Directional mass transfer of azo molecular glass microsphere induced by polarized light in aqueous immersion media

Photoinduced mass transfer of azo polymer and azo molecular glass has been intensively investigated under various light irradiation conditions simply using air as the ambient environment. In this work, in order to understand the effects of the surrounding medium on the light-induced process, azo molecular glass microspheres adhered on a substrate were immersed in water and different aqueous solutions, and their mass transfer behavior was investigated by irradiation with linearly polarized light. The microspheres in the aqueous media showed significant deformation through directional mass transfer upon light irradiation and transformed into a series of shape-anisotropic particles as revealed by microscopic observations. Compared with their counterparts upon light irradiation in air, the particles immersed in the aqueous media exhibited larger elongation parallel to the substrate and higher shape anisotropy. Optical simulation showed that this was caused by the alteration of the direction of the electric vibration of the refracted light at the medium–microsphere interface, which controlled the mass transfer behavior. On the other hand, the viscosity of the aqueous media showed no effect on the mass transfer process induced by the irradiation. The photo-thermal effect on the mass transfer behavior was ruled out as the thermal dissipation through a liquid is much more efficient than that through air. On the basis of this, this methodology was also successfully employed in the photo-fabrication of anisotropic submicron-sized periodic structures in aqueous medium. These observations can supply deep understanding of this fascinating process induced by polarized light and extend the scope of its applications.