There has been increasing interest in the use of magnetic fluids to manipulate diamagnetic particles in microfluidic devices. Focusing particles (both biological and synthetic) into a single tight stream is usually a necessary upstream operation in numerous microfluidic applications. Current methods for diamagnetic-particle focusing in magnetic fluids require either a pair of repulsive magnets or a diamagnetic sheath flow, which can work efficiently for very small particles by simply increasing the flow-rate ratio between the sheath fluid and particle suspension. This approach, however, becomes difficult to implement if particles need to be focused in both the horizontal and vertical directions for so-called three dimensional focusing. Therefore, a variety of sheathless particle-focusing approaches have been developed in microfluidic devices, which are classified as either passive or active depending on the source of the involving force. We demonstrate herein a passive, tunable, sheathless focusing of diamagnetic particles in a microchannel ferrofluid flow with a single set of overhead permanent magnets. Particles are focused into a single stream near the bottom wall of a straight rectangular microchannel, where a magnetic-field minimum is formed because of the magnetization of the ferrofluid. This focusing can be readily switched off and on by removing and replacing the permanent magnets. More importantly, the particle-focusing position can be tuned by shifting the magnets with respect to the microchannel. We perform a systematic experimental study of the parametric effects of the fluid-particle-channel system on diamagnetic particle focusing in terms of a defined particle-focusing effectiveness.
Li, Di and Xuan, Xiangchun, "Particle focusing and separation in xanthan gum solutions" (2019). Graduate Research and Discovery Symposium (GRADS). 249.