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Model of Detecting Nonmagnetic Cavities in Ferrofluid for Biological Sensing Applications

We propose a novel method for detecting large bioparticles (e.g., viruses, cells, bacteria) using solid-state magnetic field sensors and utilizing magnetic fluid (e.g., ferrofluid) to provide magnetic susceptibility contrast for the nonmagnetic bioparticle. The proposed sensing approach measures the volume of the bioparticle directly for known particle/substrate separation, effectively overcoming deficiencies with conventional magnetic tagging. An exact analytical solution was developed in bispherical coordinates for the bioparticle's field near the interface, and we compare this solution both with the first-order approximation of a dipole and its induced image, and with a numerical finite-element solution. The results indicate that the dipole-image model works well for most of the particle/substrate separations considered, however, for very small separation or large fluid permeability the dipole-image model can deviate from the exact solution