Bundling of Rod-Like Colloids via Depletion Forces*

Self assembly of rod-like colloids in to bundles can be driven by entropic processes. We use Coarse Grained Molecular Dynamics simulations to investigate using depletion forces to induce bundling in a suspension of rod-like colloids. The colloids are approximated as bead-spring polymers, with persistence lengths much larger than the length of the polymer. Depletion forces are an effective attractive force between colloids due to the presence of particles significantly smaller than the colloids, called depletants. Depletion forces are modelled as an implicit entropic force between colloids. Bundling begins to occur when the occupied volume fraction occupied by depletants is above a critical value. This volume fraction is dependent on the length and stiffness of the rod-like colloids. We investigate using this property to induce bundling only in rods above a certain length or certain persistence length.