Numerical investigation of two-stage acoustofluidic separation based on acoustic contrast factor differences

In the field of micro-particle separation, acoustic separation technology is widely applied in areas such as fluid detection and biomedicine due to its non-contact and nondestructive nature. Existing methods predominantly rely on size differences, making it difficult to effectively separate particles with diverse acoustic properties and similar sizes, particularly due to the lack of systematic utilization of both the sign and magnitude of the acoustic contrast factor. To address this, we propose a two-stage sorting chip based on surface acoustic waves (SAW) and investigate its performance through numerical simulations using the finite element method. By integrating a standing surface acoustic wave (SSAW) field in a rectangular microchannel and tilted-angle standing surface acoustic wave (TaSSAW) field in a tapered microchannel, and by combining the sign and magnitude information of the acoustic contrast factor, the chip design achieves efficient and continuous separation of polycaprolactone (PCL), polystyrene (PS), and polymethyl methacrylate (PMMA) on a single device. Theoretically, the trajectory slope is derived from particle force balance as a key parameter, which successfully characterizes the longitudinal displacement and reveals its quantitative relationships: non-monotonic dependence on the tilt angle, negative correlation with flow velocity, and positive correlation with the acoustic radiation force. The simulation results show that the scheme enables pre-separation of PCL and achieves optimal separation of PS and PMMA at a tilt angle of 10°. Furthermore, the study clarifies the coordinated regulation between flow velocity and acoustic pressure (e.g., 6 mm/s corresponds to 168 kPa). Under these optimized conditions, a maximum separation distance of approximately 600 μm is predicted. This computational study provides a new approach for multi-target sorting of same-sized particles in acoustofluidics and offers theoretical guidance for future experimental implementations.