The MT apparatus in this study was built on an inverted microscope (Olympus, IX73) similar to previously reported high-resolution MT setups (56 (link)–58 (link)). A pair of magnets (vertically aligned in opposite directions with a 1-mm gap) was placed above the stage holding a flow cell, and its vertical position and rotation were controlled by a translation stage (Physik Instrumente, M126) and a stepper motor (Autonics, A3K-S545W), respectively. The magnet axis was confirmed to be aligned to the imaging axis (within 1°) by following the motion of a free magnetic bead. Beads in a flow cell were illuminated by a red superluminescent diode (QPhotonics, QSDM-680-2) and imaged by a 100× oil-immersion objective (Olympus, UPlanSApo NA 1.40) and a high-speed complementary metal-oxide semiconductor (CMOS) camera (Mikrotron, EoSens MC-3082) grabbing 512 × 512 images at 4 kHz. The objective position was controlled by a piezo-controlled nanopositioner (Mad City Labs, Nano-F100S) to calibrate distances and to correct for drift. The images were recorded by a custom software written in LabVIEW (National Instruments), and the coordinates of beads were tracked in real time at up to 1.2 kHz. Unless necessary, measurements were performed at 100 Hz to reduce file size.