Coalescence of levitated polystyrene microspheres

The slow, laser-induced coalescence of two conjoined, polystyrene spheres levitated in a quadrupole ion trap is investigated by monitoring optical morphology dependent resonances (MDRs) appearing in the fluorescence emission spectrum. The heated bisphere is driven by surface tension to become a single, larger sphere with a volume equal to the combined volumes of the two initial spheres. In the final stage of the structural transformation the particle is a prolate spheroid whose dimensions are ascertained by analyzing frequency shifts of the non-degenerate azimuthal MDRs. The relaxation time for the deformed viscous sphere is used to estimate the polystyrene viscosity and temperature. The study highlights the feasibility of using a temperature-controlled quadrupole ion trap to investigate the coalescence dynamics of viscous microstructures free from the perturbative effects of any solvent or substrate.