Using Magnetic Fluids as a Versatile Method for Manipulating and Sorting Unlabeled Nonmagnetic Particles in a Flow

Sorting and detection technologies have become an important part of industrial and medical practice. Recently, innovation in lab-on-a-chip technologies promises smaller, less expensive, and more portable devices for these applications. Labeling efficiency, specificity and throughput are challenges that must be overcome in developing such technologies. We introduce a continuous-flow magnetic flow focusing, sorting and detection scheme for unlabeled particles on the size order of cells. Unlabeled particles are focused and sorted by size in the apparatus using the magnetophoretic force in a specially crafted high-gradient magnetic field. The magnetic scheme is orthogonal to other sorting techniques, allowing other physical properties to be explored. We demonstrate using the light pressure force from a laser to actively sort a focused stream of flowing particles and use the balance between the light pressure force and the magnetic force as an additional physical axis on which particles can be sorted. We show that the positions and distribution of the particles conform to their theoretical expectations, and use the theory to explore the limitations of this technology in practice.