On the application of refractive index matching to study the buoyancy-driven motion of spheres

Refractive index matching (RIM) is a powerful tool for multiphase flow studies as it eliminates optical distortions and enables high-fidelity tomographic measurements near solid-fluid interfaces of freely moving solids in the flow. However, by improving the RIM and optical quality, the solids become effectively invisible, preventing direct identification of their location. To address this limitation, we develop a physics-informed detection framework that locates transparent spheres within time-resolved tomographic Particle Tracking Velocimetry by combining tracer voids, vertical velocity signatures, and vortex structures into a unified optimization problem. Integrated with volumetric reconstructions, the method provides simultaneous analysis of velocity, pressure, and force on the sphere. Applied to an example case of an 11.11 mm acrylic sphere rising in a RIM sodium iodide solution, the technique reveals a clear phase-locked relation between double-thread wake structures, surface-pressure distributions, and unsteady hydrodynamic forces over half a cycle of the sphere motion in the 4R vortex shedding regime. For the first time, this enables direct calculation of drag and lift histories on a freely moving sphere. The framework can be extended to dynamic masking for improved tomographic reconstruction and pressure-field calculations, to non-spherical bodies with more complex motions, and to multi-body interactions, advancing RIM from a flow-only diagnostic to a tool for fully coupled body-wake measurements.