Apparent size correlation: A simple method to determine vertical positions of particles using conventional microscopy

This paper reports a simple and widely applicable particle tracking technique using conventional fluorescence microscopy to determine vertical positions of particles suspended in a fluid. Previous experimental work in this area has been restricted to two-dimensional intensity profiles or confocal and stereoscopic setups for three-dimensional tracking. Using a calibration-based defocusing method, our technique has been experimentally verified and compared with previously observed trends in inertial flow focusing. The technique has proven to be a simple and valuable tool for determining locations of micron-sized particles in microscale flows.