Pressure sub-node formation by interparticle acoustic radiation forces

Microscale manipulation is essential for advancing research in biophysics and biomedical engineering, with acoustic tweezers emerging as a powerful tool for non-contact and label-free particle manipulation. Despite significant progress in understanding acoustic forces on individual objects, the collective behavior of particles in confined acoustic fields remains insufficiently explored. In this study, we experimentally investigate the acoustic trapping forces acting on microscale particles (2–5 μm) commonly used for cellular and organelle micromanipulation. Using a custom-designed acoustic device, featuring an aluminum mold and opposing piezoelectric transducers, we generate a standing wave field to analyze particle trapping dynamics. Our results reveal that, as particle concentration increases, interparticle interactions critically influence pressure nodal patterning, leading to the emergence of previously unreported “pressure sub-nodes” parallel to the primary nodal planes. This novel finding challenges conventional assumptions of acoustic trapping by demonstrating that equilibrium particle configurations are not only dictated by external acoustic pressure gradients but also by particle-induced secondary forces. We provide a theoretical and experimental analysis of these forces, offering new insights into the fundamental mechanisms governing acoustic manipulation. By elucidating the interplay between acoustic radiation and interparticle forces, our work advances the understanding of acoustic manipulation, highlighting the impact of acoustic-mediated forces on node patterning, and paving the way for enhanced control in biophysical and biomedical applications.