High Speed Acoustophoresis for Multiphase Micron-Sized Particle Assembly

Abstract

Acoustophoresis assembly is a non-contact assembly method characterized by its ability to assemble micron particles in 2D and 3D mm-scale patterns and its material agonism. In this method, acoustic standing wave pressure gradients interact with the particles to generate the acoustic radiation forces that assemble the particles into the desired pattern. However, assembly times are prohibitively slow in applications with small particle sizes and high-viscosity fluids. One mitigation strategy is to increase transducer power, thereby increasing the assembly forces. However, the increased power also increases streaming, a bulk fluid flow which destroys the assembled pattern when large enough and, thus, caps transducer power, setting a floor on particle size and assembly time. This thesis presents a strategy for circumventing the streaming flow disruption- assembling particles in the streaming transient stage. During this stage, the streaming velocity is low enough to allow the assembly of particles whose sizes are otherwise too small when streaming is at steady state. Indeed, assembly experiments in the transient stage show particles assembling into a pattern that otherwise deteriorates as streaming velocity reaches a steady state. Results show that increasing transducer voltage by 50% shortened the time to peak pattern quality magnitude by 25-46%. This thesis also presents a model for using the streaming velocity measurement to predict the timing of maximum pattern quality. This thesis also presents a sensitivity analysis of the assembly time model in the literature to explain the discrepancy between the assembly time measurements and the literature model. Here, the central theme is to show where to recoup lost assembly energy and improve assembly rate without increasing streaming. The sensitivity analysis shows that assembly time is most sensitive to sound speed, where 1% sound speed variation leads to a 4% change in the assembly time. Examination of the acoustophoresis standing waves show that sound speed and vessel width variation contribute to imperfect constructive interference of the standing waves, further reducing assembly time, especially once the model includes resonance. Resonance combined with parameter measurements accounts for the discrepancy between the model and measurements.