Progress of Theoretical Physics Supplement No. 175, 2008 37 Dynamics of Colloidal Particles in Soft Matters

We developed numerical methods for studying the dynamics of colloidal particles sus-pended in complex fluids. It is essential to employ a coarse-grained model for studying slow dynamics of these systems. Our methods are based on the “fluid particle dynamics (FPD)” method, which we have developed to deal with hydrodynamic interactions in colloidal sys-tems in an efficient manner. We regard a solid particle as an undeformable fluid one. It has a viscosity much higher than the solvent, which smoothly changes to the solvent viscosity at the interface. This methods allows us to avoid troublesome boundary conditions to be satis-fied on the surfaces of mobile particles. Since we express the spatial distribution of colloids as a continuum field, we can easily introduce the order parameter describing a complex sol-vent, e.g., ion distribution for charged colloids, director field for nematic liquid crystal, and concentration for phase-separating binary fluid. Then we solve coupled dynamic equations of three relevant parameters, i.e., particle positions, flow field, and the order parameter. We demonstrate a few examples of such simulations. Recently, suspensions of colloids or nanoparticles in complex fluids hav