Precise manipulation of bio-micro/nanoparticles, including cells, platelets, bacteria, and extracellular vesicles, is critical for tumour diagnostics, infectious disease detection, and cell analysis. The acoustofluidic aggregation method shows its significant advantages in microparticle aggregation, including highly scalable, label-free, contact-free, biocompatible and non-invasive. For nanoparticle enrichment, thermophoresis has been recently proposed as an efficient way to accumulate nanovesicles in biomedical applications. Although numerous experimental and analytical studies have been undertaken to study the micro/nanoparticle aggregation mechanisms, few studies have been conducted to optimise the acoustophoresis or thermophoresis device design. Therefore, this dissertation aims to improve the efficiency and accuracy of the micro/nanoparticle aggregation devices through numerical modelling using the finite element method.

This research first conducts a numerical investigation of the acoustophoresis of microparticles suspended in a compressible liquid. The wall of the rectangular microchannel is made of Polydimethylsiloxane (PDMS), and Standing Surface Acoustic Waves (SSAW) are introduced into the channel from the bottom wall. The relative amplitude of the acoustic radiation force and the viscous drag force is evaluated for particles of different radii ranging from 0.1μm to 15μm. Only when the particle size is larger than a critical value can the particles accumulate at acoustic pressure nodes (PNs). While the displacement amplitude of SSAW impacts the time scale of particle movement, it does not influence the final position of the particles. The efficiency of the particle accumulation depends on the microchannel height, so an extensive parametric study is then undertaken to identify the optimum microchannel height. The optimum height, when normalised by the acoustic wavelength, is found to be between 0.57-0.82.

Second, a numerical model is established to investigate the laser heating parameters on thermophoretic enrichment of nanoparticles. In the thermophoresis enrichment system, a microchamber containing particle/fluid mixture is sandwiched by a glass top, from where an infrared light laser heat source is introduced, and a sapphire bottom, which has a high heat conductivity to prevent overheating. First, the radius of the final nanoparticle distribution is found to be approximately 1.25 times the laser spot radius. A reduction in the laser attenuation length leads to a reduction of the time taken by the nanoparticles to reach the steady state, but an enlarged final area over which nanoparticles are concentrated. There exists an optimum range of the attenuation length, depending on the size of the target area. We have determined the threshold particle size, which decides whether the particle motion is convection-dominated or thermophoresis-dominated. Furthermore, an increase in the laser power reduces the accumulation time of nanoparticles.

It is found from the second part of the research that the enrichment time for nanoparticles can be prolonged due to convection caused by local heat. To address this issue, a finite element (FE) model which incorporates SSAW with thermophoresis is developed in the third part. Based on the thermophoretic model from the second part, SSAW is introduced at the top of the microchamber by two pairs of interdigitated transducers (IDTs). The SSAW-induced thermoacoustic streaming can be properly controlled to move in the opposite direction of the convection, optimising its impact on thermophoresis and consequently reducing nanoparticle enrichment time. A parametric study is then conducted to examine the influence of the acoustic field on particle enrichment time with a laser power of 194 mW. With the optimised actuation condition of SSAW, the enrichment time of nanoparticles can be reduced by 61% compared to the thermophoresis enrichment without SSAW. Similar studies are then conducted with different laser powers ranging from 194 mW to 248 mW. About 61%-time reduction can be achieved for all the tested cases. The optimum magnitude of the maximum acoustic pressure increases slightly with laser powers. These findings provide insights into the design of the micro/nanoparticle aggregation devices.