Optical monitoring of low-field magnetophoretic separation of particles

We describe the design and testing of a low-cost optical sensor for monitoring separation of magnetic micro and nanoparticles in low magnetic field gradients. The sample is placed in a vertically aligned test tube, where the magnetic field gradient and the gravitational field are both in the same direction, such that the magnetophoretic velocity can be calibrated against a known value. We describe methods for extracting the magnetophoretic velocity using two different theoretical approaches, one based on linear interpolation and another based on fitting to a physical theory implementing a time-dependent version of Beer's law. We discuss strengths and weaknesses of the two approaches. We show that the sensor can be used to detect particle aggregation in polymer solutions. We also show that the magnetic field gradient may alter the size distribution of the aggregates, which is detected as larger intensity fluctuations and nonlinear response.