Structure and rheology of lyotropic liquid crystalline suspensions

Abstract

Suspensions in the form of solid–liquid dispersions are encountered in several industrial applications. Household detergents and processed foods can be modeled as a suspension of phase-separated or externally added particulate solids in a medium of mesomorphic and lyotropic gel phases. Our aim is to probe the connection between the structure and rheology of suspensions of micron-sized particles in lamellar and other translationally ordered surfactant mesophases. We have conducted experiments with a commercial rheometer to study the rheology of the pure lyotropic phases and suspensions of particles in these phases. A custom-made microscope was used in conjunction with the rheometer to study the structure of the lyotropic phases, with and without particles. Our experiments on the lamellar phase reveal that the rheological properties of the lamellar phase strongly depend on the applied shear rate rather than just on the shear history. We also observe that the addition of particles slows down the decay of the elastic modulus with shear time. This observation was related to the optical microscopy observations where we see that the particles stabilize the defect network. The shape of the frequency spectrum of storage modulus does not change with shear time or with particle concentration in the concentration regime studied. This implies that the overall structure of the lamellar phase remains unaffected by shear and added particles. The variation of the storage modulus with particle concentration is smooth, indicating no major change in the overall structure. Dynamic rheological measurements on the hexagonal phase show that there is no change in the shape of the frequency spectrum of the storage modulus with added particles. This implies that there is no change in the overall structure with added particles. Optical micrographs also indicate that there is no change in the overall structure of the mesophase at optical wavelengths. However, it is difficult to infer the local structure around a particle from light microscopy. It is also observed that there is a substantial increase in the storage modulus upon addition of particles. We have also performed lattice Monte Carlo simulations of amphiphilic systems using the Larson model. Typical equilibrium configurations in two dimensions show highly irregular interfaces, apparently caused by capillary waves. We have generated self-assembled lamellar phases in three dimensions and discussed the feasibility of studying the effect of particles on the structure of the lamellar phase using the present code. The present code can be very effectively used for studying oil–water solubility in the presence of amphiphile.