Scaling relationships for acoustic control of two-phase microstructures during direct-write printing

Acoustic forces can align and consolidate particles in fluids, enabling microstructural control of two-phase materials at time-scales compatible with direct-write printing of composites. This paper presents key scaling relationships for acoustically-assisted direct-write printing that describe characteristic time-scales for assembly and alignment of particles during printing. Critical combinations of system parameters (including particle and nozzle dimensions, acoustic excitation amplitude, viscosity, and flowrate) are defined that govern particle focusing and assembly in the print stream. The results can be used to identify combinations of printing protocols and nozzle configurations that control particle packing parallel and transverse to the print direction. [GRAPHICS] IMPACT STATEMENT We present theory and experiments demonstrating acoustic focusing in conjunction with directwrite printing for 'on-the-fly' control of two-phase microstructures, and a design framework for printing arbitrary material combinations.