Enhancing Bulk Acoustic Streaming in Silicon Channels by Wall Structuring
Mehrnaz Hashemiesfahan is a PhD student in the Department Mesoscale Chemical Systems. Promotors are prof.dr. J.G.E. Gardeniers from the Faculty of Science and Technology and prof.dr. W. De Malsche from Vrije Universiteit Brussel.
Acoustofluidics is an emerging research field wherein mixing or (bio)-particle separation is conducted. Compatibility with sensitive samples like cells, offering a gentle separation process that minimizes sample damage, makes acoustofluidic an interesting approach for particle separation and manipulation. However, scaling up acoustofluidic-based separation techniques needs further research.
This thesis presents an extensive description of microfabrication techniques devoted to acoustofluidic Silicon and glass devices, including lithography and etching steps. It discusses step-by-step fabrication, challenges, and limitations in the fabrication of these devices.
In the following chapters, the research is mainly focused on enhancing acoustic mixing by incorporating sharp corners. The effect of sharp corners, their position in the device, the angle, and the shape of the sharp edges on acoustic streaming were characterized. The sharp corners not only greatly enhance mixing and manipulation in the channel, but they can also, under specific conditions, produce a pumping effect inside the channel when the flow is induced by acoustic energy.
The next part includes a fabricated silicon micro membrane in an acoustofluidic device, and characterization of mixing in the device was done using different methods. A novel approach is introduced using an active BAW mixing method to enhance lateral transport in etched micromachined silicon devices. BAWs have previously been applied to channels for mixing in single channels, but this is the application in membrane devices. It is demonstrated that resonance is achieved with minimal influence of the pore configuration on the average lateral flow. In fact, the channels connected with porous walls under the acoustic streaming influence act as two separate channels rather than one channel, regarding the critical channel dimension. Additionally, the roughness of the microchannel walls plays a significant role in mixing. A roughened (black silicon) wall results in an increase in average streaming flow compared to a non-porous wall.
The last part of the thesis is related to the performance of the fabricated acoustofluidic devices. The effect of the temperature on the acoustofluidic setup was studied, and adding temperature control improved the performance of the system. A feedback temperature control system integrated into an acoustofluidic setup using bulk acoustic waves (BAWs) to elevate mass transfer and manipulation of particles is presented. The system performance was tested by measuring the mixing efficiency and determining the average velocity magnitude of acoustic streaming. The results showed that the integrated temperature control system kept the temperature at the set point even at high power and improved the setup performance reproducibility.
