Observations of a streak texture in the hybrid-aligned smectic-C phase.

A novel structure was observed below the smectic-A-smectic-C phase transition in a very thin open cell having an air interface above and enforced planar anchoring at the substrate below. The structure appears as periodic dark and light streaks running perpendicular to the oily streaks, which are present in the smectic-A phase [D. Coursault et al., Soft Matter, 2016, 12, 678]. These new streaks, which we call "soapy streaks", form by extending from one oily streak to the next in discrete steps, eliminating optical evidence at visible wavelengths of the oily streaks. At lower temperatures the streaks can undulate and exhibit a sawtooth-like structure; such a structure is chiral in two dimensions. A possible scenario for the origin of these streaks is presented.

Anisotropic reversible aggregation of latex nanoparticles suspended in a lyotropic nematic liquid crystal: effect of gradients of biaxial order.

We studied the anisotropic aggregation of spherical latex particles dispersed in a lyotropic liquid crystal presenting three nematic phases: calamitic, biaxial, and discotic. We observed that in the nematic calamitic phase aggregates of latex particles are formed, which become larger and anisotropic in the vicinity of the transition to the discotic phase, due to a coalescence process. Such aggregates are weakly anisotropic and up to 50 microm long and tend to align parallel to the director field. At the transition to the discotic phase, the aggregates dissociated and re-formed when the system was brought back to the calamitic phase. This shows that the aggregation is due to attractive and repulsive forces generated by the particular structure of the nematic phase. The surface-induced positional order was investigated by surface force apparatus experiments with the lyotropic system confined between mica surfaces, revealing the existence of a presmectic wetting layer around the surfaces and oscillating forces of increasing amplitude as the confinement thickness was decreased. We discuss the possible mechanisms responsible for the reversible aggregation of latex particles, and we propose that capillary condensation of the NC phase, induced by the confinement between the particles, could reduce or remove the gradient of order parameter, driving the transition of aggregates from solidlike to liquidlike and gaslike.

Forces in nematic liquid crystals constrained to the nanometer scale under hybrid anchoring conditions.

Using a surface forces apparatus we have studied two thermotropic nematic liquid crystals (5CB and ME10.5) subjected to hybrid (homeotropic/planar) anchoring conditions. A film of nematic material is constrained between two curved smooth surfaces separated by less than 2500 A . The intersurface force is nonmonotonic with the separation, being repulsive for thicknesses larger than approximately 100 A and strongly adhesive at a shorter scale. While the repulsion can be qualitatively explained by an elastic model of director deformation, including anchoring deviation at the boundaries, the attraction cannot be explained either by elasticity or by dispersive forces. The expected confinement-induced anchoring transition has not been observed for a thickness as small as 200 A .