Liquid droplets coalescing on solid surfaces are found in many applications, from inkjet printing to microfluidic devices. Whilst most work on droplet coalescence focuses only on the external dynamics, there is a growing interest in the internal flows and mixing between the fluids contained within each of the coalescing droplets. Adequate mixing is crucial in emerging printing technologies, but the process of mixing within coalescing droplets is currently not well understood, especially when the coalescence occurs on a solid surface, and at small length scales.
The coalescence of a free and a sessile droplet is investigated both experimentally and through numerical simulation using OpenFOAM. The initial configuration of the droplets ensures that their velocities at the onset of coalescence are negligible. Due to the dominant surface tension force, the free droplet is injected into the sessile droplet which drives the internal flow. The factors affecting the internal dynamics are considered, including the droplet volume ratio and surface wettability. The presence of the solid surface is shown to promote good mixing, even for modest droplet volume ratios.