Impact of the Electric Polarizability on the Transport and Collective Dynamics of Metallodielectric Janus Particles

The objective of this article is to investigate how the electric polarizability manifests on the propulsion and collective dynamics of metallodielectric Janus particles by comparing the velocity spectra under rotating and nonrotating AC fields. Janus particles were fabricated by depositing sequential layers of titanium and SiO₂ on spherical cores. Model systems of known polarizability were created by varying the thickness of titanium or by adjusting the concentration of electrolyte. We found that the spectra for propulsion velocity displayed features (amplitude and transition frequencies) that were closely matched in the electrorotation spectra. That is, the transition frequency from dielectric- to metal-side forward matched closely the peak in counterfield rotation, while the minima in propulsion velocity matched the transition frequency from counterfield to cofield rotation. Furthermore, based on electroorientation measurements for prolate Janus ellipsoids, we conclude that the propulsion velocity of spherical Janus particles reflects the real part of their polarizability. Solutions of the Poisson-Nernst-Planck equations confirm the thickness of the metal cap facilitates adjusting the behavior from metal- to dielectric-like. These traits translate into different collective behaviors, such as the ability to traverse or become part of a lattice of nonpatchy silica particles. Overall, these results provide experimental evidence to either challenge or refine existing electrokinetic models of propulsion.