Effect of Particle Composition on Dielectrophoretic Co-Assemblies around Topographic Features: Experiment and Theory

Abstract

Dielectrophoresis is a powerful method for colloidal particle assembly that is particularly suited to forming structures that can be reconfigured via changes in field conditions. Realization of functional assemblies for optical applications requires consideration of particle material properties that determine both the optical response and particles' dielectrophoretic behavior. Few studies have directly compared dielectrophoretic assembly of different materials or investigated assemblies having binary mixtures of different particle compositions. Here, we investigate the assembly of individual and binary assembly mixtures of ∼1 μm microspheres around microfabricated photoresist posts using both experiment and simulation. The particles include a semiconductor (TiO 2 ) and two insulating materials, one inorganic (SiO 2 ) and one organic [poly(methyl methacrylate) (PMMA)]. Individually, SiO 2 and PMMA assembled atop photoresist posts as the frequency and voltage were increased, displaying negative dielectrophoresis. TiO 2 particles assembled around the sides of the posts at low frequencies and atop the posts only at higher frequencies and voltages. These behaviors for all particle types were observed in both simulation and experiment. Mixtures of TiO 2 particles with either SiO 2 or PMMA, however, did not show a straightforward combination of their individual assembly properties, and their assembly depended upon the relative concentrations of the two particle types. This phenomenon brings to light an electromodulative shielding effect that can affect multiparticle assemblies. By adjusting conditions, it was possible to reconfigure assemblies of semiconducting and insulating particles to sort them to different sites or combine them together. These experiments and simulations provide insights into the assembly of particles with varying material properties, helping set the stage for the development of functional binary systems able to provide field-responsive spatial variations in refractive index and/or tunable scattering.