Mixing Dynamics of Impacting and Coalescing Droplets

Impacting and coalescing droplets are a pivotal feature of many microfluidic technologies such as reactive inkjet printing (RIJ).
This technology utilises droplet impact and coalescence to synthesise materials on a substrate.
Good mixing between the deposited droplets is required for an efficient fabrication process.
Motivated by experimental observations of limited mixing at larger length scales and the computational challenges of simulating full RIJ processes, a stepwise, physics-driven approach was adopted to isolate and understand the mechanisms governing droplet mixing.

A customised numerical framework was developed by extending the OpenFOAM solver interFoam to incorporate molecular diffusion.
This solver is rigorously validated against analytical solutions and novel experiments, demonstrating its capability to resolve internal and external droplet dynamics across a range of configurations.
Initial simulations explore the role of diffusion in picolitre-scale inkjet droplets, confirming that advective mixing is minimal, and diffusive mixing alone is slow and highly sensitive to droplet spacing, timing, and substrate properties.
Parametric studies revealed complex, non-linear dependencies that are critical for understanding and optimising printed droplet mixing.

To investigate thermal effects, high-speed imaging experiments were designed to isolate the impact of temperature gradients in non-isothermal coalescing droplets on a heated substrate.
These revealed enhanced contact line mobility, buoyancy-driven flow, and up to two orders of magnitude faster mixing compared to diffusion alone.
These findings guide the extension of the numerical framework to simulate non-isothermal droplets, incorporating heat transfer and thermophysical viscosity and density models. Simulations confirmed the temperature configuration can affect the mixing in impacting and coalescing droplets as well as the role of substrate interaction in redirecting internal jets inertia and enhancing interface folding, thus boosting mixing efficiency.

The work provides a validated, extensible toolset for investigating multi-physics droplet interactions, and offers new insights into thermally induced mixing mechanisms. These results contribute to advancing fundamental understanding and guiding the design of RIJ systems and related applications where precise droplet control and mixing are critical.