  Chiral nematic or cholesteric liquid crystals (LCs) are mesophases with
long-ranged orientational order featuring a quasi-layered periodicity imparted
by a helical configuration but lacking positional order. Doping molecular
cholesteric LCs with thin colloidal rods with a large length-to-width ratio or
disks with a large diameter-to-thickness ratio adds another level of complexity
to the system because of the interplay between weak surface boundary conditions
and bulk-based elastic distortions around the particle-LC interface. By using
colloidal disks and rods with different geometric shapes and boundary
conditions, we demonstrate that these anisotropic colloidal inclusions exhibit
biaxial orientational probability distributions, where they tend to orient with
the long rod axes and disk normals perpendicular to the helix axis, thus
imparting strong local biaxiality on the hybrid cholesteric LC structure.
Unlike the situation in achiral hybrid molecular-colloidal LCs, where biaxial
order emerges only at modest to high volume fractions of the anisotropic
colloidal particles, the orientational probability distribution of colloidal
inclusions immersed in chiral nematic hosts are unavoidably biaxial even at
vanishingly low particle volume fractions. In addition, the colloidal
inclusions induce local biaxiality in the molecular orientational order of the
LC host medium, which enhances the weak biaxiality of the LC in a chiral
nematic phase coming from the symmetry breaking caused by the presence of the
helical axis. With analytical modeling and computer simulations based on
minimizing the Landau de Gennes free energy of the host LC around the
colloidals, we explain our experimental findings and conclude that the biaxial
order of chiral molecular-colloidal LCs is strongly enhanced as compared to
both achiral molecular-colloidal LCs and molecular cholesteric LCs and is
rather unavoidable.
