(531j) Single and Ensemble Response of Colloidal Ellipsoids to a Nearby Ac Electrode

“Bottom-up” assembly of anisotropic micrometer-scale colloidal particles could be used to build structures with complex and potentially technologically relevant structures. One pathway to accomplishing bottom-up assembly is to use an external electric field, which has regularly been used to assemble isotropic particles. Herein, we used a parallel plate electrochemical cell to study the response of colloidal ellipsoids to an external electric field. Specifically, we measured the structural evolution and the aggregation kinetics of these geometrically anisotropic particles. The response of pseudo-2D layers of colloids with systematically tuned aspect ratios was measured as a function of electric field strength and frequency. We found the regions of attraction and separation were different between spheres and ellipsoids. Moreover, ellipsoidal particles showed a hysteretic behavior in aggregation and separation, meaning the critical frequency of structure transition located ~ 300 Hz – 500 Hz when increasing the frequency, but became ~ 1000 Hz – 500 Hz when decreasing frequency. With regimes of frequency matching structure of separation, only the low aspect ratio ellipsoids aligned with the direction of the electric field at some conditions. However, additional measurements suggested the rate constant for different types of colloidal particles all increased from 25 Hz and reached a maximum between 50 Hz – 100 Hz regardless of the aspect ratio. Based on these data, complementary experiments introduced a method for obtaining long-lasting deposition of ellipsoids in energetically unfavorable orientations, which will be useful as a model system for the anisotropic particles light scattering study.