Collision is merely contact with short-time duration. However, the approach of modeling is different for each. For collision or contact, two models are normally used: the hard particle model and the soft particle model. From the viewpoint of physics, collision and contact do not differ much. The hard particle model is easy to use but applicable only to the binary collision. The relation between the pre- and post collision velocities is given explicitly using the coefficient of restitution and friction coefficient. In general, particle collisions are negligible in dilute gas particulate flows. While this is true in most cases, whether particle collision should be considered or not depend not only on the particle concentration but also on the velocity fluctuation or relative velocity between individual particles. Droplet Breakup Mechanisms e main topics when studying drop breakup are the regimes of the breakup, the time of breakup, and the outcome: size, number, and velocities of the secondary droplets.