Locomotion of magnetic objects in fluids

Shubham K. Bhat

doi:10.17918/etd-2864

Manipulation of magnetic objects in fluids is a promising technology for many applications including targeted drug delivery, sorting and analysis of biological objects. Most prior work employed magnetic field gradients achieving limited control over assembly and manipulation of magnetic and non-magnetic micro-particles. However, many important questions related to magnetic manipulation technology remain open. One such issue is the controllability of positioning multiple objects during manipulation. Positioning controllability of even the simplest system such as a pair of spherical beads seems to have been largely ignored. The main goal of this thesis is to begin addressing the question of controllability of multiple magnetic objects during their manipulation. Although the ultimate applications may involve many multiple objects, only two spherical linearly magnetizable beads are considered in this thesis in order to identify important issues and potential problems. As a first step, the manipulation problem is formulated using only magnetic interactions between the beads and their interactions with a simple source of external field gradient, while neglecting hydrodynamic interactions between the beads. Newtonian behavior of the fluid is assumed and effects of boundaries are also ignored. Such a model is typical in magnetic separation literature. It is demonstrated theoretically that positions of the beads are locally controllable (in a linearized system) by using uniform field as the input only when the beads are sufficiently close to each other. This analysis clearly demonstrates that magnetic interactions between the different objects is not necessarily a nuisance, but could in fact help to control the system. It also reveals that controllability is possible only in close proximity to a source of gradient. Placing the beads in close proximity to a source of gradient is not possible in many applications such as magnetically guided drug delivery. To circumvent difficulties with the gradients, the possibility of locomotion of the bead-pair system using only uniform magnetic field as the input is considered. The advantage of such a strategy is that the external magnetic field delivers energy for the movements of the beads but no net force. The inspiration for this comes from the motility of living organisms such as leukocytes and bacteria. It is known from previous work on swimming of living and artificial swimmers that hydrodynamic interactions play a critical role. For this reason, movement of beads subject to both, magnetic and hydrodynamic interactions is considered in the second part of this thesis. Using a simple model of hydrodynamic interactions, it is demonstrated theoretically that the beads can move as a pair in any desired direction and their final positions are completely controllable using only uniform magnetic field. Subsequently, the thesis focuses on experimental demonstrations of locomotion of a pair of beads using two different types of experimental set-ups. In one, the magnetic beads are suspended in a non-magnetic fluid using thin threads. In the other, a pair of non-magnetic beads is suspended in a magnetic fluid (ferrofluid). Locomotion is demonstrated experimentally in both test-beds. In fact, it is also shown that in the second system, where the beads are suspended magnetically, the direction and magnitude of locomotion agrees quantitatively with the proposed theoretical model. Thus, the main novel and useful contribution of this thesis is that it demonstrates using both, theoretical analysis and experimental validation, that positions of beads in a pair can be controlled to a significant extent using only uniform magnetic fields. On the one hand, this suggests future strategies by which positioning of more than two objects could possibly be magnetically guided and controlled. On the other, the work carried out in this thesis is likely to find direct applications even if it proves difficult to control more than two objects. One of the most exciting of such applications involves movements of two objects inside the body for the purpose of delivery of drugs, therapies, minimally invasive surgeries and others. Although the work carried out in this thesis is only the beginning for such medical applications, the results obtained so far offer a significant hope of success.

Locomotion of magnetic objects in fluids

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Locomotion of magnetic objects in fluids

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