Directed Assembly and Manipulation of Anisotropic Colloidal Particles by External Fields

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Title: Directed Assembly and Manipulation of Anisotropic Colloidal Particles by External Fields
Author: Gangwal, Sumit
Advisors: Gregory N. Parsons, Committee Member
John F. Muth, Committee Member
Saad A. Khan, Committee Member
Orlin D. Velev, Committee Chair
Abstract: The application of external fields to anisotropic particles can be an efficient means of programmed assembly of novel materials and is a rapidly expanding research field. We report a series of studies on the assembly and manipulation of surface patterned anisotropic colloidal particles (whose surfaces are physically or chemically different) by external alternating current (AC) electric and magnetic fields. The fundamental results include the first experimental observation of induced-charge electrophoretic (ICEP) motion of asymmetric metallodielectric microspheres and the formation of novel assembled structures of these particles by dielectrophoresis (particle interaction with external AC electric ﬠeld gradients) and by magnetophoresis (migration and interaction of particles in an inhomogeneous magnetic field). The experimental and modeling techniques developed and fundamental principles uncovered could be used to engineer the processes of directed and/or programmed assembly of other types of anisotropic particles. “Janus†particles were prepared by coating dielectric, polystyrene latex microspheres with a conductive metal layer on one hemisphere. The phase space for AC electric field intensity and frequency was explored for these particles on a glass surface between two electrodes. A rich variety of metallodielectric structures and dynamics were uncovered, which are very different from those obtained from directed dielectrophoretic assembly of plain dielectric or plain conductive particles. The application of low frequency AC ﬠelds to aqueous suspensions of the Janus particles leads to unbalanced liquid flows around each half of the particle causing nonlinear, ICEP particle motion (perpendicular to the ﬠeld direction). Above 10 kHz field frequency, the metallodielectric particles assemble into new types of chain structures, where the metallized halves of neighboring particles align into lanes along the ﬠeld direction. These staggered chains were confined together to form two-dimensional metallodielectric crystals. The experimental results of the orientation of Janus particles in the electric field and the formation of staggered chains were interpreted by means of numerical simulations of the electric energy of the system. The assembly of Janus metallodielectric particles may ﬠnd applications in liquid-borne microcircuits and materials with directional electric and heat transfer. The electrokinetic motion of the particles may ﬠnd applications in microactuators and microfluidic devices. The assembly of magnetic Janus colloids (having 50% surface coating of iron on polystyrene microspheres) under the combined (and sometimes competing) dielectrophoretic and magnetophoretic forces was investigated. The structures formed by magnetic fields have the advantage that the particle interactions are bistable. They can result in permanent structures, which could be disassembled on demand by remote demagnetization and then reassembled into new stable structures, thus recycling the building blocks. The assembly of magnetic anisotropic particles may find numerous potential applications, among which are bifunctional drug delivery agents and novel flexible displays. We found that even more unusual types of new structures are formed when high frequency (> 50 kHz) AC electric fields are applied to suspensions of “patchy†particles. The microspheres, produced by glancing angle metal deposition, have either a single patch that is less than 50% of the total latex particle surface or two metallic patches on each pole of the particle. These patchy particles assemble in electric fields by interacting with each other in two or more directions, pre-programmed by the patch size and orientation. The multi-directional chains were confined together to form a percolated network of particles and lattices of unusual symmetry. Simulation results indicate that the assembly pattern of these particles into multi-directional chains is guided by quadrupolar and multipolar interactions, which allow for the future development of new strategies for highly controlled “programmed†assembly by external fields.
Date: 2010-01-12
Degree: PhD
Discipline: Chemical Engineering

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