Impacting and coalescence of droplets on a substrate solid surface are of big importance to the instituted inkjet printing industry and applications such as the noncontact printing of efficient electronics and biological materials and in the fields of microfluidic devices, microchemistry, and fast prototyping.

To shed light on the associated phenomena and mechanisms including dynamic liquid/gas interface deformation, computational Fluid Dynamics (CFD) used as an effective tool. However, the numerical simulation of thermocapillary flows involving large dynamic liquid/gas interface deformations presents significant challenges. In this direction some numerical techniques such as Volume of Fluid (VOF), Lattice Boltzmann, Phase Field, and Front Tracking have been utilized. In the present study, we selected the VOF method due to its capability of tracking complex interface shapes. The VOF method implemented in OpenFoam uses a marker function, which indicates the ratio of gas and liquid phases, and the marker function is advected by the flow. However, the accuracy of the method is not high especially for computing curvature and normal direction of interfaces.To improve this technique, some additional methods have been developed. One way of improving the VOF method is to couple it with the Level Set (LS) method which is called” Coupled Level Set and Volume of Fluid (CLSVOF)”. For our investigation, a new solver S-CLSVOF (simple Coupled Volume of Fluid with Level Set) method incorporated by Albadawi et al was compiled and used with OpenFOAM to improve computations for interface curvature and its normal direction.

Dynamics of two droplets interactions on a substrate (droplet impact on a sessile droplet) are numerically investigated using OpenFOAM. The impact speed, location of the impacting droplet, viscosity and surface tension were varied in the numerical studies. We found that when the surface tension dominates the flow, the mass canter of two droplets moves to impacting droplet side. When the inertia dominates the flow, the mass centre moves to the opposite direction.