The Optical Absorption Force Allows Controlling Colloidal Assembly Morphology at an Interface

Gaining control on particle-particle interactions and in this way on their (self)-assembled structures is essentialfor colloidal and material sciences. Currently, different strategies are described to achieve such control, however, all of them lack the spatiotemporal resolution required at the microscale. In this work, the potential of combining optical trapping and resonant photoexcitation for modifying particle-particle interactions and subsequent assembling of dye-doped particles at the solution interface is demonstrated. The particle assemblies prepared by nonresonant 1064 nm optical trapping undergo morphology changes after resonant photoexcitation of the embedded dye molecules. Depending on the physicochemical properties of interface, quick hexagonal close packing (HCP)-rearrangement or explosive dispersion of assemblies is observed at air/solution (A/S) and glass/solution interfaces, respectively. By contrast, by resonant photoexcitation only, the dispersed dye-doped particles are pushed toward the A/S interface, followed by association to yield HCP-structured assemblies. The results are rationalized by considering the optical absorption force coupled with other nonoptical forces (e.g., capillary force, dipole-dipole or electrostatic repulsion) at the solution interface. Due to the inherent spatiotemporal properties of light and electronic transition of materials, absorption force is a unique element to control and modify the structural order of particle assemblies at interfaces.