Active Assembly of Colloids with Broken Symmetries Under AC Electric Fields

Ning Wu

Department of Chemical & Biological Engineering

Colorado School of Mines

Abstract

Colloids are important for our daily life and modern technologies. Among which colloidal particles with anisotropic properties in geometry, surface functionality, and chemical composition emerge as an important family of the colloidal genome. Scientifically, the in- and out-of-equilibrium behavior of anisotropic colloids are fundamentally different from conventional particles because of their broken symmetry in particle properties, colloidal interactions, and surrounding hydrodynamic flow. Technologically, they can be used to make metamaterials with exotic optical properties and micro-/nano-scale automotive devices.

In this talk, I will discuss our recent studies on the in- and out-of-equilibrium behavior of anisotropic particles. In particular, we will show that these particles possess orientation-dependent interactions under applied electric fields. Surprisingly rich structures and crystalline arrays have been observed in experiments. Our theoretical modeling and numerical simulation demonstrate that the competition and balance between electrostatic and electro-hydrodynamic interactions dictate different phases and the assembly paths between anisotropic particles. Breaking symmetry is also an important strategy to induce propulsion of microscopic objects in low Reynolds number flow. Here, we will describe a new type of particle propulsion mechanism that is based on breaking the symmetry of electrokinetic flow using AC electric fields. We show that dielectric colloidal dimers with broken symmetry in geometry, composition, or interfacial charges can all propel in directions that are perpendicular to the applied AC electric field. The asymmetry in particle properties ultimately results in an unbalanced electrohydrodynamic flow on two sides of the particles. Consistent with scaling laws, the propulsion direction, speed, and orientation of dimers can be conveniently tuned by frequency. Beyond active motion of individual particles, the electrohydrodynamic interaction between specific parts of the asymmetric motors also drives the formation of close-packed clusters with chirality. Our studies not only provide insights in the non-equilibrium physics for active colloids, but also propose new routes for making functional materials based on the building blocks of active colloids.