Two Dimensional Acoustofluidic Manipulation of Microparticles

Abstract

Acoustically actuated microfluidic devices for bio diagnostics have gained the attention of the research community, due to the gentle and non-contact characteristics of the actuation force. The existing acoustofluidic devices have been primarily made off materials with high acoustic impedance such as glass (Z_ac≅12 MRayls ), and silicon (Z_ac≅19 MRayls ). In terms of fabrication costs and fabrication time, plastics would have been more preferable due to their rapid and easy fabrication methods such as milling, and molding into microfluidic chips. However, the drawback of using plastics in acoustofluidics is that plastics tend to have lower acoustic impedance than glass, or silicon, and are considered as lower quality resonators than e.g. glass. Nonetheless, poly-methyl methacrylate (PMMA) has a moderate acoustic impedance (Z_ac≅3 MRayls), which is higher than e.g. PDMS where its acoustic impedance is (Z_ac≅1.03 MRayls), and that has encouraged this work. In this thesis, the employment of bulk acoustic waves (BAW) on a two-dimensional acoustofluidic resonators made from glass and PMMA were compared for acoustofluidic micromanipulation applications. Polystyrene microparticles were used in the manipulation experiments for visualizing the acoustic modes of the acoustofluidic chips, where their frequency responses were recorded by the magnitude of motion achieved from particle tracking velocimetry (PTV). Firstly, a PMMA microfluidic device was fabricated by laser engraving, and ethanol thermal bonding. As a control experiment, a glass acoustofluidic chip was fabricated using femtosecond ablation (at Technical University of Braunschweig) and tested using the same parameters as the PMMA chip. Secondly, simulations using COMSOL Multiphysics have been carried out to determine numerical values of resonance frequencies of the devices, and their corresponding acoustic resonance mode shapes. In addition to glass and PMMA, simulations were conducted for an acoustofluidic device made out of PDMS. The simulation results showed that PDMS performance is poor for particle manipulation. Thirdly, preliminary experiments were contacted, where a beta version of our particle tracking velocimetry (PTV) was used and, we were able to obtain a rough frequency profile of the devices in the frequency range 60 kHz and 460 kHz. The results of the preliminary frequency response of the PMMA chip showed good agreement with the predicted frequency response using multiphysics simulations ,where its performance in both cases (simulations and experiments) was comparable to results obtained by the glass chip. Simulated results shown that PMMA has the potential to performs similar to the glass chip. However, in the preliminary experiments was observed that the power requirements needed for a particle motion of 0.5 mm in PMMA are much greater than in glass. This has caused overheating of the aqueous solution at antiresonances, due to the enormous amount of power fed to the actuator, which this did not happen in the glass chip. Additionally, the actual acoustic modes of both glass and PMMA chips do not agree with the simulated results, where the frequency difference in actual modes from theoretical is ~100 kHz for the glass chip