Directed self-assembly in toggled magnetic fields

Furst, Eric M.Directed self-assembly is a promising route to nanomanufacture materials with advanced phononic, photonic, mechanical and/or catalytic properties. In directed self- assembly, colloidal matter (proteins, polymers, nanoparticles, etc.) spontaneously form ordered structures either directly through typical colloidal forces and/or indirectly via an external field, such as a macroscopic flow or electromagnetic field. The external field modifies the interactions between constituent particles, making it possible to cre- ate structures and phases that are otherwise unobtainable in the absence of the field. If the pairwise interaction potential is too strong or applied too quickly, kinetic bot- tlenecks can halt the structural transitions in glassy, non-equilbrium states. In this thesis, we demonstrate the ability to circumvent arrested states by using suspensions of superparamagentic particles exposed to toggled magnetic fields, i.e. potentials that are cycled on and off at a fixed frequency and duty cycle. In the presence of a magnetic field, the suspension forms a percolated network, which is a kinetically arrested state. In a toggled magnetic field, the network coarsens and detaches from the edges of the sample cell and self-assembles into 250 mm sized domains, which is the predicted structure when the energetics of the suspension are minimized. We identify the optimal toggle frequency = 0.66 Hz, for self-assembling aggregates in the minimum amount of time, on the order of 500 seconds. Deviations to higher or lower frequencies cause the self-assembly time to increase exponentially. Even though the network is composed of solid magnetic particles, it deforms like a fluid over time scales much longer than the toggle frequency. Modulating the toggle frequency changes the effective viscosity of the network from a viscous, Newtonian fluid for frequency = 0.66 - 1.5 Hz to a Bingham fluid for frequency greater than 2 Hz. The deformation of the network proceeds via a Rayleigh-Plateau instability that has a characteristic wavelength. We show that it is possible to suppress the instability and control the size and spacing of the domains by confining the suspension on length scales commensurate with the instability wavelength. In addition to characterizing the deformation and evolution of the percolated network, we also study the microstructure using optical microscopy and small-angle light scattering. For the suspension to evolve into its thermodynamically predicted state, a body-centered tetragonal crystal, it is necessary to properly balance the time scale of the toggle frequency and particle relaxation during the field-off half-period of the toggle cycle. If this balance is not met, the structure remains pinned in a non- equilibrium state. The results of this thesis broaden the pathways for self-assembling thermody- namically predicted structures. Directed self-assembly with toggled fields modulate particle-particle interactions via an external field instead of fine-tuning the interactions, which often lead to kinetically arrested states. Overall, the toggled field approach pro- vides an exciting opportunity to tailor the properties of advanced materials by building ordered structures.University of Delaware, Department of Chemical and Biomolecular EngineeringPh.D