Self diffusion of particles in complex fluids: temporary cages and permanent barriers

We study the self diffusion of individual particles in dense (non-)uniform complex fluids within dynamic density functional theory and explicitly account for their coupling to the temporally fluctuating background particles. Applying the formalism to rod-like particles in uniaxial nematic and smectic liquid crystals, we find correlated diffusion in different directions: The temporary cage formed by the neighboring particles competes with permanent barriers in periodic inhomogeneous systems such as the lamellar smectic state and delays self diffusion of particles even in uniform systems. We compare our theory with recent experimental data on the self diffusion of fluorescently labelled filamentous virus particles in aqueous dispersions in the smectic phase and find qualitative agreement. This demonstrates the importance of explicitly dealing with the time-dependent self-consistent molecular field that every particle experiences.