Time-resolved 3D reconstruction of an early-stage condensing microdroplet using digital holographic microscopy

Abstract

Early-stage condensation droplets set initial conditions for subsequent growth, coalescence, and surface re-exposure, yet their time-resolved three-dimensional morphology at the microscale remains difficult to measure using conventional two-dimensional imaging techniques. Here, digital holographic microscopy (DHM) was used to reconstruct time-resolved height fields of an individual condensing microdroplet on a silane-coated glass surface, with lateral and temporal resolutions of 0.83 \({\upmu }\)m and 66 ms, respectively. The reconstructed phase fields were converted into droplet height maps, from which maximum height, footprint-equivalent diameter, integrated volume, and projected-boundary descriptors were quantified. For the representative droplet analyzed in this study, early growth departed from uniform spherical-cap-like enlargement. Vertical growth was more pronounced than lateral footprint expansion during an early growth interval, while projected-boundary analysis showed that the initially less regular footprint became more circular as growth proceeded. At three selected growth stages, threshold-aligned spherical-cap cross-estimates underestimated the DHM-measured maximum height by 10.47–17.29% and the projected-footprint area by 9.00–27.74%, while overestimating the integrated volume by 9.84–28.53%. These observations indicate that the early lateral development of the analyzed droplet did not occur through uniform self-similar expansion, but through non-uniform morphological evolution involving coupled height growth, footprint development, volume accumulation, and boundary regularization. Together, the coupled temporal measurements and direct geometric comparison demonstrate that DHM can quantify non-uniform three-dimensional morphological evolution and assess the limitations of simplified droplet geometries during early-stage condensation.