Micro magnetofluidic phenomena and applications

• Main Author: Zhu, Guiping
• Other Authors: Nguyen Nam-Trung
• Format: Theses and Dissertations
• Language: English
• Published: 2014
• Subjects: DRNTU::Engineering::Mechanical engineering::Fluid mechanics
• Online Access: https://hdl.handle.net/10356/59220
• Institution: Nanyang Technological University

This thesis presents studies on different applications of magnetic force in microfluidic devices, including the utilization of magnetism in digital and continuous microfluidics. In the study on digital microfluidics, magnetism of paramagnetic solutions were used for adjusting wetting property and dynamics of liquid on a solid surface. Experiments were carried out to investigate droplet deformation under the effect of an external uniform magnetic field. The magnetic field was applied parallel to the holding surface. Droplets with different sizes were used. Droplet geometry variation was analyzed as functions of flux density and magnetization. A numerical model was developed for predicting the deformation of ferrofluid droplet undergoing a linear magnetization in a uniform magnetic field. The discrepancy between experimental and numerical results offered a qualitative estimation of droplet behaviour. In addition, experiments were carried out with ferrofluid droplets in a permanent magnetic field. The wettability properties were investigated through contact angle variations under different magnetic field strength. The experiments were conducted using droplets with five different base diameters, and ten different magnet moving velocities ranging from 0.2 mm/s to 2 mm/s. Two basic operating regimes, sliding and disengagement, were observed. A scaling analysis was established to describe the operation regions of the sliding droplets. The critical magnet moving velocities for droplet sliding were obtained from the scaling analysis. The investigation of magnetowetting effect and magnetic manipulation of ferrofluid droplets in particular or magnetic droplets in general may have potential impacts on droplet-based microfluidics interface control of liquid lenses, and possibly e-paper technology. In the continuous microfluidics, the study was focused on investigating the interaction between a uniform magnetic field and a magnetic fluid in a microfluidic configuration. Firstly, the magnetism of paramagnetic component in microchannels was employed for spreading and transport of magnetic nanopartilcles. The experiments were carried out with two miscible fluids: mineral oil and an oil-based ferrofluid. The ferrofluid consisted of superparamagnetic nanoparticles suspended in an oil-based carrier. Under a uniform magnetic field, the superparamagnetic particles were polarized and represented magnetic dipoles. The magnetization of the magnetic nanoparticles led to a force resulting in the change of diffusion behaviour inside the microchannel. Mixing due to secondary flow close to the interface also contributed to the spreading of the ferrofluid. The magnetic force acting on the liquid/liquid interface was induced by the mismatch of magnetization between the nanoparticles and surrounding liquid in a multiphase flow system. In the investigation of magnetically-induced spreading of magnetic nanoparticles, the stream width of ferrofluid was analyzed in terms of the applied magnetic flux intensity. The flow rate ratio was varied to test the spreading performance. In addition, a numerical simulation was carried out to verify the experimental data, taking into account the effect of particles on the flow field. The mass, momentum and diffusion equations were solved by the Finite Volume Method (FVM). These phenomena would allow simple wireless controls of a microfluidic system without changing the flow rates. These phenomenon can potentially be used for focusing and sorting in cytometry. For better understanding of the magnetic nanoparticles motion under an external magnetic field, investigation was extended to ferrofluids in circular chamber with higher magnetic flux density. The flow system consisted of a water-based ferrofluid and a mixture of DI water and glycerol. The circular chamber offered a better visualization than a conventional rectangular channel due to the opaque of ferrofluid itself. The magnetic field generated a magnetic force on individual particle and thus an additional velocity. Furthermore, the mismatch in magnetization of the fluids led to instability at the interface and subsequent rapid mixing under a uniform magnetic field. The mismatch of magnetization was determined by concentration of magnetic nanoparticles. The performance of the device was analyzed based on the distribution of the magnetic nanoparticles in an interrogation window. The numerical model showed a similar trend of increasing mixing efficiency in terms of the increasing magnetic flux intensity. Almost instant mixing with an efficiency of 90% was achieved at a relatively low magnetic flux density up to 10 mT. The mixing concept demonstrated here potentially provides a wireless solution for a lab-on-a-chip system that is low-cost, robust, free of induced heat, and independent of pH level or ion concentration.