a specific vibrational mode contribution to the radiation force determined by appropriate selection of the HOBB parameters.

Corona patterns around inclusions in freely suspended smectic films

We discuss the structure and physical origin of corona patterns observed around solid or liquid spherical inclusions in freely suspended smectic films. Such patterns are observed when droplets or solid beads of micrometer size are sprayed onto the films. They are found in the smectic C phase and in the smectic A phase above such a smectic C phase, but disappear, for example, at the transition into a lower-temperature smectic B phase. We show that these structures are equivalent to splay domains found in the meniscus of freely suspended films, originating from surface-induced spontaneous splay.

Inclusions in free standing smectic liquid crystal films

Colloidal inclusions in thin free standing liquid crystal films are ideal model systems for 2D anisotropic dispersions. Different types of self-organization in chain and lattice structures have been observed. The orientational elasticity of the anisotropic matrix and capillary forces are the dominating interaction mechanisms between solid or liquid inclusions, the director field, and dislocations of the films. We give an overview of the progress in this field, focussing on different inclusion types and their interactions in thermotropic smectic films.

Interactions of quadrupolar nematic colloids

We present experimental and theoretical study of colloidal interactions in quadrupolar nematic liquid crystal colloids, confined to a thin planar nematic cell. Using the laser tweezers, the particles have been positioned in the vicinity of other colloidal particles and their interactions have been determined using particle tracking video microscopy. Several types of interactions have been analyzed: (i) quadrupolar pair interaction, (ii) the interaction of an isolated quadrupole with a quadrupolar chain, and (iii) the interaction of an isolated quadrupolar colloidal particle with a two-dimensional (2D) quadrupolar crystallite. In all cases, the interactions are of the order of several 100k(B)T for 2 microm particles, which gives rise to relatively stable 2D colloidal crystals. The experimental results are compared to the predictions of Landau-de Gennes theory and we find a relatively good qualitative agreement.