Mixing between coalescing droplets

Abstract

When printed droplets coalesce, internal flows determine the extent of fluid mixing on short time scales. Mixing may be desirable, such as to bring chemical reactants (e.g. in reactive inkjet printing) or biological agents together, but may also lead to undesirable colour bleeding in graphical printing. We will summarise the key results of our research, which reveals a variety of ways to assess and tune the amount of mixing between droplets deposited on solid surfaces. Surprisingly, lateral separation can have a non-trivial effect on mixing between successively-deposited droplets; large separations can lead to localised free surface flows that may enhance colour bleed. Surface tension gradients (Marangoni flow) can either enhance or supress mixing, whilst substrate wettability (dynamic contact angles) can significantly alter the resulting internal flow structures. We demonstrate the latter both experimentally and numerically (using OpenFOAM). Experimentally, we show how colour-change reactions can be used to reveal internal flow structures and the real-time extent of fluid mixing.