Smart surface dissymmetrization of microparticles driven by laser photochemical deposition

Dissymmetric nano/microsized spheres are very appealing because controlled dissymmetry brings an additional degree of freedom for the synthesis of a new generation of materials with spatially separated chemical properties, We explore this aspect by extending to spherical surfaces the application field of lithographic techniques that was up to now essentially limited to planar and cylindrical substrates. The method proposed uses a strongly focused laser beam to generate dissymmetric coatings on microparticles by micro-photochemical deposition in a reactive solution. This is experimentally illustrated by considering the photochemical reduction of chromate ions induced by a continuous Ar laser wave to "nucleate" and grow a dissymmetry on the surface of silica beads dispersed in a chromate solution. When properly rescaled, the coating growth laws measured at different laser excitations are reduced to a single master behavior that implies a simple strategy to control and predict the desired dissymmetry from its dynamics. The versatility of the technique is then demonstrated by scanning the beam (i) to tailor microscale patterning on one hemisphere and (ii) to assemble beads into ordered structures. Owing to its flexibility, the method can easily be extended to the coating of different types of particles and various photochemical reactions.