A Microfluidic Quadrupole (IMAGE) McGill University Caption The quadrupole is formed by injecting fluids from two source apertures (the pluses) and aspirating back in two sink apertures (the minuses). Fluorescent beads are used to trace the path of the flow much as iron filings can trace the path of the magnetic field around a magnet. Credit McGill University Usage Restrictions None License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.