(749c) High Throughput Acoustically Driven Self-Assembly of Microfluidic Colloidal Crystals

Manufacturing defect-free ordered materials is immensely important for producing phononic and photonic crystals for energy applications or to serve as waveguides for optical computing. External fields (e.g., electric, magnetic, flow) that drive self-assembly are a form of bottom-up manufacturing, where micro- or nanoscale building blocks assemble spontaneously to form complex shapes. Drawbacks to current approaches include low throughput (~10 particles per second or less), the formation of permanent defects (e.g., grain boundaries, point defects, line defects) during assembly and the high cost associated with fabricating devices to produce external fields. In this presentation, we demonstrate a high throughput (~300 particles per second) self-assembly process based on acoustically-driven assembly of microparticles using of-the-shelf components without the need to access a clean room.

Fifteen micron polystyrene particles dispersed in water are continuously introduced to glass-capillary microfluidic flow cell with a syringe pump. A piezoelectric element attached to the size of the flow cell is driven to generate an acoustic standing wave that drives particles to assemble at acoustic nodes. An experimental state diagram shows the effect that fluid flow rate and applied acoustic pressure have the assembled microstructure. Highly ordered, continuous crystals are observed to assemble in less than a minute with a throughput rate of several hundred particles per second were observed under various pressure and flow rates. Particle tracking software is used to identify the location of microparticles assembled in ordered structures. This data is used to as part of a distribution analysis to understand the compressive influence that the acoustic pressure has on the observed structures. The flow cell throughput is quantified using a micro-particle image velocimetry analysis of optical video microscopy data. The degree of order within the assembled structures is quantified by comparing the number of nearest neighbors to theoretical expectations for an ideal ordered structure.

Future work is focused on mitigating defect formation in our ordered crystal structures. Acoustic assembly is dynamically controllable, allowing us to understand the kinetic effects that fluid flow and acoustic pressure have on dislocation dynamics. We aim to develop real-time annealing techniques to remove defects during crystal formation.