  Controlling the assembly of polyelectrolytes and surfactant at liquid-liquid
interfaces offers new ways to fabricate soft materials with specific physical
properties. However, little is known of the relationships between the kinetics
of interfacial assembly, structural and rheological properties of such
interfaces. We studied the kinetics at water-oil interface of the assembly of a
positively charged biopolymer, chitosan, with an anionic fatty acid using a
multi-scale approach. The growth kinetics of the membrane was followed by
interfacial rheometry and space- and time- resolved dynamic light scattering.
This set of techniques revealed that the interfacial complexation was a
multi-step process. At short time-scale, the interface was fluid and made of
heterogeneous patches. At a 'gelation' time, the surface elastic modulus and
the correlation between speckles increased sharply meaning that the patches
percolated. Confocal and electron microscopy confirmed this picture, and
revealed that the basic brick of the membrane was sub-micrometric aggregates of
chitosan / fatty acid.
