Local mechanical properties of a hyperswollen lyotropic lamellar phase

The local and slow dynamics of a colloidal particle dispersed in a hyperswollen lyotropic lamellar phase was studied by passive and active microrheological methods. According to observation by a particle-tracking video microscopy, the motion of a particle follows a normal diffusion process in a lamellar phase with a large interlayer distance. However, trap-diffusion motion was also observed for a small interlayer distance. This characteristic motion was discussed by consideration of the time evolution of mean square displacement, the van Hove self-correlation function and the non-Gaussian parameter. These indicate that the dynamics of a particle becomes spatially heterogeneous in a lamellar phase whose interlayer distance is close to the size of the particle. By the measurement of local viscosity using optical tweezers, Newtonian behavior was observed at a high shear rate. In a lamellar phase with a small interlayer distance, non-Newtonian behavior was also observed at a low shear rate. The dependences of effective viscosity on the interlayer distance obtained by both methods are in agreement. The effective viscosity in the lamellar phase was found to be four to five times larger than that of water even for a large interlayer distance. Possible reasons for the increase in effective viscosity are discussed quantitatively.