Fabrication and Electric Field-Driven Active Propulsion of Patchy Microellipsoids

Abstract

Active colloids are a synthetic analogue of biological microorganisms that consume external energy to swim through viscous fluids. Such motion requires breaking the symmetry of the fluid flow in the vicinity of a particle; however, it is challenging to understand how surface and shape anisotropies of the colloid lead to a particular trajectory. Here, we attempt to deconvolute the effects of particle shape and surface anisotropy on the propulsion of model ellipsoids in alternating current (AC) electric fields. We first introduce a simple process for depositing metal patches of various shapes on the surfaces of ellipsoidal particles. We show that the shape of the metal patch is governed by the assembled structure of the ellipsoids on the substrate used for physical vapor deposition. Under high-frequency AC electric field, ellipsoids dispersed in water show linear, circular, and helical trajectories which depend on the shapes of the surface patches. We demonstrate that features of the helical trajectories such as the pitch and diameter can be tuned by varying the degree of patch asymmetry along the two primary axes of the ellipsoids, namely longitudinal and transverse. Our study reveals the role of patch shape on the trajectory of ellipsoidal particles propelled by induced charge electrophoresis. We develop heuristics based on patch asymmetries that can be used to design patchy particles with specified nonlinear trajectories.

Field-Induced Assembly and Propulsion of Colloids

“…Glancing angle vapor deposition allows the fabrication of patchy particles with even lower symmetry than the Janus hemispherical configuration. This technique was recently used to demonstrate the unique role of surface anisotropy in encoding complex motions under ac electric fields. , Spherical polystyrene microspheres coated with low-symmetry patches of gold swim in nonlinear 3D trajectories. Notably, a triangular patch allows particles to move in a helical path similar to the swimming of spermatozoa and other microorganisms .…”

“…Polystyrene ellipsoids are readily obtained by stretching spherical microparticles to a specific aspect ratio. , Such anisotropic particles can then be rendered patchy using the same metal vapor deposition techniques used for microspheres but yielding more complex patch shapes. Lee et al reported a way to fabricate and characterize such particles based on the patch asymmetry, measured from the metal coverage of the transverse and longitudinal axis of the ellipsoid . These active particles access linear, circular, and helical motions based on the type of gold patch on their surface (Figure c) .…”

“…Lee et al reported a way to fabricate and characterize such particles based on the patch asymmetry, measured from the metal coverage of the transverse and longitudinal axis of the ellipsoid. 10 These active particles access linear, circular, and helical motions based on the type of gold patch on their surface (Figure 7c). 10 For the helical motion, as the longitudinal symmetry of the patch increases, the helical pitch generally decreases.…”

“…Research in colloidal assembly and propulsion has greatly benefited from the recent progress in the synthesis and fabrication of particles with reduced symmetry. − As will be further discussed below, the use of external electric and magnetic fields often requires the presence of an electrically conductive or magnetic domain . Depositing thin patches of metal on the surface of particles is one of the most common ways for experimentalists to introduce such domains in colloidal systems. , These patchy particles are most often polymer or silica microspheres with thin layers (∼10 1 nm) of metal such as gold, iron, or nickel deposited on selected areas to form patches on the surface.…”

Rigidity Dictates Spontaneous Helix Formation of Thermoresponsive Colloidal Chains in Poor Solvent

“…Recently, heterodimeric nanoclusters designed to promote specific intercluster interactions have been shown to assemble into double helical strands . Patchy microellipsoids have been recently shown to follow helical trajectories in an AC electric field . Helicity has technological implications, for example, in functional optical materials − and in nanomechanical systems. − Further, colloids are excellent model systems to examine the physics of self-assembly: micrometer-sized colloidal particles exhibit Brownian motion and are sufficiently large to allow visualization using optical microscopy.…”