Abstract
In the present work we perform Monte Carlo simulations in the isothermal-isobaric ensemble to study defect topologies formed in a cholesteric liquid crystal due to the presence of a spherical colloidal particle. Topological defects arise because of the competition between anchoring at the colloidal surface and the local director. We consider homogeneous colloids with either local homeotropic or planar anchoring to validate our model by comparison with earlier lattice Boltzmann studies. Furthermore, we perform simulations of a colloid in a twisted nematic cell and discuss the difference between induced and intrinsic chirality on the formation of topological defects. We present a simple geometrical argument capable of describing the complex three-dimensional topology of disclination lines evolving near the surface of the colloid. The presence of a Janus colloid in a cholesteric host fluid reveals a rich variety of defect structures. Using the Frank free energy we analyze these defects quantitatively indicating a preferred orientation of the Janus colloid relative to the cholesteric helix.
Highlights
It is well-known that liquid-crystal molecules in the nematic phase exhibit no long-range positional order of their centers-of-mass but tend to align their longer axes parallel to a preferred direction speci ed by the global director n^0.1 If a colloid is immersed in such a homogeneously ordered nematic phase the order is perturbed locally
Because the threshold value for l+(r) does not affect the length of the disclination lines shown in Fig. 5, we propose the following line of arguments to unravel the apparent discrepancy between Monte Carlo (MC) data and the point defects that would form ideally
This is because in the case of a Janus colloid the orientation of its antithetic surfaces with respect to the local orientation of the director eld at the position of the colloid's center of mass enters the analysis as a new parameter w which we introduce via n^0(0)$uJ 1⁄4 cos w where uJ is a unit vector describing the orientation of the Janus colloid
Summary
It is well-known that liquid-crystal molecules (mesogens) in the nematic phase exhibit no long-range positional order of their centers-of-mass but tend to align their longer axes parallel to a preferred direction speci ed by the global director n^0.1 If a colloid is immersed in such a homogeneously ordered nematic phase the order is perturbed locally. If a chemically homogeneous spherical colloid with locally homeotropic surface anchoring (i.e., anchoring of mesogens parallel with respect to the colloid's local surface normal) is placed in a nematic host phase the well known Saturn ring defect topology forms around the colloid's equator.[6]. If two such colloids come sufficiently close, the defect lines interact and can entangle the colloidal dimer in several ways.[9,10].
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