In the first part of the project, bulk acoustic waves (BAWs) were generated using a silicon-based chip. During the development of the BAW setup, several disadvantages became apparent. Based on the typical streaming pattern with four vortices, the BAWs can only enhance mass transport through the sensor from half the height of the flow cell, thereby limiting the system’s overall improvement. Silicon shows several disadvantages (expensive, hard sealing, etc.). Alternatively, Eckart streaming relies on the dissipation of acoustic energy within the bulk of the liquid. This concept was tested, and particle mixing was observed. However, the process faced challenges related to limited reproducibility and efficiency.
Sharp-edge streaming was the next pursued approach. With a well-designed geometry, the entire liquid flowing above the sensor can be actively mixed. It is not frequency and dimension dependent, it is cheap and easy for reversible sealing.
High-power acoustic streaming significantly enhances mass transfer, but requires active cooling because of the PZT temperature rising. We integrated a closed-loop temperature control system into the acoustofluidic setup. A thermocouple interfaced with an Arduino microcontroller regulated a Peltier module, maintaining the system at target temperature. The system could avoid a temperature rise when a voltage up to 150 Vpp was applied. If needed a more powerful Peltier element can be used to improve this performance further.
Alternative designs were used to enhance mixing. Transparent chips, glass rectangles were used to visualize and analyze acoustic streaming.
In some designs streaming vortices could be generated across a wide range of frequencies, and the operating regime had limited influence (single vs sweep).
DNA bonding was used as a test mechanism and enhance with acoustic mixing. Acoustic waves were generated up to 65 V. A positive impact was observed during the experiments.
In theory, enhancing mass transport can only improve diffusion-limited systems. Whether a system is diffusion-limited depends on its kinetics: if diffusion is faster than reaction, system is reaction-limited. Conversely, the system is diffusion-limited.
In the tested system, we hypothesize that DNA diffusion was in given cases faster than its reaction. Even though the setup could not be consistently tested on a diffusion-limited reaction, the potential of the technology was clearly demonstrated. The ability to actively mix liquids for sensing enhancement was developed, and the chip geometry was optimized.
Based on these findings, the final phase of the project focused on identifying application domains where the technology offers a clear advantage, specifically, systems known to operate in a diffusion-limited regime. This project led to several (at this stage confidential) follow-up proposals targeting high impact fields wherein detection and enhanced mass transport is critical.
