Droplet coalescence occurs in a variety of settings, from natural to emerging technologies, such as reactive inkjet printing and lab-on-a-chip microfluidic devices. The internal dynamics during coalescence determine the extent of fluid mixing within the coalesced droplet, with efficient mixing often critical to the application’s viability. The fluid properties of the precursor droplets can significantly affect the internal dynamics, especially in the case that each droplet does not consist of the same fluid. Surface tension disparities are especially interesting, since they affect the internal dynamics both through the additional contribution to Laplace pressure and Marangoni flow.
This works investigates the coalescence of a sessile and an impacting droplet of different surface tension, in contact with a solid substrate. Two colour high speed cameras are used to capture the internal dynamics from both the side and below. This arrangement exposes both the detail of the flow through the depth of the droplet (side view), in addition to the intricate flow structures without distortion from the droplet’s curved surface (bottom view), allowing surface and internal phenomena to be identified. Our results demonstrate significantly faster mixing compared to droplets with identical fluid properties, uncovering various intriguing internal flow mechanisms. Multiple fluid configurations and lateral separations are studied, with our results having practical implications for enhancing mixing within impacting and coalescing droplets of different surface tension.