Magneto-capillary dynamics of particles at curved liquid interfaces

The ability to manipulate colloidal particles with magnetic fields has profound applications both in industry and academic research ranging from automobile shock absorbers to robotic micro-surgery. Many of these applications use field gradients to generate forces on magnetic objects. Such methods are limited by the complexity of the required fields and by the magnitude of the forces generated. Spatially uniform fields only apply torques, but no forces, on magnetic particles. However, by coupling the particles' orientation and location, even static uniform fields can drive particle motion. We demonstrate this idea using particles adsorbed at curved liquid interfaces. We first review the intersection between active colloidal particles and (passive) particles at the fluid-fluid interface (chapter 1), followed by the introduction of magnetism, magnetic manipulation, and magnetic Janus particle fabrication techniques (chapter 2). In chapter 3, we use magnetic Janus particles with amphiphilic surface chemistry adsorbed at the spherical interface of water drop in decane as a model system to study particle response to a uniform field. Owing to capillary constraints, Janus particles adsorbed at curved interfaces will move in a uniform magnetic field to align their magnetic moment parallel to the applied field. This phenomenon is labeled as the magneto-capillary effect in this thesis. As explained quantitatively by a simple model, the effective magnetic force on the particle induced by static uniform field scales linearly with the curvature of the interface. For particles adsorbed on small droplets such as those found in emulsions, these magneto-capillary forces can far exceed those due to magnetic field gradients in both magnitude and range. The time-varying fields induce more complex particle motions that persist as long as the field is applied (chapter 4). Depending on the angle and frequency of a precessing field, particles orbit the drop poles or zig-zag around the drop equator. Magneto-capillary effects are not limited to Janus particles. Similar behaviors are observed in commercially available carbonyl iron particles. Periodic particle motion at the liquid interface can drive fluid flows inside the droplets, which may be useful for enhancing mass transport in droplet micro-reactors. The magneto-capillary effect at curved liquid interfaces offers new capabilities in magnetic manipulation: even static uniform fields can propel magnetic particles and the use of time-varying fields leads to steady particle motions of increasing complexity. These experimental demonstrations and the quantitative models that accompany them should both inspire and enable continued innovations in the use of magnetic fields to drive active processes in colloid and interface science. The final chapter highlights some specific directions for future work in this area.