  The precise arrangement of microscopic objects is critical to the development
of functional materials and ornately patterned surfaces. Here, we present an
acoustics-based method for the rapid arrangement of microscopic particles into
organized and programmable architectures, which are periodically spaced within
a square assembly chamber. This macroscale device employs two-dimensional bulk
acoustic standing waves to propel particles along the base of the chamber
toward pressure nodes or antinodes, depending on the acoustic contrast factor
of the particle, and is capable of simultaneously creating thousands of
size-limited, isotropic and anisotropic assemblies within minutes. We pair
experiments with Brownian dynamics simulations to model the migration kinetics
and assembly patterns of spherical microparticles. We use these insights to
predict and subsequently validate the onset of buckling of the assemblies into
three-dimensional clusters by experiments upon increasing the acoustic pressure
amplitude and the particle concentration. The simulations are also used to
inform our experiments for the assembly of non-spherical particles, which are
then recovered via fluid evaporation and directly inspected by electron
microscopy. This method for assembly of particles offers several notable
advantages over other approaches (e.g., magnetics, electrokinetics and optical
tweezing) including simplicity, speed and scalability and can also be used in
concert with other such approaches for enhancing the types of assemblies
achievable.

Soft Condensed Matter