This study experimentally investigates the three-dimensional settling process of centimeter-sized spherical particles in still water using convergent binocular vision technology. The results reveal that the number of settling particles significantly influences the settling behavior when particles are released side by side. Individual particles initially settle along nearly straight paths before randomly deflecting and returning after reaching maximum deviation points. In contrast, twin particles settle synchronously in an almost mirror-image manner, with random deflection largely suppressed—a phenomenon referred to as the mutual support phenomenon (MSP) is this study. The outermost two particles in triplet and quadruplet configurations also exhibit a typical MSP during the settling process. However, the middle particle in the triplet configuration behaves similarly to an individual particle, while the middle two particles in the quadruplet configuration display more complicated settling trajectories due to asymmetric influences from neighboring particles. Additionally, particle image velocimetry measurements are conducted in specific cases to explore the possible mechanisms underlying the random deflection of individual particles and the MSP observed in twin particles.

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• Fluid-particle interactions
• Particle interactions