Interactions of quadrupolar nematic colloids

Abstract

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.

Lassoing saddle splay and the geometrical control of topological defects

“…3E. This scale dependence of the defect structure is analogous to the physics of spherical colloids with homeotropic anchoring--Saturn ring defects become more stable for smaller system sizes compared with those of companion point defects (3,23). In our case, a thinner hole substrate has a greater density of splay distortion near the hole rims, favoring the expansion of the central point defect into a ring, allowing the splay distortion to be canceled near the rim.…”

“…A 10 × 10 array of holes with radius of 50 μm is prepared by drilling a Mylar sheet using IPG Microsystem's IX-255 UV excimer laser in the low fluence setting, provided by the University of Pennsylvania's Quattrone Nanofabrication Facility (QNF). The Mylar is coated with silicon tetrachloride (SiCl 4 ) through vapor deposition for the surface to be treated with DMOAP (Sigma-Aldrich) to obtain strong homeotropic anchoring (3,32,35). Coverslips coated with indium tin oxide (ITO; SPI Supplies), for the application of an electric field across the sample, are treated to have oriented planar anchoring by spin coating a thin layer of PVA (SigmaAldrich), which is subsequently baked at 80°C for 1 h, then rubbed with a velvet cloth in the desired direction (35).…”

“…liquid crystals | topological defects | saddle splay | disclinations T he investigation of mechanisms, both chemical and geometrical, to control and manipulate defects in liquid crystals (LCs) is essential for the use of these defects in the hierarchical self-assembly (1-3) of photonic and metamaterials (4,5), as well as for studies in low-dimensional topology (3,(6)(7)(8)(9)(10). For instance, the disclination line networks characteristic of blue phases (11,12) have been proposed to organize colloidal inclusions (4,13).…”

Nematic colloids, topology and photonics

“…These assemblies are very robust and can be transported as a whole. Several other two-dimensional colloidal crystals have been demonstrated, including quadrupolar colloidal crystals [16] and binary colloidal crystals made up of dipolar and quadrupolar colloids, shown in figure 3f [17]. When small and big colloidal particles are mixed together in the nematic LC, smaller particles tend to decorate the topological defects around bigger particles and hierarchical structures are formed, as presented in figure 3e [19].…”

“…The dipolar and quadrupolar forces between nematic colloidal particles were used to assemble one-dimensional colloidal chains (figure 3a,b) and two-dimensional nematic colloidal crystals of different symmetries [7,8,16,17], shown in figure 3. A two-dimensional colloidal crystal made up of dipolar colloids is presented in figure 3c.…”