Coalescence of Sessile Drops: the Role of Gravity, Interfacial Tension and Surface Wettability

Coalescence of liquid drops is important in many natural and industrial processes, such as raining, inkjet printing and coating applications. The coalescence for sessile drops is more complicated due to the additional interplay between the drops and solid surface. This work examines the impact of gravity, interfacial tensions and wetting properties on both the static and dynamic aspects of the coalescence of sessile drops. In the presence of gravity, seven dimensionless parameters are identified to describe the axisymmetric configuration of a compound sessile drop after coalescence. A stability criterion is established based on the perturbation of Laplacian shape and the stability criterion is numerically evaluated in the zero Bond number limit. Surface Evolver simulations and experiments are performed for compound sessile drops at small and intermediate Bond numbers. Both simulations and experiments agree closely with the zero Bond number analysis, exhibiting a small discrepancy at intermediate Bond number. For the dynamics of sessile drop coalescence, experiments are performed for miscible fluids with similar surface tensions but different densities and viscosities. The coalescence behavior shows three distinctive stages with well separated timescales: an initial stage of fast bridge healing process, an intermediate stage of advective motion for fluids with different densities, and a final stage of diffusion. A dimensional analysis shows that the flow behavior for the advective motion resembles gravity current. A more detailed analytical model based on the lubrication approximation is conducted and demonstrates good qualitative agreement with the advective motion during the sessile drop coalescence.