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 texture" of a lyotropic lamellar phase. The onion texture is obtained by prolonged steady shearing of a lamellar sample. Depending on the size of the particles (compared to the onions), their volume fraction, and the stage during the preparation process at which the particles are added, we find three structurally distinct classes of composite. These are "stuffed onions," in which the particles art: sequestered at the center of the onions; "decorated onions," in which, as well as replacing the onion core, the particles decorate the polyhedral lattice of edges between the onions: and onion/particle "alloys." The latter are formed when the particles are added late in the shearing procedure, in which case the (preexisting) onions remain intact and the particles reside entirely in the interstitial regions between them. (At high enough particle densities these regions occupy a significant volume fraction, and the onions revert from a polyhedral to a spherical shape). Although structurally distinct, these three materials all have remarkably similar rheological properties (at least in the range of volume fractions of dopant particles studied here, less than or similar to 4%), which, to within experimental 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].