Magnetic Programmable Polymer-Nanoparticle Composite for Microstructure Actuation

Abstract

In this dissertation, I introduce a new magnetic nanocomposite material system and in situ fabrication process that is not shape limited and allows the programming of heterogeneous magnetic anisotropy at the microscale. The key idea is to combine the self-assembling behavior of superparamagnetic nanoparticles, which have stronger magnetization than that of general paramagnetic materials, with a spatially modulated photopatterning process. By repetitively tuning the nanoparticle assembly and fixing the assembled state using photopolymerization, I fabricate microactuators for which all parts move in different directions under a homogeneous magnetic field. To show the feasibility of this concept, I demonstrate polymeric nanocomposite actuators capable of two dimensional and three-dimensional complex actuations that have rarely been achieved using conventional microactuators. This approach greatly simplifies the manufacturing process and also offers effective rules for designing novel and complex microcomponents using a nanocomposite material with engineered magnetic anisotropy.

First, I investigate the self-assembling behavior of both ferromagnetic magnetite nanoparticles and superparamagnetic nanoparticles using Monte Carlo simulation. Magnetic materials used to fabricate magnetic polymer composite include ferrimagnetic magnetite nanoparticles with 50nm of averaged diameter and superparamagnetic magnetite nanoparticles with 280nm of averaged diameter. Magnetic particle interactions, that critically affect to the self-assembling behavior of the magnetic nanoparticles, such as particle-field interaction, particle-particle dipole interaction, magnetic anisotropy and steric layer repulsion are considered. I adopt cluster-moving Monte Carlo simulation method to analyze the magnetic self-assembly of magnetic nanoparticles and investigate the self-assembling behavior of the magnetite nanoparticles varying the intensity of the applied magnetic field during the chain formation and the concentration of the magnetic nanoparticles. The result shows that the well-defined magnetic chains are formed as both the intensity of the applied magnetic field and the magnetic nanoparticle concentration increase.

Also, a novel method to fabricate magnetic nanoparticle embedded polymer composite microstructure is introduced. Briefly, the combination of photocurable polymer and magnetic nanoparticles is photopolymerized to immobilize the various states of magnetic nanoparticles. I especially adopt a system called optofluidic maskless lithography system to fabricate various shapes of polymeric microstructures within a second. Also, I develop a model system to describe the actuation of a magnetic polymer composite. The magnetic torque, the derivative of system energy, of the composite microstructure embedding magnetic chains is calculated based on the expanded Monte Carlo simulation result. And, the steady state elastic modulus of the magnetic composite microbeam is induced by utilizing the simulated torque and cantilever bending experiment result. The movement of cantilever type microstructure is investigated at equilibrium state that the magnetic torque equals to the mechanical restoring torque.

As an application, I demonstrate multiaxial microactuators. Polymeric microcomponents are widely used in microelectromechanicalsystems (MEMS) and lab-on-a-chip devices, but they suffer from the lack of complex motion, effective addressability and precise shape control. To address these needs, I fabricated polymeric nanocomposite microactuators driven by programmable heterogeneous magnetic anisotropy. Spatially modulated photopatterning was applied in a shape independent manner to microactuator components by successive confinement of self-assembled magnetic nanoparticles in a fixed polymer matrix. By freely programming the rotational axis of each component, I demonstrate that the polymeric microactuators can undergo predesigned, complex two- and three dimensional motion.

Finally I also introduce a novel color changing microactuators based on the self-assembling behavior of the magnetic nanoparticles. I propose a color-tunable microactuator utilizing the optical and magnetic behaviors of one-dimensionally assembled superparamagnetic nanoparticles that play the role of a one-dimensional Bragg reflector and establish a magnetic easy axis. By combining these properties with rapid photopolymerization, I developed red, blue, and green micropixels whose colors could be tuned by the application of an external magnetic field. This strategy offers very simple methods for the fabrication and operation of soft color tunable surfaces with high resolution.