Ultrasonic microparticle alignment and direct ink writing using glass capillaries

We envision implementing direct ink writing for 3-D printing while aligning microfibers in the resin using standing wave ultrasonics; the aligned fibers would control desired mechanical properties such as strength and ductility, and 3-D printing would match the mechanical properties to the particular part geometry. At this time we work with highviscosity fluids as a physical simulant of representative resins, and spherical polystyrene microparticles or glass microrods instead of microfibers. In this paper we show experimental results using square glass capillaries (with interior dimensions ranging from 0.4 to 1.0 mm) as our microfluidic systems, which are inherently well-suited by their geometry to act as print nozzles, sandwiched between two piezoceramic plates that generate the ultrasonic standing waves. We report experimental data for particle alignment as we change from our initial test fluid, water, to high-viscosity fluids. Similarly, we report experimental data of the fluid behavior pertinent to direct ink writing; we enforce controlled volumetric flow rates (which correspond to print speeds) for high-viscosity fluids under pressurized flow through glass capillaries of varying cross-sectional areas and varying lengths, observing and measuring the approximate ink line width and height. Our use of commercially available square glass capillaries (sandwiched between piezoceramic transducers that are driven at frequencies away from transducer resonance) is novel and distinguishes our approach from that of other research groups; the underlying physics of our devices differs from that of Lund-type acoustic resonators.