Enhancing and visualising mixing between impacting droplets*

Ways to efficiently mix the fluids of subsequently-deposited droplets are urgently needed to unlock the burgeoning potential of droplet-based applications such as reactive inkjet printing and open microfluidics. In our previous work, we demonstrated the potential of Marangoni flow to stretch & fold internal fluid interfaces between coalescing droplets of simple fluids, and therefore induce good advective mixing. Now, by combining color high-speed imaging with a passive color-change reaction that highlights mixed regions, we can probe (diffusive) mixing dynamics and assess the true extent of mixing. Furthermore, we use this new tool to demonstrate how surfactants can be employed as an additive to impart Marangoni flow by modifying the dynamic surface tension of coalescing droplets without changing the underlying fluid. This development offers a way to efficiently mix droplets in applications where the underlying functional fluid (e.g. a particular chemical reactant) should be maintained.