Volume 80, Number 4, November 2007
Article Number 46001
Number of page(s) 5
Section Condensed Matter: Structural, Mechanical and Thermal Properties
Published online 17 October 2007
EPL, 80 (2007) 46001
DOI: 10.1209/0295-5075/80/46001

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

R. Pratibha, N. V. Madhusudana and B. K. Sadashiva

Raman Research Institute - C. V. Raman Avenue, Bangalore 560080, India

received 23 July 2007; accepted in final form 19 September 2007; published November 2007
published online 17 October 2007

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 $\omega _{0}$ with nb. The novel stripe pattern is extremely soft, as evidenced by the occurrence of isolated lattice disclinations, whose nature is dictated by $\omega _{0}$. 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.

61.30.-v - Liquid crystals.
61.30.Jf - Defects in liquid crystals.
68.18.Jk - Phase transitions.

© Europhysics Letters Association 2007