Special issues No. 4,
Pages: 49-50,
https://doi.org/10.69646/1csst29
1st Conference on Space Science and Technology in Serbia
Published by: Astronomical Observatory Belgrade

The Droplet Coalescence (DropCoal) device operated aboard the ISS under residual gravity 10⁻⁵-10⁻⁶ g, providing a unique platform to study inertial-capillary bridge dynamics and surface capillary wave propagation, free from buoyancy and convection. Water droplets (D = 2-5 mm, Oh ≈ 0.002-0.004) were generated at coaxial needles and brought into contact under controlled conditions; high-speed video at 8000 fps captured bridge formation for equal-size and asymmetric pairs, as well as capillary wave packets launched at the moment of contact and propagating along the droplet surface. When two liquid droplets first contact, a liquid bridge forms and grows rapidly under surface tension. Theory predicts that in the inertial-capillary regime (Ohnesorge number Oh ≪ 1) the bridge neck radius follows ݑ侮푡) = ݐ氝ᅨݑᯰ휏)1/2, where ݜ｠(ݜ찝ᅳ/ݜ/2 is the inertial time scale. Testing this scaling law requires gravity-free conditions; on Earth, buoyancy deforms the droplets and drives convective flows that interfere with the measurement. Eight experimental configurations were studied: four equal-size droplet pairs (D = 2, 3, 4, 5 mm) and four asymmetric pairs (size ratios up to 2.5:1), with up to eight replicates each. Power-law fits yield a universal exponent n = 0.49 ± 0.03 across all eight configurations, in excellent agreement with the theoretical value of 0.5. The dimensionless prefactor C = 1.08-1.22 for all experiments, consistent with recent numerical simulations. For asymmetric pairs, normalising by the harmonic-mean effective radius ݑ尝ᒰ푓ݑ㽠2ݑ屰푅2/ (ݑ屠+ ݑ岩 and the corresponding inertial time ݜﰝᒰ푓ݑ㽠(ݜ찝ᅰ푒ݑ㰝ᓳ/ݜ/2 collapses all data onto the same universal curve as equal-size pairs, with no statistically significant dependence of C or n on size ratio up to 2.5:1. Capillary wave packets launched at the moment of coalescence are clearly resolved propagating along the droplet interface across all configurations, offering an independent probe of the inertial-capillary dynamics beyond the bridge region. Volume-of-Fluid (VOF) simulations are performed with OpenFOAM (interIsoFoam) in an axisymmetric 2D geometry, using a 10 µm mesh resolution in the bridge region. A key feature is initialising each simulation from the experimentally measured pre-coalescence droplet contour rather than an ideal sphere, which is expected to improve quantitative agreement with experiment. Preliminary bridge-radius curves from simulation will be presented alongside the experimental results, providing a first validation of the numerical approach for microgravity binary pinned droplets coalescence. Acknowledgements. The authors gratefully acknowledge the European Space Agency for funding the DropCoal mission, supporting the development and deployment of the payload on the International Space Station. The work of the Romanian authors was supported by the Romanian Ministry of Education and Research, through UEFISCDI, project number PN-IV-P7-7.1-PED-2024-1995, within PNCDI IV, and by the Romanian National Nucleu Program LAPLAS VII, contract no. 30N/2023.