Assembly and propulsion of colloidal particles under combined electric and magnetic fields

Abstract

Colloids are reminiscent of atoms and molecules with larger dimensions. Under external fields, they tend to get organized in a hierarchical order that shares the same fundamental aspect of functional materials and living organisms. The primary goal of this thesis is to explore the combined impacts of electric and magnetic fields on colloidal assembly and actuation. We first report a strategy combining a planar magnetic field with a one-dimensional electric field to assemble and actuate linear chains made of paramagnetic microspheres. While a horizontal chain lying on the substrate is symmetric fore and aft and does not translate, a two-dimensional magnetic field can tilt the chain with an angle relative to the substrate. A superimposed alternating-current electric field leads to the propulsion of tilted chains along the substrate due to an unbalanced electrohydrodynamic flow. Using the magnetic field for steering and electric field for driving, we reveal a new propulsion mechanism that breaks the symmetry of hydrodynamic flow by manipulating the orientation of a microscopic object. We have also applied electric and magnetic fields orthogonally to magnetic microspheres, which allow us to independently control the magnitude and direction of dipolar attraction and repulsion between the particles. As a result, we obtain various kinds of well-aligned hierarchical structures based on the building blocks of microspheres and colloidal clusters of trimers, tetramers, heptamers, and nonamers. Our results demonstrate the potential in using combined fields to make diversified types of highly aligned structures for applications in high strength composites, optical materials, and battery electrodes.