DropCoal: An instrument to study the drop coalescence in microgravity

• Droplet coalescence studied in microgravity aboard the International Space Station • Compact device enables controlled coalescence of millimetre-scale droplets • Custom needle design ensures stable droplet pinning in low gravity • High-speed imaging captures coalescence and early mixing dynamics • Initially results confirm device readiness for operation in microgravity conditions Coalescence of liquid droplets and the subsequent mixing of their contents play a significant role in many natural and technological environments, including raindrop formation, fuel combustion, and drug-delivery applications. Studying droplet coalescence and mixing in microgravity enables the isolation of fluid-mechanical phenomena that are otherwise masked on Earth by gravity. This paper presents the design, development, and commissioning of a microgravity experiment dedicated to the investigation of droplet coalescence and mixing aboard the International Space Station (ISS). The experiment investigates interacting droplet pairs with diameters between 2 mm and 5 mm and controlled approach velocities ranging from 0.01 mm s − 1 to 1 mm s − 1 . A compact, space-qualified instrument combining custom-engineered and selected commercial components was developed, tested on Earth, and qualified according to ESA and NASA requirements. The payload was installed in the ICE Cubes Facility within the Columbus module of the ISS. During a two-week commissioning phase, repeatable generation of millimetre sized spherical droplets and controlled relative velocities were demonstrated. Preliminary analysis of recorded video data reveals clear coalescence dynamics, including capillary-wave propagation and bridge-oscillation patterns. Observations of the mixing process confirm that microgravity conditions isolate surface-tension-dominated behaviour in the absence of buoyancy effects, thereby enabling direct observation of capillary-driven coalescence without gravitationally induced deformation. The test and validation protocols performed during commissioning confirm the functionality of the payload and the viability of the instrument operational plan, indicating that the forthcoming data sets will enable characterisation of capillary-driven coalescence and mixing in microgravity conditions.