Morphology control of dynamic optical matter of gold nanoparticles fabricated by optical trapping in printed microchannels.

Optical trapping at interfaces has emerged as a valuable research topic in the study of colloidal particles and soft matter. Objects are drawn from the irradiated cone-like region toward the laser focus, generating flow patterns beyond the focal area. Localized heating at the focus induces coupled effects on surface tension, capillary forces, and Marangoni convection. Furthermore, optical propagation and scattering of the trapping laser beyond the focus can lead to the formation of large assemblies along the interface, extending well beyond the laser beam itself. For gold nanoparticles, a single large swarming assembly forms, with individual nanoparticles exhibiting vivid fluctuations. In this study, we investigate the swarming assembly as a non-linearly evolving optical matter using a plastic microchannel. The original structure undergoes transformations into pressed, square, unidirectional, triangular, elongated rectangular, or even twisted assemblies. In addition, the photothermal effects of the optical matter are analyzed in the context of a local anisotropic heater. This phenomenon not only suggests potential applications but also offers valuable insights for advancing new technologies.