Internal dynamics of coalescing droplets

Internal flows determine the extent of advective mixing (stretching and folding of fluid interfaces) between coalescing droplets, effectively defining the initial condition for molecular diffusion to homogenise the fluid. Efficient mixing is essential in applications from inkjet printing to microfluidic devices, especially where a chemical reaction or biological process occurs between the droplet fluids. In emerging technologies, coalescing droplets often have different fluid properties, are in contact with a substrate and are surrounded by gas. An improved fundamental understanding of the internal dynamics in such cases, as contributed by this work, is both of significant physical interest and essential for improving mixing efficiency in applications. This combined numerical and experimental work considers surface-tension-dominated coalescing droplets in two primary configurations: initially-static free and sessile droplets; impacting and sessile droplets with varied lateral separation. Two high-speed imaging experimental setups were designed and constructed, including one featuring synchronised colour cameras yielding simultaneous front and bottom views. A quantitatively-validated customised numerical simulation code was developed within OpenFOAM, utilising the Kistler dynamic contact angle model (including contact angle hysteresis) to capture substrate wettability and a conserved passive scalar to assess advective mixing. The conditions leading to the formation of internal and surface jets between coalescing droplets are determined, where jet formation can significantly improve mixing efficiency. In particular, the effect of substrate wettability (principally via capillary waves) and fluid properties on internal flows are systematically studied in tandem. A mechanism of internal jet formation between free and sessile coalescing droplets, at volume ratios very different from those accepted for free droplets, is identified. A mechanism of surface jet formation between impacting and sessile droplets with a large lateral separation is elucidated, in which jet formation and mixing is controlled via Marangoni flow for droplets of different surface tension. Moreover, it is shown that diffusive mixing can be passively assessed via colour-change reactions, which are in turn used to identify efficient mixing mechanisms.