Three-dimensional imaging of liquid crystal structures and defects by means of holographic manipulation of colloidal nanowires with faceted sidewalls

Abstract

We use nanowires with faceted sidewalls for mapping of the patterns of three-dimensional orientational order and defect structures. In chiral nematics, the nanowires follow the local average orientation of rod-shaped molecules. When spatially translated by use of holographic optical tweezers in three dimensions, they mediate direct nondestructive visualization of the helicoidal ground-state structures, edge and screw dislocations, and kinks, as well as enable non-contact manipulation of these defects. We probe interactions of faceted nanowires with different defects and demonstrate their spontaneous selfalignment along the cores of singular defect lines.

Active Shape-Morphing Elastomeric Colloids in Short-Pitch Cholesteric Liquid Crystals

“…Within a far-field approximation, even spherical particles with strong boundary conditions induce dipolar or quadrupolar n(r)-distortions arising due to the anisotropic nature of the host medium and particle-induced satellite defects, such as point defects and disclination loops [1,2]. Although many methods for self-assembly of anisotropic particles in isotropic solvents have been introduced [4], LC hosts bring a number of unique capabilities, such as the well-defined long-range alignment of anisotropic particles with respect to n controlled by external fields [3,[5][6][7][8][9][10].…”

“…Along with symmetry considerations, this brings about an expansion into multipole series and electrostatic analogies between long-distance colloidal interactions and that of multipoles in electrostatics [1][2][3][4][5][6][7][8][9][10][11]36]. The ground state in a CLC is strongly twisted and a direct parallel with the homogeneous nematic ground state is impossible because homogeneity of the ground state is a prerequisite of the symmetry-based considerations [11].…”

“…Shape of particles dictates colloidal self-assembly [3,[7][8][9][10][11][12][13][14][15] and may enable practical utility of these fascinating LC-colloidal systems. Although complex-shaped particles can be fabricated using photolithography [3,6], two-photon polymerization [10], nanocrystal growth [8], and wet chemical synthesis [7,9], these approaches provide nematic dispersions of non-responsive particles with no means for altering their shape, alignment, locomotion, and self-assembly [3,[5][6][7][8].…”

Self-assembly of colloidal particles in deformation landscapes of electrically driven layer undulations in cholesteric liquid crystals

“…An externally applied magnetic field H induces the magnetic moment m of a given ferrite 5 nanoparticle to favor one direction, which may not necessarily align collinearly with H. Since the ferrite nanoparticle orientations are mechanically coupled to the epoxy matrix of the SPMB microsphere, magnetic moment interactions with the applied magnetic field prompt transient SPMB rotation and alignment of its m to eventually point along H; this interaction subsequently mechanically aligns the SPMB such that its net magnetic moment locks collinear to H and follows its rotation [49,50] during our experiments detailed below. In addition to superparamagnetic beads, we also use melamine resin (Sigma-Aldrich Chemistry) and polystyrene (PS, from Bangs Laboratories, Inc.) spheres of 7 and 5 μm in diameter, respectively, as well as GaN nanowires nominally 10 μm in length and 300 nm in diameter [51,52]. All colloidal particles were dispersed in a CLC host either via mechanical mixing or solvent exchange, with both methods producing comparable quality of dilute colloidal dispersions [49][50][51][52].…”

“…In addition to superparamagnetic beads, we also use melamine resin (Sigma-Aldrich Chemistry) and polystyrene (PS, from Bangs Laboratories, Inc.) spheres of 7 and 5 μm in diameter, respectively, as well as GaN nanowires nominally 10 μm in length and 300 nm in diameter [51,52]. All colloidal particles were dispersed in a CLC host either via mechanical mixing or solvent exchange, with both methods producing comparable quality of dilute colloidal dispersions [49][50][51][52].…”