Description
The analysis of biological and environmental samples is of fundamental importance for human health and safety. In particular, the ability to extract target-sized particles from complex suspensions is highly relevant across several fields, ranging from environmental monitoring to biomedical applications (Caragnano et al., 2025). Especially when only limited sample volumes are available, the development of efficient microfluidic particle sorting techniques becomes a key requirement. Among the available techniques, surface acoustic wave (SAW)–based acoustophoresis represents an attractive solution due to its label-free and contactless approach (Friend & Yeo, 2011; Lenshof et al., 2012). One metric for quantifying the efficiency of these devices is the ability to accurately predict particle trajectories. In SAW-based microfluidic devices, this task remains challenging, as particle motion results from the interplay between acoustic radiation forces and hydrodynamic drag, and depends on multiple strongly coupled parameters, including particle size and material properties, interaction with the acoustic field, flow conditions, and device geometry (Bruus, 2012).
In this work, we present a fully coupled three-dimensional multiphysics numerical model for the prediction of particle trajectories in standing SAW (SSAW) microfluidic devices. The model sequentially couples piezoelectric actuation of the substrate, acoustic wave propagation in the fluid, steady-state laminar flow, and time-domain particle tracing under the combined action of acoustic radiation and drag forces. This approach enables a quantitative description of particle motion under realistic operating conditions, overcoming the limitations of simplified analytical and two-dimensional models commonly adopted in the literature (Fakhfouri, Devendran, Ahmed, et al., 2018; Fakhfouri, Devendran, Albrecht, et al., 2018; Guo et al., 2015). Numerical simulations are performed for polystyrene (PS) particles with diameters of 6 and 20 μm in distilled water, selected to match the characteristic size range of blood cells in perspective of future blood-sorting applications. Simulations are carried out at discrete operating conditions, and are used to identify the operating point yielding the most pronounced lateral particle deflection within the explored conditions.
Experimental measurements are conducted using fluorescent PS microspheres in a SSAW-based microfluidic device operated at its resonance frequency. Particle trajectories are extracted from fluorescence microscopy images, and the lateral deflection is quantified with respect to a reference trajectory obtained in the absence of SSAW excitation. In our operating conditions, simulated and measured lateral deflections are found to be in good agreement, with relative discrepancies on the order of 5–10%, compatible with the experimental uncertainty associated with trajectory extraction and averaging.