Rapid microfluidic acoustic sorting of Microplastics in synthetic seawater: A Design and Simulation Study

The increasing prevalence of microplastics in marine environments necessitates advanced separation technologies capable of operating in realistic aqueous matrices such as seawater. In this study, a surface acoustic wave (SAW) based acoustofluidic microdevice is numerically designed and investigated for the selective manipulation and separation of polyethylene (PE), polypropylene (PP) and Nylon-6 (N6) microplastics suspended in synthetic seawater. The device employs a lithium niobate (LiNbO₃) substrate integrated with gold interdigitated transducers (IDTs) to generate standing surface acoustic waves over excitation frequencies ranging from 0.6 to 2 MHz. Finite element simulations using COMSOL Multiphysics® resolve the coupled electromechanical and acoustofluidic interactions enabling detailed analysis of acoustic pressure fields, substrate displacement, electric potential distribution, acoustic streaming and time-resolved particle trajectories. The results reveal pronounced frequency-dependent acoustophoretic behavior governed by particle size, density, compressibility and acoustic contrast factor relative to synthetic seawater. Among the investigated microplastics, PP exhibits the largest net lateral displacement and strongest nodal focusing followed by PE while N6 shows the weakest response due to its smaller size and reduced acoustic radiation force. Time-resolved trajectory analysis identifies 1 MHz as the optimal operating frequency providing the most effective balance between acoustic radiation forces and hydrodynamic drag and yielding maximum inter-stream lateral separation. At this frequency, all binary combinations (PE-PP, PE-N6 and PP-N6) demonstrate stable spatial divergence and distinct outlet partitioning without trajectory overlap. Higher frequencies induce excessive nodal confinement leading to reduced separation resolution. These findings establish frequency-dependent design criteria for SAW-driven microplastic separation in synthetic seawater and demonstrate that moderate-frequency acoustofluidic operation of 1 MHz enables label-free and contactless separation of environmentally relevant microplastic mixtures.