Magnetic microparticles are often used in laboratory settings for the separation of biological material. These microparticles can also be useful in microfluidic settings because of their small size and the possibility of manipulating them with magnetic forces. Typical microfluidic flows have low Reynolds and Bond numbers, such that viscosity and interfacial tension dominate the dynamics of the system. However, body forces can become important when magnetic forces act on the magnetic microparticles.

This dissertation describes several examples where the magnitude of the body forces on magnetic microparticles are comparable to viscous and capillary forces. Specifically, four cases are described where magnetic forces either deflect individual particles across fluid streamlines and fluid-fluid interfaces, or assemble clusters of particles through a magnetic-capillary interaction.

The first example (Chapter 2) documents a systematic study of a particle’s deflection in a microchannel as a function of the particle size and susceptibility, the external magnet’s strength, and the fluid’s speed and viscosity. We describe experimental results of particles flowing in a single phase fluid. The results lead to the development of a theoretical model that captures the relationship between the individual variables and the particle’s deflection.

Chapter 3 describes a set of experiments where a second immiscible fluid phase is introduced to the microfluidic channel and the two fluids form a co-flow. Particles from the magnetic suspension are forced through the fluid-fluid interface and coated with a thin layer of the initial phase. The thickness of the coating layer is controlled by the magnitude of the flow speed. The number of particles captured in a single cluster is adjusted by varying the strength of the magnetic field or the liquid-liquid interfacial tension.

Chapter 4 combines the microfluidic co-flow experiments with a mathematical model of the magnetic force on a single particle. By deflecting the particles and observing their behavior at the fluid-fluid interface − the particles either pass through or are trapped − it is possible to obtain an estimate of the interfacial tension between the two fluids. The proposed measurement technique is useful for ultralow interfacial tensions, and an experiment is described where a range of interfacial tensions, O(10⁻⁶ − 10⁻⁵) N/m, are measured for an oil-water mixture with varying surfactant concentrations.

The final example (Chapter 5) describes the bulk deformation of a liquid-air interface by an aggregate of magnetic particles. As a permanent magnet is brought closer to the interface, the deformation grows. Above a threshold of magnetic force, the interface destabilizes and forms a jet. Mathematical models are developed that predict the shape of the deformation and the transition to a jet.