Oscillatory flow for contactless particle trapping

Although several contact-free trapping techniques exist in microfluidics - such as optical, acoustic, and dielectrophoretic - these approaches often face tradeoffs including: limited area, fine tuning, or special buffers. Here, we introduce the microfluidic oscillatory asymmetrical trap (MOAT), an oscillatory Reynolds number-dependent phenomenon that overcomes these limitations. The MOAT enables stable, contact-free trapping under a bias flow through pressure oscillations, but only in devices with streamwise geometric asymmetry. Our investigation of this phenomenon involves experiments conducted on a 3D-printed plane expansion chip and associated numerical simulations. We measure trapping efficiency and strength, propose a physical explanation, and outline a parameter space in which this phenomenon occurs. Notably, trapping behavior manifests across diverse devices and particle types, spanning from plastic beads to cell lines. Trapping efficiency is highest when particle streamlines intersect the trap regions, meaning that upstream pre-focusing-by e.g. inertial focusing, as employed here, enhances contactless trapping.