A study of ferrohydrodynamics under traveling magnetic field excitations

In this thesis, a new ferrofluid pumping scheme based on traveling magnetic fields is studied. The modeling and the simulation of the ferrofluid pumping under sinusoidally time-varying and spatially traveling magnetic fields are presented. A second order finite difference scheme in both Cartesian and Cylindrical coordinates is used to calculate the flow and spin velocities of the ferrofluid in a one-dimensional case. The two-dimensional case is simulated by using a commercial multi-physics package COMSOL. It is shown theoretically that, under traveling magnetic field excitation, maximum flow velocity is achieved when the product of the excitation wave number and the height of the ferrofluid channel (Cartesian case) or the radius of the ferrofluid channel (Cylindrical case) is unity. Once geometric dimensions are chosen, maximum pumping is achieved when the excitation frequency is the reciprocal of the overall relaxation time constant of the magnetic nanoparticles. In order to verify the theory, macro-scale experiments of this ferrofluid actuation scheme are conducted. The results show that oil-based EFH1 ferrofluids can be pumped up to 7.4 mm/sec under 9 kA/m magnetic fields with a maximum pressure drop of 4.7 Pa. Furthermore, micro-scale experimental study of this ferrofluid actuation scheme and a pathogen detection scheme based on the ferrofluid actuation are also presented. Stopped flow pressure measurements show very good agreement between the simulation and the experimental data. A prototype of the pathogen sensor based on ferrohydrodynamics is fabricated and tested. In the initial experiments, avidin-coated microbeads (3.6 μm in diameter) are used to emulate the geometry of the pathogen cells. The result shows that as few as 66400 microbeads/ml concentration can be detected within minutes. In the end, a new microfluidic mixing concept based on ferrohydrodynamic actuation is demonstrated. The mixer shows 8-12 fold improvement in mixing effectiveness over the diffusive mixing