A collection device for various-sized microparticles that uses four serial acoustic separations: Working toward microplastic emission prevention

Abstract

Small plastic debris particles less than 5 mm in size called microplastics (MPs) are an emerging global ecological issue. This study developed a high enrichment microfluidic device to collect various-sized microparticles that uses four serial acoustic separations. To adopt the device for use with up to 200-µm MPs, the microchannel was widened by lowering the excitation frequency. The microfluidic device was designed for a 3.2-fold enrichment at each junction and a total 105-fold enrichment at the four junctions. The microfluidic network of the device was designed on the basis of a hydraulic-electric analogy and it worked even though only a single pump was used without multiple precise flow controllers. The collection performance of the device was evaluated based on the total collection rate that in turn was based on microscopic observations at the four junctions and the actual collection rate and the actual enrichment ratio obtained by measuring the effluents from the outlets. First, the device was evaluated separately using microparticles of 5, 10, 15, 25, 50, and 200 µm in diameter. The total collection rates were over 90% except for the 5-µm microparticles which seemed to be too small to manipulate acoustically at the lowered frequency. Finally, the device was evaluated with two example mixtures representing small MPs ranging from 200 µm down to 25 µm and very small MPs ranging from 25 µm down to 10 µm. Both the total collection rates and the actual collection rates ranged from 70% to 90%. However, the actual enrichment ratios ranged from half the designed value of 105 to the design value itself as some microparticles were slowed down and a few seemed to become trapped and attached to the microchannel walls by acoustic radiation force. Therefore, the microfluidic device was judged to be applicable to MP removal applications after prefiltration through a coarse mesh, while the MP analysis applications were judged to require improvement to inhibit the microparticle attachment used in 2D focusing. In conclusion, the serial acoustic separations could be a promising approach to highly enrich and remove various-sized MPs from environmental samples.