SPIRAL TEXTURES IN LYOTROPIC LIQUID-CRYSTALS - 1ST-ORDER TRANSITION BETWEEN NORMAL HEXAGONAL AND LAMELLAR GEL PHASES

The first order transition between the normal hexagonal phase (H(alpha )) and lamellar gel phase (L(beta) L(beta', L(delta), ... type) in lyo tropic liquid crystals of binary surfactant/water systems is investiga ted. Structural transformations and epitaxial relations are investigat ed by small-angle X-ray scattering on powdered and oriented samples. B y slow evaporation of water, growth of the gel layered structure from the two-dimensional packing of surfactant cylinders of the hexagonal m esophase in the presence of a solid wall reveals a spectacular new tex ture composed of interwoven spirals. It is demonstrated that the layer s grow from the rods of the hexagonal phase, in planes coplanar with t he hexagonal packing and perpendicular to the wall. The configuration is such that line wedge disclinations of strength s = + 1/2 of the hex agonal phase are preserved through the phase transition. Estimates of the radii for the developable domain and cores, and also for the bendi ng elastic constant are obtained. A mechanism for the phase transforma tion is discussed in view of topological structural transformations an d a modification of the short-range order associated to the disorder/o rder transition of the configuration of the paraffinic chains.