  After rainfall, pine needles are observed to float on the surface of small
puddles. As the water evaporates, these pine needles exhibit spontaneous
self-assembly into distinct clusters. Motivated by this natural phenomenon, we
conducted experimental investigations into the dynamic evolution of particles
during the shrinkage of a liquid surface. Our experiments revealed that
particles tend to aggregate, forming distinct clusters as the liquid boundary
shrinks. We systematically examined the emergence of these clusters and
explored how their sizes and numbers evolve with changes in packing fraction.
Furthermore, we 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. Our study contributes to a better
understanding of self-assembly phenomena involving macroscopic particles on
shrinking liquid surfaces.
