  Acoustic-directed assembly is a modular and flexible bottom-up technique with
the potential to pattern a wide range of materials. Standing acoustic waves
have been previously employed for patterning preformed metal particles,
however, direct patterning of metallic structures from precursors remains
unexplored. Here, we investigate utilization of standing waves to exert control
over chemical reaction products, while also exploring their potential in the
formation of multi-layered and composite micro-structures. Periodic metallic
micro-structures were formed in a single step, simplifying microstructure
fabrication. Concentric structures were obtained by introducing a metal
precursor salt and a reducing agent into a cylindrical piezoelectric resonator
that also served as a reservoir. In addition, we introduce an innovative
approach to directly fabricate metallic multi-layer and composite structures by
reducing different metal ions or adding nanoparticles during the reduction
step. Fewer steps are needed, compared with other methods, and there is no need
to stabilize the nanoparticles or to ensure chemical affinity between the
metallic matrix and inorganic nanoparticles. This innovative approach is
promising for production of complex microstructures with enhanced functionality
and controlled properties.
