Experimental and Numerical Study for the Coalescence Dynamics of Vertically Aligned Water Drops in Oil

In this paper, we propose an experimental and numerical investigation for the impact of the surface tension and the continuous phase viscosity on the dynamics of the liquid bridge during the coalescence process in liquid-liquid systems. A specific configuration of a sessile drop in direct contact with another drop placed over it has been studied. Calculating the redefined Reynolds number Re, it is found that for all studied cases, the coalescence process is dominated by the inertial force. The first step of the work was the validation of the numerical model that has been performed in an axisymmetric coordinate system. This has been done by the comparison between numerical and experimental results obtained in the framework of experimental series realized in parallel for two different liquid-liquid (LL) systems: water drops in silicone oil (SilOil M40.165) and water drops in sunflower oil. A good agreement was found between different results for numerous parameters used for comparisons. It is found that for the first stages of the coalescence (at the start of the drops merging), for a given drop's viscosity, the dynamics of the dimensionless liquid bridge is conducted by the viscosity of the continuous phase where it is illustrated that the more the surrounded viscosity is large, the lower the rate of the liquid bridge growth, the lower the earlier radial velocity of the bridge, and the higher the external capillary pressure generated around the bridge. Moreover, it is depicted that the impact of the surface tension starts appearing after the complete development of the liquid bridge where it is observed that for the same surrounding phase viscosity, the propagation of the capillary wave is faster for a LL system with higher surface tensions than those of lower surface tensions.