The approach of two unequal or equal sized fluid particles is modeled using a simple parallel film model, giving expressions for interaction time and maximum film area, disregarding all external forces. The interaction time is found independent of the initial approach velocity and it explains why the effective virtual mass coefficient changes with the radius ratio of particles in the model of Chesters and Hofman (1982). A more general parallel film model is also developed based on the balance of forces acting on the particles and shows how initial velocity, radius ratio and buoyancy affect the interaction time, approach velocity and film area. Both models are compared to the experimental data of Scheele and Leng (1971).