Acoustic field controlled patterning and assembly of anisotropic particles

• Acoustic waves are used to align and order anisotropic particles in suspensions.
• Microrods form isolated chains, parallel columns, and brick-and-mortar structures.
• Particle arrays are preserved in solid hydrogels and in solution via DNA-binding.
• Our acoustic assembly model accurately predicts column spacing and stability.
• This model is broadly applicable to various particle shapes and two-phase systems.

Highly-organized particle assembly at the microscale offers the potential to transform fields ranging from medical diagnostics to materials-by-design. Here, we demonstrate tunable ordering and alignment of microscale particles, including ∼10×50 μm glass rods and SU-8 brick and bowtie particles, via acoustic excitation, which produces columns with controlled spacing or highly-regular ‘brick-and-mortar’ packing. The method does not require surface functionalization and is broadly agnostic to colloidal chemistry or particle properties. We show that particle anisotropy promotes alignment and ordering even in the presence of flow, and can be used to increase the efficiency of particle delivery and sorting. Acoustically-assembled microstructures are preserved via photopolymerization of hydrogel matrices or binding with DNA ’mortar;’ the latter provides sufficient stability and flexibility to fold ribbon-like structures in solution by tuning the direction of the acoustic waves. Significant insight into pattern spacing and stability is provided by models which incorporate primary acoustic focusing and secondary acoustic scattering forces. These results suggest that acoustic excitation may be a powerful complement to diffusion-controlled self-assembly and enable hierarchical fabrication; e.g., micron-scale anisotropic crystals formed via self-assembly of smaller particles can be rapidly transported large distances, aligned and packed together to quickly assemble much larger objects.

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