Acoustic control of microstructures during direct ink writing of two-phase materials

•A device to manipulate microparticle placement within epoxy-based 3D printing lines using acoustic focusing is proposed.•Decreasing the matrix viscosity improves control over particle placement and worsens form holding.•The deposition process does not greatly perturb particle placement and packing. New methods to 3D print multiphase materials with tailored microstructures could expand additive manufacturing capabilities to include structures with unprecedented complexity. One promising technique to control microstructure is acoustic focusing, a method by which particles in a fluid are manipulated by acoustic waves. By combining acoustic focusing with direct ink writing (where fluid inks are solidified after extrusion), the distribution of particles in fluid matrices can be modified throughout print lines. Here, we describe the design space for acoustic focusing with direct ink writing and explore the associated trade-offs, for inks composed of epoxy resin, silica, acetone, and glass microspheres. Increasing silica content increases the viscosity, widening the spatial distribution of particles, but it improves the preservation of printed shapes. Increasing the amplitude of the acoustic wave and decreasing the printing speed allow for narrower distributions of particles in printed lines but increase the time and energy needed for the print. To quantify these trade-offs, we conducted a combinatorial study of four factors: fumed silica and acetone loadings in epoxy-based matrices (which control ink rheology), print speed, and acoustic wave amplitude. We show that after deposition, particle distributions within focused lines are preserved; particle-poor regions on the edges of focused lines spread out farther than the particle-rich regions in the middle. The present study relating device operating parameters, ink properties and printed microstructures structure provide key insights to future designs of actuated print nozzles, which target new microstructures enabled by field-assisted control.