Abstract: The droplet motion evolution model was established based on the lubrication theory for two identical surfactant-containing droplets on the topographic substrate. The coalescing process of the droplets on topographic substrate was simulated, and the effects of spacing, wave number and base height on the characteristics of coalescing were examined. The results show that the two droplets meet and superimpose each other under the capillary force, and finally form a “new” droplet at t_f. The dynamics of droplet over time are divided into five stages; while the surfactant concentration experiences the three stages in a short period; increasing spacing a extends the coalescing time of droplets; whereas increasing the wave number k leads droplet coalescing to be unstable; raising base height D shortens the coalescing time.

Key words: coalescence, surfactant, initial spacing, wave number, base height, "new"droplet