Structure and rheology of composite soft solids: Particles in lamellar phases

We study the structure and rheology of new classes of composite soft solid materials, formed by incorporation of hard particles into the "onion textur e" of a lyotropic lamellar phase. The onion texture is obtained by prolonge d steady shearing of a lamellar sample. Depending on the size of the partic les (compared to the onions), their volume fraction, and the stage during t he preparation process at which the particles are added, we find three stru cturally distinct classes of composite. These are "stuffed onions," in whic h the particles art: sequestered at the center of the onions; "decorated on ions," in which, as well as replacing the onion core, the particles decorat e the polyhedral lattice of edges between the onions: and onion/particle "a lloys." The latter are formed when the particles are added late in the shea ring procedure, in which case the (preexisting) onions remain intact and th e particles reside entirely in the interstitial regions between them. (At h igh enough particle densities these regions occupy a significant volume fra ction, and the onions revert from a polyhedral to a spherical shape). Altho ugh structurally distinct, these three materials all have remarkably simila r rheological properties (at least in the range of volume fractions of dopa nt particles studied here, less than or similar to 4%), which, to within ex perimental error, do not differ from those of the pure onion system without particles. All of these structures are metastable but have lifetimes long compared to the case where particles are added to a lamellar phase in which the onion texture is not present. [S1063-651X(99)12103-2].