Stripes in biaxial smectic-A liquid crystals arising from spontaneous surface charge modulation

We report the discovery of a periodic stripe pattern consisting of alternate domains of uniaxial and biaxial smectic-A phases when a binary mixture made of rod-like and bent-core molecules is taken between rubbed glass plates. The same mixture usually exhibits only the biaxial smectic-A phase with a uniform concentration when the plates are not rubbed. The apolar director nb corresponding to the bow axes of the bent-core molecules aligns along the rubbing direction in the stripes with the wave vector of the pattern making a finite angle ω0 with nb. The novel stripe pattern is extremely soft, as evidenced by the occurrence of isolated lattice disclinations, whose nature is dictated by ω0. We argue that the stripes owe their origin to the underlying spatial modulation of surface charges generated on the glass plates by the rubbing action. Though the stripe structure is surface-induced the stripe spacing decreases with increase of sample thickness. Our calculations indicate that the novel origin also leads to the counter intuitive dependence of the stripe spacing on sample thickness