(485e) Electrohydrodynamic Flow Around a Colloidal Sphere Under AC Electric Fields: A Combined Theoretical and Experimental Study

Particles under an AC electric field can assemble into a variety of complex structures and exhibit tunable locomotion. Electrohydrodynamic (EHD) flow around a spherical particle is essential to understand the assembly and propulsion behavior of both isotropic and asymmetric particles. Although possessing very similar physical properties such as size, zeta potential, and dielectric constant, silica particles form two-dimensional close-packed hexagonal crystals in deionized water under an AC electric field that is applied perpendicularly to the conducting substrate, while polystyrene spheres repel each other. To understand this, we incorporated both the low-frequency dielectric dispersion (LFDD) model and Stern-layer conductance in the theoretical model of EHD flow around a dielectric spherical particle. The LFDD model allowed us to correctly capture the dynamic polarization of particles at low frequencies, a critical component to quantitatively predict both the direction and magnitude of EHD flow. We systematically examined the impacts of frequency, zeta potential, Stern-layer conductance, salt concentration, and particle diameter on the EHD flow. We found that particles with high zeta potential, small diameter, or immersed in low salt centration solution tend to have extensile EHD flow surrounding them because they enhance the surface conductivity in the double layer and make the particles effectively more conductive than the medium. On the experimental side, by using nano-tracers, we revealed that the opposite EHD flow directions underlie different assembly behavior of polystyrene and silica particles. We demonstrated that the EHD flow around silica particles is contractile while it is extensile around polystyrene particles. We further measured the Stern-layer conductance of polystyrene spheres experimentally and used it in the theoretical model to calculate the EHD flow surrounding them, which matched well with experiments in terms of flow direction and magnitude. Therefore, the incorporation of LFDD model and Stern-layer conductance resolve the puzzle that EHD flow surrounding a particle with moderate zeta potentials can be extensile. Our calculation on spherical particles, when applying it to a scaling law, can also successfully predict the propulsion direction and speed of asymmetric dimers. The enhanced understanding of EHD flow around individual spheres markedly facilitates the studies of colloidal active matter and their collective behaviors under electric fields.