Optical manipulation of colloids and defect structures in anisotropic liquid crystal fluids

Abstract

Optical trapping in anisotropic fluids such as liquid crystals shows inherently different behavior compared to that in isotropic media. Anisotropic optical and visco-elastic properties of these materials result in direction-sensitive and polarization-dependent interaction of the focused laser beam with colloidal inclusions, defects and structures of long-range molecular order, providing new means of non-contact optical control. Optical trapping properties are further enriched by laser-induced realignment of the optical axis that can be observed in these liquid crystalline materials at relatively low trapping laser powers. Optical manipulation of particles and defects in these anisotropic fluids is of immense importance for their fundamental study and from the standpoint of technological applications such as light-directed colloidal self-assembly and generation of tunable photonic architectures in liquid crystals. We review the basic physical mechanisms related to optical trapping in anisotropic liquid crystalline fluids and demonstrate how it can be employed in quantitative studies of colloidal interactions and both topological and mechanical properties of defects.

Shape-Dependent Oriented Trapping and Scaffolding of Plasmonic Nanoparticles by Topological Defects for Self-Assembly of Colloidal Dimers in Liquid Crystals

“…At short distances < R np , the topological defect trapping force F tdt is expected to increase linearly with the displacement 26,27 Δr of the nanoparticle from the equilibrium position in the elastic trap following…”

“…19 For R np comparable to the defect core size, the energy reduction is mostly due to the energy of the displaced isotropic defect core, but the contribution of the elastic part of the defect energy increases further with increasing R np . Using the calculated reduction energy ΔW R for different nanoparticles and defects ( 27 ) at the sample plane do not exert optical trapping forces that would be strong enough to remove nanoparticles from the topological defect traps once they are entrapped. The stiffness of these topological defect traps is κ tdt ≥ k B T/Δr 2 , where Δr ∼ 10 nm is the maximum experimentally observed displacement of the nanoparticle from the center of the defect trap, precise measurement of which is limited by the resolution of videomicroscopy.…”

Large-area optoelastic manipulation of colloidal particles in liquid crystals using photoresponsive molecular surface monolayers

“…However, conventional optical manipulation does not simultaneously achieve the required high resolution and the large-area control, owing to the need for tight focusing of high-power beams (21)(22)(23). Although high-throughput, largearea optical manipulation has been achieved by use of opticallydirected electrophoretic, dielectrophoretic, and other forces (23)(24)(25)(26)(27)(28)(29)(30), these approaches are often restricted to specific types of particles, require application of fields in addition to the use of light, and cannot be applied to large-area manipulation in anisotropic LC fluids (30).…”

Direct observation of coupling between orientation and flow fluctuations in a nematic liquid crystal at equilibrium

“…(4). The averaged value of c is 0.30 × 10 4 , where we used V = 3.6 × 10 −14 m 3 , K 2 = 7.9 pN, and K 3 = 15.4 pN [18]. Next, we show the mean-squared displacements (MSDs) of the particle along the x and y directions at a height of z = d/4.…”