Density map of collision between two bubble-droplets. It is seen that the collision can be elastic for two bubble-droplets, but many particles in q₂ component transit to q₁ component and transit back perfectly during the collision process. The parameters are λ 1 = λ 3 = 0.4 , λ 2 = λ 4 = 0.6 , k 1 = k 2 = 1 , c 1 = c 2 = 3 , α 1 = α 4 = − 3 , α 2 = α 3 = 3 .