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Out-of-equilibrium behavior of colloidal particles under AC electric fields
Yang, Xingfu
Yang, Xingfu
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2019
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2020-09-27
Abstract
Starting from more than a decade ago, the assembly of colloidal particles under electric fields becomes a burgeoning topic. It offers a versatile platform for creating ordered metamaterials and manufacturing micro-/nano-devices. While interacting with the electric field, anisotropic particles exhibit unique behaviors due to asymmetric properties. Significant advances have been made in the manipulation and assembly of anisotropic particles, but fabricating novel structures in a controlled fashion remains an enduring challenge. This thesis primarily focuses on investigating the out-of-equilibrium behaviors of anisotropic particles under AC electric fields. As the simplest out-of-equilibrium example, isotropic particles of different compositions have been observed to induce contractile or extensile hydrodynamic flow under an AC electric field. To understand this phenomenon, we incorporate both the low-frequency dielectric dispersion (LFDD) model and Stern-layer conduction in the calculation of the electrohydrodynamic (EHD) flow around a spherical particle and compare the theoretical predictions with experimental measurements. The LFDD model captures the dynamic polarization of particles at low frequencies, and the incorporation of Stern-layer conductance resolves the puzzle that EHD flow surrounding particles with moderate zeta potential can be extensile. Experimentally, we demonstrate that the opposite EHD flow directions dictate the different assembly behaviors of polystyrene and silica microspheres. We quantitively measure the Stern-layer conductivity of polystyrene particles and use this information to successfully predict both direction and magnitude of its surrounding EHD flow, which match well with experiments. With the understanding of EHD flow around a sphere, we further investigate the propulsion and active assembly of polystyrene dimers under AC electric fields. We study the collective behaviors of colloidal dimers actuated by a perpendicularly applied AC electric field, which controls the electrohydrodynamic flow at the sub-particle level. Although these motors experience strong dipolar repulsion from each other and are highly active, surprisingly, they assemble into a family of planar clusters with handedness. We show that this type of unusual structure arises from the contractile hydrodynamic flow around small lobes but extensile flow around the large lobes. By fine-tuning the surface charge asymmetry on particles and salt concentration in solution, we demonstrate the ability to control their collective behaviors on demand. Finally, we investigate the impact of geometry and chemical composition on two-dimensional propulsion of asymmetric particles under both parallel- and coplanar-electrode setups in the high frequency regime (~100 kHz - MHz). We find that a metallo-dielectric Janus sphere exhibits reversed motion where the metallic lobe orients forward when the substrate is either conducting or insulating. On the coplanar-electrode setup, the metallo-dielectric dimers change their orientations from parallel to perpendicular to the applied field at a critical frequency, which is the same one under which the dimers reverse their propulsion directions. More importantly, we have also discovered that asymmetric all-dielectric particles can propel in the high-frequency regime and its propulsion direction is opposite to that at low frequencies. Interestingly, the particles are also lifted above the conducting substrate while propelling. Our experiments using nanoparticle tracers further reveal strong and contractile flow around each lobe, suggesting an unbalanced electrohydrody-namic flow originated from the electroosmosis of double layer near the electrode. This new propulsion mechanism greatly extends the availability of active colloids and may inspire new design of active matters.
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