The Impact of Stern‐Layer Conductivity on the Electrohydrodynamic Flow Around Colloidal Motors under an Alternating Current Electric Field

Particles under a perpendicularly applied alternating current (AC) electric field assemble into complex structures and exhibit tunable locomotion. Although they possess very similar physical properties, silica particles form two‐dimensional (2D) close‐packed crystals in deionized water, whereas polystyrene (PS) spheres repel each other. Using nanoparticle tracers, it is shown that the electrohydrodynamic (EHD) flow around silica particles is contractile, whereas it is extensile around PS particles. The Stern‐layer conductivities of PS and silica spheres are further measured experimentally and used in theoretical models to calculate the EHD flow surrounding them, which matches well with experiments. Therefore, the incorporation of Stern‐layer conductivity resolves the puzzle that EHD flow surrounding a particle with moderate zeta potentials is extensile. The impacts of zeta potential, Stern‐layer conductivity, salt concentration, and particle size on the EHD flow are examined herein. It is found that particles with high zeta potential, small diameter, or immersed in low salt concentration solutions tend to have extensile EHD flow surrounding them because the enhanced surface conductivity in the double layer makes the particles effectively more polarizable than the solvent. Herein, it is further shown that asymmetric EHD motors made from PS and silica particles exhibit behaviors that are consistent with the model predictions. Due to differences in the Stern‐layer conductivity, the electrohydrodynamic (EHD) flow around a silica microsphere is contractile, whereas it is extensile around a polystyrene (PS) microsphere. This resolves the puzzle that particles with moderate zeta potentials can generate extensile EHD flow. Furthermore, asymmetric EHD motors made from PS and silica particles exhibit behaviors that are consistent with the model predictions.