Competition between Capillarity, Layering and Biaxiality in a Confined Liquid Crystal Articles uri icon

publication date

  • May 2010

start page

  • 89

end page

  • 101

issue

  • 1

volume

  • 32

international standard serial number (ISSN)

  • 1292-8941

electronic international standard serial number (EISSN)

  • 1292-895X

abstract

  • The effect of confinement on the phase behaviour and structure of fluids made of biaxial hard particles (cuboids) is examined theoretically by means of Onsager second-order virial theory
    in the limit where the long particle axes are frozen in a mutually
    parallel configuration. Confinement is induced by two
    parallel planar hard walls (slit-pore geometry), with particle long
    axes perpendicular to the walls (perfect homeotropic
    anchoring). In bulk, a continuous nematic-to-smectic transition takes
    place, while shape anisotropy in the (rectangular) particle
    cross-section induces biaxial ordering. As a consequence, four
    bulk phases, uniaxial and biaxial nematic and smectic
    phases, can be stabilised as the cross-sectional aspect ratio is varied.
    On confining the fluid, the nematic-to-smectic transition is
    suppressed, and either uniaxial or biaxial phases, separated
    by a continuous transition, can be present. Smectic ordering
    develops continuously from the walls for increasing particle
    concentration (in agreement with the supression of
    nematic-smectic second-order transition at confinement), but first-order
    layering transitions, involving structures with n and n
    + 1 layers, arise in the confined fluid at high concentration.
    Competition between layering and uniaxial-biaxial ordering
    leads to three different types of layering transitions, at
    which the two coexisting structures can be both uniaxial, one uniaxial
    and another biaxial, or both biaxial. Also, the interplay
    between molecular biaxiality and wall interactions is very subtle:
    while the hard wall disfavours the formation of the biaxial
    phase, biaxiality is against the layering transitions, as we have
    shown by comparing the confined phase behaviour of cylinders
    and cuboids. The predictive power of Onsager theory is checked
    and confirmed by performing some calculations based on
    fundamental-measure theory.