Acoustofluidics for Micro and Nanoplastics Enrichment towards Environmental and Drinking Water Monitoring : A Story of Sound and Soul

Plastics and their derivatives have forever changed the nature of human activity. Their various material advantages, such as versatility, low cost, and ease of fabrication, have made them essential for virtually all industries. While their resistance to degradation is often considered one of their greatest traits, it has also been found to be a significant danger. As proper disposal methods fail, large plastics slowly degrade into micro and nanoplastics. These contaminants are extremely small, hard to detect, and challenging to remove from both the environment and consumer goods. This leads to their accumulation in all ecosystems and in the human body, raising questions for policymakers and society at large about how dangerous these contaminants actually are. For this reason, novel monitoring solutions must be developed to enable micro and nanoplastic sample preparation and identification, so that the lifecycle of plastics can be studied and their impact on human life can be better understood. To address the challenge of managing contaminants at these size ranges, acoustofluidics – a fusion of acoustic actuation and microfluidics – is a promising candidate due to its ability to manipulate particles even during flow. The work presented in this thesis focuses on developing an acoustofluidic platform, termed the EchoGrid, capable of trapping micro and nanoplastics at throughputs traditionally considered unattainable for both microfluidics and acoustofluidics, in a way that is compatible with endpoint analysis. In Paper I, we presented the EchoGrid as a novel device capable of enriching microplastics at high flow rates. Additionally, we developed the silica-enhanced seed particle method to address samples with low concentrations of microplastics, while further increasing the flow rate. We also evaluated the complex manner in which microplastics of different sizes organize themselves around a silica cluster. Status: Accepted. Reference: Costa, M., Hammarström, B., Van Der Geer, L., Tanriverdi, S., Joensson, H. N., Wiklund, M., & Russom, A. (2024). Echogrid: High-throughput acoustic trapping for enrichment of environmental microplastics. Analytical Chemistry, 96(23), 9493–9502. In Paper II, we reported on the EchoTilt, a microfluidic method using the EchoGrid to maximize nanoplastic capture by manipulating the way the acoustic field interacts with the sample flow lines. This was achieved by altering the angle at which the transducer was integrated with the microchannel, an angle determined through the use of simulation and algorithms. Here, we also demonstrated that the silica-enhanced seed particle method can capture very small nanoplastics, as small as 25 nm, at high throughput. Status: Submitted to Micromachines. In Paper III, we presented a study using fluorescence imaging and Raman spectroscopy to examine the geometry and internal structure of the acoustic clusters levitated by the EchoGrid, investigating how microparticles of different sizes organize themselves around and within the silica clusters. We also successfully detected different types of plastic in the same acoustic cluster, paving the way for handling complex, real-life samples. Status: Manuscript In Paper IV, we employed an elasto-inertial microchannel for size-based separation using a non-Newtonian fluid, achieving highly selective microparticle focusing relevant to environmental and biomedical applications. This comprehensive study evaluated the impact of particle size, flow rate, viscoelasticity, and channel dimensions on the ultimate focusing positions of the particles. Status: Accepted. Reference: Tanriverdi, S., Cruz, J., Habibi, S., Amini, K., Costa, M., Lundell, F., Mårtensson, G., Brandt, L., Tammisola, O., & Russom, A. (2024). Elasto-inertial focusing and particle migration in high aspect ratio microchannels for high-throughput separation. Microsystems & Nanoengineering, 10(1), 87. In Paper V, we extended elasto-inertial separation to nanoparticles, highlighting how size affects focusing quality when combined with different concentrations of viscoelastic fluid. Additionally, we successfully achieved focusing of biomedical nanoparticles essential for medical diagnostics at size ranges not seen before in microfluidics. Status: Manuscript