Predicting the Anchoring of Liquid Crystals at a Solid Surface: 5-Cyanobiphenyl on Cristobalite and Glassy Silica Surfaces of Increasing Roughness

Abstract

We employ atomistic molecular dynamics simulations to predict the alignment and anchoring strength of a typical nematic liquid crystal, 4-n-pentyl-4'-cyano biphenyl (5CB), on different forms of silica. In particular, we study a thin (~20 nm) film of 5CB supported on surfaces of crystalline (cristobalite) and amorphous silica of different roughness. We find that the orientational order at the surface and the anchoring strength depend on the morphology of the silica surface and its roughness. Cristobalite yields a uniform planar orientation and increases the order at the surface with respect to the bulk whereas amorphous glass has a disordering effect. Despite the low order at the amorphous surfaces, a planar orientation is established with a persistence length into the film higher than the one obtained for cristobalite.

Single Molecule Studies on Dynamics in Liquid Crystals

“…This influence is reduced for the isotropic no-PDI, leading to a qualitative agreement of experimental results with the hydrodynamic model in case of no-PDI [35]. Recent atomistic molecular dynamics (MD) simulations for a 24 nm thick nematic 5CB film on amorphous silica showed a crossover from planar to substrate-normal alignment within 15 nm from the substrate [50]. For smectic 8CB the situation is more complex and thus MD simulations are still lacking.…”

Inhomogeneous relaxation dynamics and phase behaviour of a liquid crystal confined in a nanoporous solid

“…Computer simulations on LCs in thin film and pore geometry can give here important complementary insights [13,24,[41][42][43][44][45][46][47][48][49][50]. These studies indicate pronounced spatial heterogeneities, in particular interface-induced molecular layering and radial gradients both in the orientational order and reorientational dynamics in cylindrical pore geometry [51].…”

From Chiral Islands to Smectic Layers: A Computational Journey Across Sexithiophene Morphologies on C60

“…Since mesophases of rod-like molecules typically show a planar alignment on regular surfaces, [ 60,61 ] our aim was to explore the opportunities offered by high temperature annealing for producing an interface more suitable for solar cell applications than the one obtained by vapor deposition, namely one with T6 layers parallel to the C 60 surface.…”

“…The choice of a NpT simulation is aimed at avoiding the effect of the interface with vacuum, that typically induces a homeotropic alignment on rod-like mesogens, and may propagate for some nanometers inside the sample. [ 60,61 ] Before discussing the results, it is worth recalling that, with the force fi eld used here, melting from the crystal to a layered LC phase (smectic A) occurs at 580 K for bulk samples, and it is followed by the transition to a nematic phase at 610 K and fi nally to the isotropic liquid at 670 K. [ 28 ] Already from the snapshot in Figure 5 a, it appears that our attempt to obtain a liquid crystalline phase of T6 with planar alignment on C 60 was successful, with T6 molecules forming smectic layers extending from the bottom to the top surface. A further analysis of the phase director of the liquid crystal phase demonstrates that its orientation is templated by the surface, as it coincides with the C 60 [110] direction.…”

“…A similar interfacial ordering effect has been previously observed for glassy quaterthiophene on fullerene [ 14 ] or for liquid crystals on atomically fl at inorganic surfaces. [ 60,61 ] Such an increased structuring at the interface, which persists also at room temperature (vide infra), is likely to strongly affect charge separation kinetics and energetics. [9][10][11] …”