By: Philippe Cluzeau
From: CNRS Bordeaux
At: Instituto de Investigação Interdisciplinar, Anfiteatro
NOTE: DIFFERENT DAY OF THE WEEK!
In the recent decade, investigations of pattern formation and self-organization of inclusions in liquid crystal free standing films have been the subject of intensive experimental and theoretical investigations. InÂ such liquid crystal free standing films, the spreading of micrometric liquid or solid inclusions induces a subsequent interface deformation around the inclusions: the so called corrona pattern. Nevertheless, up to now, the physical origin of the radial stripes pattern visible in the meniscus surrounding the inclusion remains misunderstood.
We investigate extensively the corrona pattern using an interferometric technique (Phase Shifting Interferometry ) combined with polarised and depolarised optical microscopy. Our interferograms reveal without ambiguity that the radial stripe patterns observed in the Smectic-C (or Smectic-C*) meniscus correspond to an undulation of the LC-air interface (amplitude ranging from 30 to 100nm) and was not only originating from surface-induced spontaneous splay [2, 3]. In the thicker part of the meniscus, the undulations propagate themselve in two orthogonal directions and surface exhibits a âegg box-likeâ structure.
In order to confirm these optical results, we perform an AFM investigation of free-standing films meniscus in the Smectic-C phase: These pictures of the LC-air interface also reveal a periodic undulation of the smectic layers (amplitude ranging from 30 to 100nm according to places in good agreement with interferometric technique).
Lastly, a very basic theoritical model based on the well-known dilatation induced instability [4,5] is developped and turns out to be compatible with our results for it gives correct orders of magnitude of the undulations wavelengths. From this model, we discuss the physical origin of the corona pattern.
 D. W. Robinson and G. T. Reed, Interferogram analysis(Institute of Physics Publishing, Bristol) 1993.
 K. Harth and R. Stannarius, Eur. Phys. J. E, i2008-10404-6 (2009).
 M. Conradi, P. Ziherl, A. Sarlah, I. Musevic, Eur. Phys. J. E 20, 231 (2006).
 C. Rosenblatt, R. Pindak, N.A. Clark, R.B. Meyer, Phys. Rev. Lett. 42, 1220 (1979).
 N. Clark and R. B. Meyer, Appl. Phys. Lett. 22, 493 (1973).