Internal jets can vastly improve mixing efficiency during droplet coalescence. For free droplets, jet formation depends on geometry, initial configuration and fluid properties. When at least one of the coalescing droplets is on a substrate, we systematically demonstrate that the three-phase contact line and substrate wettability are crucial too. In particular, using both high-speed imaging and quantitatively validated numerical simulations incorporating the Kistler dynamic contact angle model (with hysteresis) we investigate internal jet formation during the coalescence of an initially static free droplet and a sessile droplet of the same fluid. We identify and elucidate a mechanism of jet formation arising for surprisingly low sessile to free droplet volume ratios, showing that the presence of a substrate can improve mixing efficiency. Moreover, this mechanism is found to depend on substrate wettability with the importance of advancing contact angle subordinated to that of receding contact angle. Droplet geometry and fluid properties are also considered to thoroughly explain the dynamics observed.
Supported by the EPSRC Centre for Doctoral Training in Fluid Dynamics at the Uni. of Leeds [Grant No. EP/L01615X/1].