The primary goal of this project is to develop new approaches to fabricate clusters of colloids with broken symmetries in geometry, configuration, and composition. These asymmetric properties will then be exploited to study two representative out-of- and in-equilibrium behavior of colloidal clusters, which are fundamentally different from conventional colloids.

First, the assembly of colloidal molecules from a binary mixture of polystyrene spheres was investigated under an alternating current electric field. The rich family of assembled oligomers typically consists of a large sphere surrounded by a few smaller petal particles, driven by the dipolar attraction between large and small particles. The number of satellite particles increased with the increasing size ratio of the constituent particles. For a given size ratio, it decreased with the increasing frequency of the applied field. These trends have also been correctly captured by computing the electric energy of different oligomers based on induced dipolar and double-layer interactions. This method provided a robust way to produce a family of colloidal molecules with well-defined geometry and high yield.

In addition to microspheres, the propulsion and assembly of colloidal clusters made of asymmetric dimers were also studied. It was demonstrated that incorporating Stern-layer conductivity resolved the puzzle that the electrohydrodynamic (EHD) flow surrounding a particle with moderate zeta potentials can be extensile. Asymmetric EHD motors made from polystyrene and silica particles exhibited behaviors consistent with the model predictions. In the high-frequency regime, a new phenomenon was discovered. All-dielectric asymmetric particles can propel along the substrate. In particular, polystyrene-silica dimers propelled with the polystyrene lobe forward at an elevated height above the electrode. With the help of nanoparticle tracers, strong and contractile hydrodynamic flow surrounding both lobes was observed. Since the frequency regime for active motion is very close to the inverse of the time scale for ions migrating in and out of the double layer, it is hypothesized that the hydrodynamic flow is originated from the electro-osmosis of induced charges within the double layer near the electrode can potentially be the mechanism. This new propulsion mechanism greatly expands the material library for designing active motors for both fundamental and practical studies.

A better understanding of the EHD flow surrounding colloidal dimers was further utilized to assemble them into colloidal clusters with chirality, which have potential use as sensors to detect molecules or metamaterials with exotic optical properties. Applying electric fields can assemble dielectric dumbbell-shaped particles coated with a thin magnetic layer into chiral clusters with an equal right- and left-handedness population. However, with the superposition of an additional rotating magnetic field, the chirality of the clusters changed according to the rotating direction of the magnetic field. By controlling the direction of the magnetic field, the cluster chirality can be reversibly switched. This finding provides a convenient route to produce chiral colloidal clusters with single-handedness. 

Beyond small colloidal clusters, flexible magnetic colloidal chains were also fabricated. They are inspired by microscale swimmers that utilize long filaments or slender bodies for propulsion since one-dimensional chains are the simplest structure that can be bent, twisted, braided, or folded into a wide range of geometrically or topologically complex morphologies. As the result of their reconfigurability, the long colloidal chains can switch propulsion mode from free-swimming to surface-enabled translation, enabling navigation through complex 3D environments such as channels that mimic arteries, veins, and capillaries. The demonstrated shape change and adaptability are ubiquitous in natural systems and are necessary for microbot navigation in complex environments.

This project supported one master student and one postdoctoral fellow. It also partially supported three Ph. D. students. In addition, we have mentored two REU students in the summers and three undergraduate students at Colorado School of Mines. Seven peer-reviewed papers have been published. Three additional manuscripts are under preparation. More than twenty talks were given based on the work performed in this project. Research findings in this work have been incorporated into a book coedited by the PI, “Anisotropic Particle Assemblies” (Elsevier, 2018), and a graduate-level course entitled “Engineering of Soft Materials.” The PI and his graduate students also prepared learning modules on colloids, surfactants, and emulsions and presented them to participants in the Summer Camp for Kids with Dyslexia and Summer Workshop on Energy Education for STEM high school Teachers.

Last Modified: 08/16/2021
Modified by: Ning Wu