Mixing dynamics between inkjet-printed droplets*

This talk presents a 3D numerical simulation framework within OpenFOAM to capture the mixing dynamics between impacting and coalescing droplets in real inkjet printing conditions. Understanding these dynamics enables optimisation of several fabrication processes, e.g. Reactive Inkjet Printing, which rely on adequate mixing of droplets.  A diffusive transport equation for a conserved scalar within the hydrodynamic model for droplet coalescence is included, enabling the key physical mixing processes to be simulated.

Simulations are validated against previously-published millimetric experimental data and used to quantify the importance of molecular diffusion in mixing inkjet droplets (~50µm). Our results show that typical inkjet printed droplets of the same fluid do not homogenise on a short time scale, regardless of the lateral offset. We use our simulations to outline the conditions in which faster mixing could be expected, and demonstrate that the size of the internal interface between the droplets is proportional to the short-term mixing that occurs, presenting a method to control droplet mixing.