1ST OBSERVATIONS OF A SPIRAL INSTABILITY AT THE SMECTIC-A-CHOLESTERIC TRANSITION UNDER AN ELECTRIC-FIELD

Archimedian spiral wave instabilities have been discovered in chemical [1] and biological systems [2]. We present here the first example of such an instability pattern, encountered in a physical medium [3]. This instability is directly observed by polarized optical microscopy, on a positive dielectric anisotropy smectic A sample with homeotropic organization between parallel glass slides. The arm of the spiral is probably constituted of a 180-degrees Bloch wall, separating indistinguishable smectic domains, and incorporating progressively the helicity of the cholesteric phase, excluded from the smectic.