Loading...
Manipulation of colloidal particles under electric and magnetic fields
Zhu, Xingrui
Zhu, Xingrui
Citations
Altmetric:
Advisor
Editor
Date
Date Issued
2024
Date Submitted
Keywords
Collections
Research Projects
Organizational Units
Journal Issue
Embargo Expires
Abstract
Colloidal particles have garnered significant attention over the past few decades. Beyond their everyday applications in food science, lubricants, and cosmetics, they also serve as fundamental building blocks for advanced materials. An interesting subset of these particles, known as anisotropic particles, with their asymmetric geometric, interfacial, or compositional properties, have demonstrated remarkable potential across a spectrum of applications, ranging from self- and directed assembly to the development of microrobots. Extensive research efforts have been dedicated to synthesizing, assembling, and actuating anisotropic particles using external fields. However, a knowledge gap exists concerning the combined effects of electric and magnetic fields on both the assembly and active motion of colloidal particles. The convergence of these fields promises to address several unresolved issues in particle assembly and motion. This thesis is dedicated to exploring new propulsion and assembly behaviors of colloidal particles under the influence of electric and magnetic fields.
Previously, we found that colloidal dimers with asymmetric shapes can propel along the substrate when subjected to a perpendicular electric field. However, their moving directions are random. Inspired by the separate engine and steering wheel systems in automobiles, we use orthogonally applied alternate-current electric field and direct-current magnetic field to control the dimer’s speed and direction independently. To this end, we first synthesize magnetic dimers by coating dopamine-functionalized nanoparticles on geometrically asymmetric polystyrene dimers. We then characterize their static and dynamic susceptibilities by measuring the hysteresis diagram and rotation speed experimentally and comparing them with theoretical predictions. The dimers can align their long axes quickly with a planar direct-current magnetic field, allowing us to control the particles’ orientation accurately. The propulsion speed of dimers is tuned by an alternating-current electric field applied perpendicularly to the substrate. As a result, we can direct the particle’s motion with pre-designed trajectories of complex shapes. Our bulk-synthesis approach has the potential of making other types of magnetically anisotropic particles.
The synthesized magnetic dimers can be further assembled into chiral clusters under AC electric fields. However, similar to their molecular counterparts, these assemblies often result in racemic mixtures. We invent an approach to obtain single-handed clusters from colloidal dimers using orthogonal electric and magnetic fields. By superimposing a planar rotating magnetic field, we break the image symmetry so that one chirality is favored over the other. By adjusting the magnetic field’s direction and strength, as well as the electric field frequency, we can not only control the handedness of a cluster precisely but also induce uniform chirality in initially achiral clusters when exposed solely to the electric field. This work demonstrates the potential of integrating external fields and provides a viable way to create reconfigurable chiral colloidal structures.
Alternating current electric fields can assemble microspheres into various colloidal clusters, but the mechanisms driving this process remain unclear. We investigate how particle concentration, salt concentration, and electric field frequency influence the formation and transformation of these clusters. By experimentally measuring the strengths of dipolar and electrohydrodynamic interactions and analyzing the balance of these forces under different conditions, we explain the observed morphological changes in the clusters. Moreover, we find that as the frequency increases at high particle concentrations, colloidal tetramers transform into square-shaped pentamers, which can further assemble into square or sigma-phase arrays–complex structures that are difficult to achieve with isotropic particles.
Overall, this thesis explores the multifaceted realm of colloidal particles and their dynamic response to external fields, offering fundamental insights toward practical applications across various domains.
Associated Publications
Rights
Copyright of the original work is retained by the author.