Self-assembling clusters of particles on a shrinking liquid surface

After rainfall, pine needles often float on the surface of small puddles. As the water evaporates, they self-assemble into distinct clusters. Motivated by this natural phenomenon, we experimentally investigate the dynamic evolution of synthetic particles as the liquid surface shrinks in area. Our experiments demonstrate the tendency of particles to aggregate, forming distinct clusters as the liquid boundary shrinks. We systematically examine the emergence of these clusters and explore how their sizes and numbers evolve with changes in the packing fraction. We also analyze particle rotation during the process and discuss the formation of the final configuration comprising clusters of various orientations. Complementary numerical simulations demonstrate qualitative agreement with our experimental findings. This study sheds additional light on the self-assembly of macroscopic particles in a dynamically evolving medium.