Shape transitions of sedimenting confined droplets and capsules: from oblate to bullet-like geometries

The transport and deformation of confined droplets and flexible capsules are central to diverse phenomena and applications, from biological flows in microcapillaries to industrial processes in porous media. We combine experiments and numerical simulations to investigate their shape dynamics under varying levels of confinement and particle flexibility. A transition from an oblate to a bullet-like shape is observed at a confinement threshold, independent of flexibility. A fluid-structure interaction analysis reveals two regimes: a pressure-dominated and a viscous-dominated regime. For highly flexible particles, the pressure-dominated regime prevails and the deformation is enhanced. These findings offer new insights into the transport of flexible particles in confined environments, with implications for biomedical applications, filtration technologies, and multiphase fluid mechanics.